Harper's Biochemistry 26th ED, Schemes and Mind Maps for Medical Biochemistry
Djordje.Stankovic
Djordje.Stankovic

Harper's Biochemistry 26th ED, Schemes and Mind Maps for Medical Biochemistry

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Harper’s Illustrated Biochemistry

a LANGE medical book

twenty-sixth edition

Robert K. Murray, MD, PhD Professor (Emeritus) of Biochemistry University of Toronto Toronto, Ontario

Daryl K. Granner, MD Joe C. Davis Professor of Biomedical Science Director, Vanderbilt Diabetes Center Professor of Molecular Physiology and Biophysics and of Medicine Vanderbilt University Nashville, Tennessee

Peter A. Mayes, PhD, DSc Emeritus Professor of Veterinary Biochemistry Royal Veterinary College University of London London

Victor W. Rodwell, PhD Professor of Biochemistry Purdue University West Lafayette, Indiana

Lange Medical Books/McGraw-Hill Medical Publishing Division

New York Chicago San Francisco Lisbon London Madrid Mexico City Milan New Delhi San Juan Seoul Singapore Sydney Toronto

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Harper’s Illustrated Biochemistry, Twenty-Sixth Edition

Copyright © 2003 by The McGraw-Hill Companies, Inc. All rights reserved. Printed in the United States of America. Except as permitted under the United States Copyright Act of 1976, no part of this publication may be reproduced or distributed in any form or by any means, or stored in a data base or retrieval system, without the prior written permission of the publisher.

Previous editions copyright © 2000, 1996, 1993, 1990 by Appleton & Lange; copyright © 1988 by Lange Medical Publications.

2 3 4 5 6 7 8 9 0 DOC/DOC 0 9 8 7 6 5 4 3

ISBN 0-07-138901-6 ISSN 1043-9811

Notice

Medicine is an ever-changing science. As new research and clinical experience broaden our knowledge, changes in treat- ment and drug therapy are required. The authors and the publisher of this work have checked with sources believed to be reliable in their efforts to provide information that is complete and generally in accord with the standards accepted at the time of publication. However, in view of the possibility of human error or changes in medical sciences, neither the authors nor the publisher nor any other party who has been involved in the preparation or publication of this work warrants that the information contained herein is in every respect accurate or complete, and they disclaim all responsibility for any er- rors or omissions or for the results obtained from use of the information contained in this work. Readers are encouraged to confirm the information contained herein with other sources. For example and in particular, readers are advised to check the product information sheet included in the package of each drug they plan to administer to be certain that the information contained in this work is accurate and that changes have not been made in the recommended dose or in the contraindications for administration. This recommendation is of particular importance in connection with new or infre- quently used drugs.

This book was set in Garamond by Pine Tree Composition The editors were Janet Foltin, Jim Ransom, and Janene Matragrano Oransky. The production supervisor was Phil Galea. The illustration manager was Charissa Baker. The text designer was Eve Siegel. The cover designer was Mary McKeon. The index was prepared by Kathy Pitcoff. RR Donnelley was printer and binder.

This book is printed on acid-free paper.

ISBN-0-07-121766-5 (International Edition) Copyright © 2003. Exclusive rights by the McGraw-Hill Companies, Inc., for manufacture and export. This book cannot be re-exported from the country to which it is consigned by McGraw-Hill. The International Edition is not available in North America.

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Authors

David A. Bender, PhD

Sub-Dean Royal Free and University College Medical School, Assistant Faculty Tutor and Tutor to Med- ical Students, Senior Lecturer in Biochemistry, De- partment of Biochemistry and Molecular Biology, University College London

Kathleen M. Botham, PhD, DSc

Reader in Biochemistry, Royal Veterinary College, University of London

Daryl K. Granner, MD

Joe C. Davis Professor of Biomedical Science, Director, Vanderbilt Diabetes Center, Professor of Molecular Physiology and Biophysics and of Medicine, Vander- bilt University, Nashville, Tennessee

Frederick W. Keeley, PhD

Associate Director and Senior Scientist, Research Insti- tute, Hospital for Sick Children, Toronto, and Pro- fessor, Department of Biochemistry, University of Toronto

Peter J. Kennelly, PhD

Professor of Biochemistry, Virginia Polytechnic Insti- tute and State University, Blacksburg, Virginia

Peter A. Mayes, PhD, DSc

Emeritus Professor of Veterinary Biochemistry, Royal Veterinary College, University of London

Robert K. Murray, MD, PhD

Professor (Emeritus) of Biochemistry, University of Toronto

Margaret L. Rand, PhD

Scientist, Research Institute, Hospital for Sick Chil- dren, Toronto, and Associate Professor, Depart- ments of Laboratory Medicine and Pathobiology and Department of Biochemistry, University of Toronto

Victor W. Rodwell, PhD

Professor of Biochemistry, Purdue University, West Lafayette, Indiana

P. Anthony Weil, PhD

Professor of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nash- ville, Tennessee

vii

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Preface

ix

The authors and publisher are pleased to present the twenty-sixth edition of Harper’s Illustrated Biochemistry. Review of Physiological Chemistry was first published in 1939 and revised in 1944, and it quickly gained a wide readership. In 1951, the third edition appeared with Harold A. Harper, University of California School of Medicine at San Fran- cisco, as author. Dr. Harper remained the sole author until the ninth edition and co-authored eight subsequent edi- tions. Peter Mayes and Victor Rodwell have been authors since the tenth edition, Daryl Granner since the twentieth edition, and Rob Murray since the twenty-first edition. Because of the increasing complexity of biochemical knowl- edge, they have added co-authors in recent editions.

Fred Keeley and Margaret Rand have each co-authored one chapter with Rob Murray for this and previous edi- tions. Peter Kennelly joined as a co-author in the twenty-fifth edition, and in the present edition has co-authored with Victor Rodwell all of the chapters dealing with the structure and function of proteins and enzymes. The follow- ing additional co-authors are very warmly welcomed in this edition: Kathleen Botham has co-authored, with Peter Mayes, the chapters on bioenergetics, biologic oxidation, oxidative phosphorylation, and lipid metabolism. David Bender has co-authored, also with Peter Mayes, the chapters dealing with carbohydrate metabolism, nutrition, diges- tion, and vitamins and minerals. P. Anthony Weil has co-authored chapters dealing with various aspects of DNA, of RNA, and of gene expression with Daryl Granner. We are all very grateful to our co-authors for bringing their ex- pertise and fresh perspectives to the text.

CHANGES IN THE TWENTY-SIXTH EDITION

A major goal of the authors continues to be to provide both medical and other students of the health sciences with a book that both describes the basics of biochemistry and is user-friendly and interesting. A second major ongoing goal is to reflect the most significant advances in biochemistry that are important to medicine. However, a third major goal of this edition was to achieve a substantial reduction in size, as feedback indicated that many readers pre- fer shorter texts.

To achieve this goal, all of the chapters were rigorously edited, involving their amalgamation, division, or dele- tion, and many were reduced to approximately one-half to two-thirds of their previous size. This has been effected without loss of crucial information but with gain in conciseness and clarity.

Despite the reduction in size, there are many new features in the twenty-sixth edition. These include:

• A new chapter on amino acids and peptides, which emphasizes the manner in which the properties of biologic peptides derive from the individual amino acids of which they are comprised.

• A new chapter on the primary structure of proteins, which provides coverage of both classic and newly emerging “proteomic” and “genomic” methods for identifying proteins. A new section on the application of mass spectrometry to the analysis of protein structure has been added, including comments on the identification of covalent modifica- tions.

• The chapter on the mechanisms of action of enzymes has been revised to provide a comprehensive description of the various physical mechanisms by which enzymes carry out their catalytic functions.

• The chapters on integration of metabolism, nutrition, digestion and absorption, and vitamins and minerals have been completely re-written.

• Among important additions to the various chapters on metabolism are the following: update of the information on oxidative phosphorylation, including a description of the rotary ATP synthase; new insights into the role of GTP in gluconeogenesis; additional information on the regulation of acetyl-CoA carboxylase; new information on receptors involved in lipoprotein metabolism and reverse cholesterol transport; discussion of the role of leptin in fat storage; and new information on bile acid regulation, including the role of the farnesoid X receptor (FXR).

• The chapter on membrane biochemistry in the previous edition has been split into two, yielding two new chapters on the structure and function of membranes and intracellular traffic and sorting of proteins.

• Considerable new material has been added on RNA synthesis, protein synthesis, gene regulation, and various as- pects of molecular genetics.

• Much of the material on individual endocrine glands present in the twenty-fifth edition has been replaced with new chapters dealing with the diversity of the endocrine system, with molecular mechanisms of hormone action, and with signal transduction.

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• The chapter on plasma proteins, immunoglobulins, and blood coagulation in the previous edition has been split into two new chapters on plasma proteins and immunoglobulins and on hemostasis and thrombosis.

• New information has been added in appropriate chapters on lipid rafts and caveolae, aquaporins, connexins, dis- orders due to mutations in genes encoding proteins involved in intracellular membrane transport, absorption of iron, and conformational diseases and pharmacogenomics.

• A new and final chapter on “The Human Genome Project” (HGP) has been added, which builds on the material covered in Chapters 35 through 40. Because of the impact of the results of the HGP on the future of biology and medicine, it appeared appropriate to conclude the text with a summary of its major findings and their implica- tions for future work.

• As initiated in the previous edition, references to useful Web sites have been included in a brief Appendix at the end of the text.

ORGANIZATION OF THE BOOK

The text is divided into two introductory chapters (“Biochemistry & Medicine” and “Water & pH”) followed by six main sections.

Section I deals with the structures and functions of proteins and enzymes, the workhorses of the body. Because almost all of the reactions in cells are catalyzed by enzymes, it is vital to understand the properties of enzymes before considering other topics.

Section II explains how various cellular reactions either utilize or release energy, and it traces the pathways by which carbohydrates and lipids are synthesized and degraded. It also describes the many functions of these two classes of molecules.

Section III deals with the amino acids and their many fates and also describes certain key features of protein ca- tabolism.

Section IV describes the structures and functions of the nucleotides and nucleic acids, and covers many major topics such as DNA replication and repair, RNA synthesis and modification, and protein synthesis. It also discusses new findings on how genes are regulated and presents the principles of recombinant DNA technology.

Section V deals with aspects of extracellular and intracellular communication. Topics covered include membrane structure and function, the molecular bases of the actions of hormones, and the key field of signal transduction.

Section VI consists of discussions of eleven special topics: nutrition, digestion, and absorption; vitamins and minerals; intracellular traffic and sorting of proteins; glycoproteins; the extracellular matrix; muscle and the cy- toskeleton; plasma proteins and immunoglobulins; hemostasis and thrombosis; red and white blood cells; the me- tabolism of xenobiotics; and the Human Genome Project.

ACKNOWLEDGMENTS

The authors thank Janet Foltin for her thoroughly professional approach. Her constant interest and input have had a significant impact on the final structure of this text. We are again immensely grateful to Jim Ransom for his excel- lent editorial work; it has been a pleasure to work with an individual who constantly offered wise and informed alter- natives to the sometimes primitive text transmitted by the authors. The superb editorial skills of Janene Matragrano Oransky and Harriet Lebowitz are warmly acknowledged, as is the excellent artwork of Charissa Baker and her col- leagues. The authors are very grateful to Kathy Pitcoff for her thoughtful and meticulous work in preparing the Index. Suggestions from students and colleagues around the world have been most helpful in the formulation of this edition. We look forward to receiving similar input in the future.

Robert K. Murray, MD, PhD Daryl K. Granner, MD

Peter A. Mayes, PhD, DSc Victor W. Rodwell, PhD

Toronto, Ontario Nashville, Tennessee London West Lafayette, Indiana March 2003

x / PREFACE

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Contents

iii

Authors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix

1. Biochemistry & Medicine Robert K. Murray, MD, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2. Water & pH Victor W. Rodwell, PhD, & Peter J. Kennelly, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

SECTION I. STRUCTURES & FUNCTIONS OF PROTEINS & ENZYMES . . . . . . . . . . . . . . . . . . . 14

3. Amino Acids & Peptides Victor W. Rodwell, PhD, & Peter J. Kennelly, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4. Proteins: Determination of Primary Structure Victor W. Rodwell, PhD, & Peter J. Kennelly, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

5. Proteins: Higher Orders of Structure Victor W. Rodwell, PhD, & Peter J. Kennelly, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

6. Proteins: Myoglobin & Hemoglobin Victor W. Rodwell, PhD, & Peter J. Kennelly, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

7. Enzymes: Mechanism of Action Victor W. Rodwell, PhD, & Peter J. Kennelly, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

8. Enzymes: Kinetics Victor W. Rodwell, PhD, & Peter J. Kennelly, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

9. Enzymes: Regulation of Activities Victor W. Rodwell, PhD, & Peter J. Kennelly, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

SECTION II. BIOENERGETICS & THE METABOLISM OF CARBOHYDRATES & LIPIDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

10. Bioenergetics: The Role of ATP Peter A. Mayes, PhD, DSc, & Kathleen M. Botham, PhD, DSc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

11. Biologic Oxidation Peter A. Mayes, PhD, DSc, & Kathleen M. Botham, PhD, DSc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

12. The Respiratory Chain & Oxidative Phosphorylation Peter A. Mayes, PhD, DSc, & Kathleen M. Botham, PhD, DSc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

13. Carbohydrates of Physiologic Significance Peter A. Mayes, PhD, DSc, & David A. Bender, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

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14. Lipids of Physiologic Significance Peter A. Mayes, PhD, DSc, & Kathleen M. Botham, PhD, DSc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

15. Overview of Metabolism Peter A. Mayes, PhD, DSc, & David A. Bender, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

16. The Citric Acid Cycle: The Catabolism of Acetyl-CoA Peter A. Mayes, PhD, DSc, & David A. Bender, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

17. Glycolysis & the Oxidation of Pyruvate Peter A. Mayes, PhD, DSc, & David A. Bender, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136

18. Metabolism of Glycogen Peter A. Mayes, PhD, DSc, & David A. Bender, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

19. Gluconeogenesis & Control of the Blood Glucose Peter A. Mayes, PhD, DSc, & David A. Bender, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153

20. The Pentose Phosphate Pathway & Other Pathways of Hexose Metabolism Peter A. Mayes, PhD, DSc, & David A. Bender, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163

21. Biosynthesis of Fatty Acids Peter A. Mayes, PhD, DSc, & Kathleen M. Botham, PhD, DSc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173

22. Oxidation of Fatty Acids: Ketogenesis Peter A. Mayes, PhD, DSc, & Kathleen M. Botham, PhD, DSc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180

23. Metabolism of Unsaturated Fatty Acids & Eicosanoids Peter A. Mayes, PhD, DSc, & Kathleen M. Botham, PhD, DSc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190

24. Metabolism of Acylglycerols & Sphingolipids Peter A. Mayes, PhD, DSc, & Kathleen M. Botham, PhD, DSc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197

25. Lipid Transport & Storage Peter A. Mayes, PhD, DSc, & Kathleen M. Botham, PhD, DSc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205

26. Cholesterol Synthesis, Transport, & Excretion Peter A. Mayes, PhD, DSc, & Kathleen M. Botham, PhD, DSc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219

27. Integration of Metabolism—the Provision of Metabolic Fuels David A. Bender, PhD, & Peter A. Mayes, PhD, DSc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231

SECTION III. METABOLISM OF PROTEINS & AMINO ACIDS . . . . . . . . . . . . . . . . . . . . . . . . . 237

28. Biosynthesis of the Nutritionally Nonessential Amino Acids Victor W. Rodwell, PhD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237

29. Catabolism of Proteins & of Amino Acid Nitrogen Victor W. Rodwell, PhD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242

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30. Catabolism of the Carbon Skeletons of Amino Acids Victor W. Rodwell, PhD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249

31. Conversion of Amino Acids to Specialized Products Victor W. Rodwell, PhD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264

32. Porphyrins & Bile Pigments Robert K. Murray, MD, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270

SECTION IV. STRUCTURE, FUNCTION, & REPLICATION OF INFORMATIONAL MACROMOLECULES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286

33. Nucleotides Victor W. Rodwell, PhD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286

34. Metabolism of Purine & Pyrimidine Nucleotides Victor W. Rodwell, PhD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293

35. Nucleic Acid Structure & Function Daryl K. Granner, MD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303

36. DNA Organization, Replication, & Repair Daryl K. Granner, MD, & P. Anthony Weil, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314

37. RNA Synthesis, Processing, & Modification Daryl K. Granner, MD, & P. Anthony Weil, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341

38. Protein Synthesis & the Genetic Code Daryl K. Granner, MD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 358

39. Regulation of Gene Expression Daryl K. Granner, MD, & P. Anthony Weil, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374

40. Molecular Genetics, Recombinant DNA, & Genomic Technology Daryl K. Granner, MD, & P. Anthony Weil, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396

SECTION V. BIOCHEMISTRY OF EXTRACELLULAR & INTRACELLULAR COMMUNICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415

41. Membranes: Structure & Function Robert K. Murray, MD, PhD, & Daryl K. Granner, MD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415

42. The Diversity of the Endocrine System Daryl K. Granner, MD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 434

43. Hormone Action & Signal Transduction Daryl K. Granner, MD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 456

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SECTION VI. SPECIAL TOPICS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474

44. Nutrition, Digestion, & Absorption David A. Bender, PhD, & Peter A. Mayes, PhD, DSc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474

45. Vitamins & Minerals David A. Bender, PhD, & Peter A. Mayes, PhD, DSc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 481

46. Intracellular Traffic & Sorting of Proteins Robert K. Murray, MD, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 498

47. Glycoproteins Robert K. Murray, MD, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 514

48. The Extracellular Matrix Robert K. Murray, MD, PhD, & Frederick W. Keeley, PhD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 535

49. Muscle & the Cytoskeleton Robert K. Murray, MD, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 556

50. Plasma Proteins & Immunoglobulins Robert K. Murray, MD, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 580

51. Hemostasis & Thrombosis Margaret L. Rand, PhD, & Robert K. Murray, MD, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 598

52. Red & White Blood Cells Robert K. Murray, MD, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 609

53. Metabolism of Xenobiotics Robert K. Murray, MD, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 626

54. The Human Genome Project Robert K. Murray, MD, PhD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 633

Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 639

Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 643

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Biochemistry & Medicine 1

1

Robert K. Murray, MD, PhD

biochemistry is increasingly becoming their common language.

A Reciprocal Relationship Between Biochemistry & Medicine Has Stimulated Mutual Advances

The two major concerns for workers in the health sci- ences—and particularly physicians—are the understand- ing and maintenance of health and the understanding and effective treatment of diseases. Biochemistry im- pacts enormously on both of these fundamental con- cerns of medicine. In fact, the interrelationship of bio- chemistry and medicine is a wide, two-way street. Biochemical studies have illuminated many aspects of health and disease, and conversely, the study of various aspects of health and disease has opened up new areas of biochemistry. Some examples of this two-way street are shown in Figure 1–1. For instance, a knowledge of protein structure and function was necessary to eluci- date the single biochemical difference between normal hemoglobin and sickle cell hemoglobin. On the other hand, analysis of sickle cell hemoglobin has contributed significantly to our understanding of the structure and function of both normal hemoglobin and other pro- teins. Analogous examples of reciprocal benefit between biochemistry and medicine could be cited for the other paired items shown in Figure 1–1. Another example is the pioneering work of Archibald Garrod, a physician in England during the early 1900s. He studied patients with a number of relatively rare disorders (alkap- tonuria, albinism, cystinuria, and pentosuria; these are described in later chapters) and established that these conditions were genetically determined. Garrod desig- nated these conditions as inborn errors of metabo- lism. His insights provided a major foundation for the development of the field of human biochemical genet- ics. More recent efforts to understand the basis of the genetic disease known as familial hypercholesterol- emia, which results in severe atherosclerosis at an early age, have led to dramatic progress in understanding of cell receptors and of mechanisms of uptake of choles- terol into cells. Studies of oncogenes in cancer cells have directed attention to the molecular mechanisms involved in the control of normal cell growth. These and many other examples emphasize how the study of

INTRODUCTION

Biochemistry can be defined as the science concerned with the chemical basis of life (Gk bios “life”). The cell is the structural unit of living systems. Thus, biochem- istry can also be described as the science concerned with the chemical constituents of living cells and with the reac- tions and processes they undergo. By this definition, bio- chemistry encompasses large areas of cell biology, of molecular biology, and of molecular genetics.

The Aim of Biochemistry Is to Describe & Explain, in Molecular Terms, All Chemical Processes of Living Cells

The major objective of biochemistry is the complete understanding, at the molecular level, of all of the chemical processes associated with living cells. To achieve this objective, biochemists have sought to iso- late the numerous molecules found in cells, determine their structures, and analyze how they function. Many techniques have been used for these purposes; some of them are summarized in Table 1–1.

A Knowledge of Biochemistry Is Essential to All Life Sciences

The biochemistry of the nucleic acids lies at the heart of genetics; in turn, the use of genetic approaches has been critical for elucidating many areas of biochemistry. Physiology, the study of body function, overlaps with biochemistry almost completely. Immunology employs numerous biochemical techniques, and many immuno- logic approaches have found wide use by biochemists. Pharmacology and pharmacy rest on a sound knowl- edge of biochemistry and physiology; in particular, most drugs are metabolized by enzyme-catalyzed reac- tions. Poisons act on biochemical reactions or processes; this is the subject matter of toxicology. Biochemical ap- proaches are being used increasingly to study basic as- pects of pathology (the study of disease), such as in- flammation, cell injury, and cancer. Many workers in microbiology, zoology, and botany employ biochemical approaches almost exclusively. These relationships are not surprising, because life as we know it depends on biochemical reactions and processes. In fact, the old barriers among the life sciences are breaking down, and

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2 / CHAPTER 1

disease can open up areas of cell function for basic bio- chemical research.

The relationship between medicine and biochem- istry has important implications for the former. As long as medical treatment is firmly grounded in a knowledge of biochemistry and other basic sciences, the practice of medicine will have a rational basis that can be adapted to accommodate new knowledge. This contrasts with unorthodox health cults and at least some “alternative medicine” practices, which are often founded on little more than myth and wishful thinking and generally lack any intellectual basis.

NORMAL BIOCHEMICAL PROCESSES ARE THE BASIS OF HEALTH

The World Health Organization (WHO) defines health as a state of “complete physical, mental and so- cial well-being and not merely the absence of disease and infirmity.” From a strictly biochemical viewpoint, health may be considered that situation in which all of the many thousands of intra- and extracellular reactions that occur in the body are proceeding at rates commen- surate with the organism’s maximal survival in the physiologic state. However, this is an extremely reduc- tionist view, and it should be apparent that caring for the health of patients requires not only a wide knowl- edge of biologic principles but also of psychologic and social principles.

Biochemical Research Has Impact on Nutrition & Preventive Medicine

One major prerequisite for the maintenance of health is that there be optimal dietary intake of a number of chemicals; the chief of these are vitamins, certain amino acids, certain fatty acids, various minerals, and water. Because much of the subject matter of both bio- chemistry and nutrition is concerned with the study of various aspects of these chemicals, there is a close rela- tionship between these two sciences. Moreover, more emphasis is being placed on systematic attempts to maintain health and forestall disease, ie, on preventive medicine. Thus, nutritional approaches to—for exam- ple—the prevention of atherosclerosis and cancer are receiving increased emphasis. Understanding nutrition depends to a great extent on a knowledge of biochem- istry.

Most & Perhaps All Disease Has a Biochemical Basis

We believe that most if not all diseases are manifesta- tions of abnormalities of molecules, chemical reactions, or biochemical processes. The major factors responsible for causing diseases in animals and humans are listed in Table 1–2. All of them affect one or more critical chemical reactions or molecules in the body. Numerous examples of the biochemical bases of diseases will be en- countered in this text; the majority of them are due to causes 5, 7, and 8. In most of these conditions, bio- chemical studies contribute to both the diagnosis and treatment. Some major uses of biochemical investiga- tions and of laboratory tests in relation to diseases are summarized in Table 1–3.

Additional examples of many of these uses are pre- sented in various sections of this text.

Table 1–1. The principal methods and preparations used in biochemical laboratories.

Methods for Separating and Purifying Biomolecules1

Salt fractionation (eg, precipitation of proteins with ammo- nium sulfate)

Chromatography: Paper; ion exchange; affinity; thin-layer; gas-liquid; high-pressure liquid; gel filtration

Electrophoresis: Paper; high-voltage; agarose; cellulose acetate; starch gel; polyacrylamide gel; SDS-polyacryl- amide gel

Ultracentrifugation Methods for Determining Biomolecular Structures

Elemental analysis UV, visible, infrared, and NMR spectroscopy Use of acid or alkaline hydrolysis to degrade the biomole-

cule under study into its basic constituents Use of a battery of enzymes of known specificity to de- grade the biomolecule under study (eg, proteases, nucle-

ases, glycosidases) Mass spectrometry Specific sequencing methods (eg, for proteins and nucleic

acids) X-ray crystallography

Preparations for Studying Biochemical Processes Whole animal (includes transgenic animals and animals

with gene knockouts) Isolated perfused organ Tissue slice Whole cells Homogenate Isolated cell organelles Subfractionation of organelles Purified metabolites and enzymes Isolated genes (including polymerase chain reaction and

site-directed mutagenesis) 1Most of these methods are suitable for analyzing the compo- nents present in cell homogenates and other biochemical prepa- rations. The sequential use of several techniques will generally permit purification of most biomolecules. The reader is referred to texts on methods of biochemical research for details.

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BIOCHEMISTRY & MEDICINE / 3

BIOCHEMISTRY

MEDICINE

Lipids

Athero- sclerosis

Proteins

Sickle cell anemia

Nucleic acids

Genetic diseases

Carbohydrates

Diabetes mellitus

Figure 1–1. Examples of the two-way street connecting biochemistry and medicine. Knowledge of the biochemical molecules shown in the top part of the diagram has clarified our understanding of the diseases shown in the bottom half—and conversely, analyses of the diseases shown below have cast light on many areas of biochemistry. Note that sickle cell anemia is a genetic disease and that both atherosclerosis and diabetes mellitus have genetic components.

Table 1–2. The major causes of diseases. All of the causes listed act by influencing the various biochemical mechanisms in the cell or in the body.1

1. Physical agents: Mechanical trauma, extremes of temper- ature, sudden changes in atmospheric pressure, radia- tion, electric shock.

2. Chemical agents, including drugs: Certain toxic com- pounds, therapeutic drugs, etc.

3. Biologic agents: Viruses, bacteria, fungi, higher forms of parasites.

4. Oxygen lack: Loss of blood supply, depletion of the oxygen-carrying capacity of the blood, poisoning of the oxidative enzymes.

5. Genetic disorders: Congenital, molecular. 6. Immunologic reactions: Anaphylaxis, autoimmune

disease. 7. Nutritional imbalances: Deficiencies, excesses. 8. Endocrine imbalances: Hormonal deficiencies, excesses. 1Adapted, with permission, from Robbins SL, Cotram RS, Kumar V: The Pathologic Basis of Disease, 3rd ed. Saunders, 1984.

Table 1–3. Some uses of biochemical investigations and laboratory tests in relation to diseases.

Use Example

1. To reveal the funda- Demonstration of the na- mental causes and ture of the genetic de- mechanisms of diseases fects in cystic fibrosis.

2. To suggest rational treat- A diet low in phenylalanine ments of diseases based for treatment of phenyl- on (1) above ketonuria.

3. To assist in the diagnosis Use of the plasma enzyme of specific diseases creatine kinase MB

(CK-MB) in the diagnosis of myocardial infarction.

4. To act as screening tests Use of measurement of for the early diagnosis blood thyroxine or of certain diseases thyroid-stimulating hor-

mone (TSH) in the neo- natal diagnosis of con- genital hypothyroidism.

5. To assist in monitoring Use of the plasma enzyme the progress (eg, re- alanine aminotransferase covery, worsening, re- (ALT) in monitoring the mission, or relapse) of progress of infectious certain diseases hepatitis.

6. To assist in assessing Use of measurement of the response of dis- blood carcinoembryonic eases to therapy antigen (CEA) in certain

patients who have been treated for cancer of the colon.

Impact of the Human Genome Project (HGP) on Biochemistry & Medicine

Remarkable progress was made in the late 1990s in se- quencing the human genome. This culminated in July 2000, when leaders of the two groups involved in this effort (the International Human Genome Sequencing Consortium and Celera Genomics, a private company) announced that over 90% of the genome had been se- quenced. Draft versions of the sequence were published

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in early 2001. It is anticipated that the entire sequence will be completed by 2003. The implications of this work for biochemistry, all of biology, and for medicine are tremendous, and only a few points are mentioned here. Many previously unknown genes have been re- vealed; their protein products await characterization. New light has been thrown on human evolution, and procedures for tracking disease genes have been greatly refined. The results are having major effects on areas such as proteomics, bioinformatics, biotechnology, and pharmacogenomics. Reference to the human genome will be made in various sections of this text. The Human Genome Project is discussed in more detail in Chapter 54.

SUMMARY

• Biochemistry is the science concerned with studying the various molecules that occur in living cells and organisms and with their chemical reactions. Because life depends on biochemical reactions, biochemistry has become the basic language of all biologic sci- ences.

• Biochemistry is concerned with the entire spectrum of life forms, from relatively simple viruses and bacte- ria to complex human beings.

• Biochemistry and medicine are intimately related. Health depends on a harmonious balance of bio- chemical reactions occurring in the body, and disease reflects abnormalities in biomolecules, biochemical reactions, or biochemical processes.

• Advances in biochemical knowledge have illumi- nated many areas of medicine. Conversely, the study of diseases has often revealed previously unsuspected aspects of biochemistry. The determination of the se- quence of the human genome, nearly complete, will have a great impact on all areas of biology, including biochemistry, bioinformatics, and biotechnology.

• Biochemical approaches are often fundamental in il- luminating the causes of diseases and in designing appropriate therapies.

• The judicious use of various biochemical laboratory tests is an integral component of diagnosis and moni- toring of treatment.

• A sound knowledge of biochemistry and of other re- lated basic disciplines is essential for the rational practice of medical and related health sciences.

REFERENCES

Fruton JS: Proteins, Enzymes, Genes: The Interplay of Chemistry and Biology. Yale Univ Press, 1999. (Provides the historical back- ground for much of today’s biochemical research.)

Garrod AE: Inborn errors of metabolism. (Croonian Lectures.) Lancet 1908;2:1, 73, 142, 214.

International Human Genome Sequencing Consortium. Initial se- quencing and analysis of the human genome. Nature 2001:409;860. (The issue [15 February] consists of articles dedicated to analyses of the human genome.)

Kornberg A: Basic research: The lifeline of medicine. FASEB J 1992;6:3143.

Kornberg A: Centenary of the birth of modern biochemistry. FASEB J 1997;11:1209.

McKusick VA: Mendelian Inheritance in Man. Catalogs of Human Genes and Genetic Disorders, 12th ed. Johns Hopkins Univ Press, 1998. [Abbreviated MIM]

Online Mendelian Inheritance in Man (OMIM): Center for Med- ical Genetics, Johns Hopkins University and National Center for Biotechnology Information, National Library of Medi- cine, 1997. http://www.ncbi.nlm.nih.gov/omim/

(The numbers assigned to the entries in MIM and OMIM will be cited in selected chapters of this work. Consulting this exten- sive collection of diseases and other relevant entries—specific proteins, enzymes, etc—will greatly expand the reader’s knowledge and understanding of various topics referred to and discussed in this text. The online version is updated al- most daily.)

Scriver CR et al (editors): The Metabolic and Molecular Bases of In- herited Disease, 8th ed. McGraw-Hill, 2001.

Venter JC et al: The Sequence of the Human Genome. Science 2001;291:1304. (The issue [16 February] contains the Celera draft version and other articles dedicated to analyses of the human genome.)

Williams DL, Marks V: Scientific Foundations of Biochemistry in Clinical Practice, 2nd ed. Butterworth-Heinemann, 1994.

4 / CHAPTER 1

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Water & pH 2

5

Victor W. Rodwell, PhD, & Peter J. Kennelly, PhD

BIOMEDICAL IMPORTANCE

Water is the predominant chemical component of liv- ing organisms. Its unique physical properties, which in- clude the ability to solvate a wide range of organic and inorganic molecules, derive from water’s dipolar struc- ture and exceptional capacity for forming hydrogen bonds. The manner in which water interacts with a sol- vated biomolecule influences the structure of each. An excellent nucleophile, water is a reactant or product in many metabolic reactions. Water has a slight propensity to dissociate into hydroxide ions and protons. The acidity of aqueous solutions is generally reported using the logarithmic pH scale. Bicarbonate and other buffers normally maintain the pH of extracellular fluid be- tween 7.35 and 7.45. Suspected disturbances of acid- base balance are verified by measuring the pH of arter- ial blood and the CO2 content of venous blood. Causes of acidosis (blood pH < 7.35) include diabetic ketosis and lactic acidosis. Alkalosis (pH > 7.45) may, for ex- ample, follow vomiting of acidic gastric contents. Regu- lation of water balance depends upon hypothalamic mechanisms that control thirst, on antidiuretic hor- mone (ADH), on retention or excretion of water by the kidneys, and on evaporative loss. Nephrogenic diabetes insipidus, which involves the inability to concentrate urine or adjust to subtle changes in extracellular fluid osmolarity, results from the unresponsiveness of renal tubular osmoreceptors to ADH.

WATER IS AN IDEAL BIOLOGIC SOLVENT

Water Molecules Form Dipoles

A water molecule is an irregular, slightly skewed tetra- hedron with oxygen at its center (Figure 2–1). The two hydrogens and the unshared electrons of the remaining two sp3-hybridized orbitals occupy the corners of the tetrahedron. The 105-degree angle between the hydro- gens differs slightly from the ideal tetrahedral angle, 109.5 degrees. Ammonia is also tetrahedral, with a 107- degree angle between its hydrogens. Water is a dipole, a molecule with electrical charge distributed asymmetri- cally about its structure. The strongly electronegative

oxygen atom pulls electrons away from the hydrogen nuclei, leaving them with a partial positive charge, while its two unshared electron pairs constitute a region of local negative charge.

Water, a strong dipole, has a high dielectric con- stant. As described quantitatively by Coulomb’s law, the strength of interaction F between oppositely charged particles is inversely proportionate to the di- electric constant ε of the surrounding medium. The di- electric constant for a vacuum is unity; for hexane it is 1.9; for ethanol it is 24.3; and for water it is 78.5. Water therefore greatly decreases the force of attraction between charged and polar species relative to water-free environments with lower dielectric constants. Its strong dipole and high dielectric constant enable water to dis- solve large quantities of charged compounds such as salts.

Water Molecules Form Hydrogen Bonds

An unshielded hydrogen nucleus covalently bound to an electron-withdrawing oxygen or nitrogen atom can interact with an unshared electron pair on another oxy- gen or nitrogen atom to form a hydrogen bond. Since water molecules contain both of these features, hydro- gen bonding favors the self-association of water mole- cules into ordered arrays (Figure 2–2). Hydrogen bond- ing profoundly influences the physical properties of water and accounts for its exceptionally high viscosity, surface tension, and boiling point. On average, each molecule in liquid water associates through hydrogen bonds with 3.5 others. These bonds are both relatively weak and transient, with a half-life of about one mi- crosecond. Rupture of a hydrogen bond in liquid water requires only about 4.5 kcal/mol, less than 5% of the energy required to rupture a covalent O H bond.

Hydrogen bonding enables water to dissolve many organic biomolecules that contain functional groups which can participate in hydrogen bonding. The oxy- gen atoms of aldehydes, ketones, and amides provide pairs of electrons that can serve as hydrogen acceptors. Alcohols and amines can serve both as hydrogen accep- tors and as donors of unshielded hydrogen atoms for formation of hydrogen bonds (Figure 2–3).

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6 / CHAPTER 2

2e

H

H

105°

2e

O H H

H

H O

O H

O H H

H H O H

O H

O H H

H

Figure 2–2. Left: Association of two dipolar water molecules by a hydrogen bond (dotted line). Right: Hydrogen-bonded cluster of four water molecules. Note that water can serve simultaneously both as a hy- drogen donor and as a hydrogen acceptor.

Figure 2–1. The water molecule has tetrahedral geometry.

H

H

OOCH2CH3 H

OOCHCH3 H

H

CH2 CH3

HO

R

R

N

II

III

C

R

RI

2

Figure 2–3. Additional polar groups participate in hydrogen bonding. Shown are hydrogen bonds formed between an alcohol and water, between two molecules of ethanol, and between the peptide carbonyl oxygen and the peptide nitrogen hydrogen of an adjacent amino acid.

Table 2–1. Bond energies for atoms of biologic significance.

Bond Energy Bond Energy Type (kcal/mol) Type (kcal/mol)

O—O 34 O==O 96 S—S 51 C—H 99 C—N 70 C==S 108 S—H 81 O—H 110 C—C 82 C==C 147 C—O 84 C==N 147 N—H 94 C==O 164

INTERACTION WITH WATER INFLUENCES THE STRUCTURE OF BIOMOLECULES

Covalent & Noncovalent Bonds Stabilize Biologic Molecules

The covalent bond is the strongest force that holds molecules together (Table 2–1). Noncovalent forces, while of lesser magnitude, make significant contribu- tions to the structure, stability, and functional compe- tence of macromolecules in living cells. These forces, which can be either attractive or repulsive, involve in- teractions both within the biomolecule and between it and the water that forms the principal component of the surrounding environment.

Biomolecules Fold to Position Polar & Charged Groups on Their Surfaces

Most biomolecules are amphipathic; that is, they pos- sess regions rich in charged or polar functional groups as well as regions with hydrophobic character. Proteins tend to fold with the R-groups of amino acids with hy- drophobic side chains in the interior. Amino acids with charged or polar amino acid side chains (eg, arginine, glutamate, serine) generally are present on the surface in contact with water. A similar pattern prevails in a phospholipid bilayer, where the charged head groups of

phosphatidyl serine or phosphatidyl ethanolamine con- tact water while their hydrophobic fatty acyl side chains cluster together, excluding water. This pattern maxi- mizes the opportunities for the formation of energeti- cally favorable charge-dipole, dipole-dipole, and hydro- gen bonding interactions between polar groups on the biomolecule and water. It also minimizes energetically unfavorable contact between water and hydrophobic groups.

Hydrophobic Interactions

Hydrophobic interaction refers to the tendency of non- polar compounds to self-associate in an aqueous envi- ronment. This self-association is driven neither by mu- tual attraction nor by what are sometimes incorrectly referred to as “hydrophobic bonds.” Self-association arises from the need to minimize energetically unfavor- able interactions between nonpolar groups and water.

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WATER & pH / 7

While the hydrogens of nonpolar groups such as the methylene groups of hydrocarbons do not form hydro- gen bonds, they do affect the structure of the water that surrounds them. Water molecules adjacent to a hy- drophobic group are restricted in the number of orien- tations (degrees of freedom) that permit them to par- ticipate in the maximum number of energetically favorable hydrogen bonds. Maximal formation of mul- tiple hydrogen bonds can be maintained only by in- creasing the order of the adjacent water molecules, with a corresponding decrease in entropy.

It follows from the second law of thermodynamics that the optimal free energy of a hydrocarbon-water mixture is a function of both maximal enthalpy (from hydrogen bonding) and minimum entropy (maximum degrees of freedom). Thus, nonpolar molecules tend to form droplets with minimal exposed surface area, re- ducing the number of water molecules affected. For the same reason, in the aqueous environment of the living cell the hydrophobic portions of biopolymers tend to be buried inside the structure of the molecule, or within a lipid bilayer, minimizing contact with water.

Electrostatic Interactions

Interactions between charged groups shape biomolecu- lar structure. Electrostatic interactions between oppo- sitely charged groups within or between biomolecules are termed salt bridges. Salt bridges are comparable in strength to hydrogen bonds but act over larger dis- tances. They thus often facilitate the binding of charged molecules and ions to proteins and nucleic acids.

Van der Waals Forces

Van der Waals forces arise from attractions between transient dipoles generated by the rapid movement of electrons on all neutral atoms. Significantly weaker than hydrogen bonds but potentially extremely numer- ous, van der Waals forces decrease as the sixth power of the distance separating atoms. Thus, they act over very short distances, typically 2–4 Å.

Multiple Forces Stabilize Biomolecules

The DNA double helix illustrates the contribution of multiple forces to the structure of biomolecules. While each individual DNA strand is held together by cova- lent bonds, the two strands of the helix are held to- gether exclusively by noncovalent interactions. These noncovalent interactions include hydrogen bonds be- tween nucleotide bases (Watson-Crick base pairing) and van der Waals interactions between the stacked purine and pyrimidine bases. The helix presents the charged phosphate groups and polar ribose sugars of

the backbone to water while burying the relatively hy- drophobic nucleotide bases inside. The extended back- bone maximizes the distance between negatively charged backbone phosphates, minimizing unfavorable electrostatic interactions.

WATER IS AN EXCELLENT NUCLEOPHILE

Metabolic reactions often involve the attack by lone pairs of electrons on electron-rich molecules termed nucleophiles on electron-poor atoms called elec- trophiles. Nucleophiles and electrophiles do not neces- sarily possess a formal negative or positive charge. Water, whose two lone pairs of sp3 electrons bear a par- tial negative charge, is an excellent nucleophile. Other nucleophiles of biologic importance include the oxygen atoms of phosphates, alcohols, and carboxylic acids; the sulfur of thiols; the nitrogen of amines; and the imid- azole ring of histidine. Common electrophiles include the carbonyl carbons in amides, esters, aldehydes, and ketones and the phosphorus atoms of phosphoesters.

Nucleophilic attack by water generally results in the cleavage of the amide, glycoside, or ester bonds that hold biopolymers together. This process is termed hy- drolysis. Conversely, when monomer units are joined together to form biopolymers such as proteins or glyco- gen, water is a product, as shown below for the forma- tion of a peptide bond between two amino acids.

While hydrolysis is a thermodynamically favored re- action, the amide and phosphoester bonds of polypep- tides and oligonucleotides are stable in the aqueous en- vironment of the cell. This seemingly paradoxic behavior reflects the fact that the thermodynamics gov- erning the equilibrium of a reaction do not determine the rate at which it will take place. In the cell, protein catalysts called enzymes are used to accelerate the rate

O +H3N

O

NH

H2O

OH + H

+H3N NH

O–

O–

O

O

Alanine

Valine

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8 / CHAPTER 2

of hydrolytic reactions when needed. Proteases catalyze the hydrolysis of proteins into their component amino acids, while nucleases catalyze the hydrolysis of the phosphoester bonds in DNA and RNA. Careful control of the activities of these enzymes is required to ensure that they act only on appropriate target molecules.

Many Metabolic Reactions Involve Group Transfer

In group transfer reactions, a group G is transferred from a donor D to an acceptor A, forming an acceptor group complex A–G:

The hydrolysis and phosphorolysis of glycogen repre- sent group transfer reactions in which glucosyl groups are transferred to water or to orthophosphate. The equilibrium constant for the hydrolysis of covalent bonds strongly favors the formation of split products. The biosynthesis of macromolecules also involves group transfer reactions in which the thermodynamically un- favored synthesis of covalent bonds is coupled to fa- vored reactions so that the overall change in free energy favors biopolymer synthesis. Given the nucleophilic character of water and its high concentration in cells, why are biopolymers such as proteins and DNA rela- tively stable? And how can synthesis of biopolymers occur in an apparently aqueous environment? Central to both questions are the properties of enzymes. In the absence of enzymic catalysis, even thermodynamically highly favored reactions do not necessarily take place rapidly. Precise and differential control of enzyme ac- tivity and the sequestration of enzymes in specific or- ganelles determine under what physiologic conditions a given biopolymer will be synthesized or degraded. Newly synthesized polymers are not immediately hy- drolyzed, in part because the active sites of biosynthetic enzymes sequester substrates in an environment from which water can be excluded.

Water Molecules Exhibit a Slight but Important Tendency to Dissociate

The ability of water to ionize, while slight, is of central importance for life. Since water can act both as an acid and as a base, its ionization may be represented as an intermolecular proton transfer that forms a hydronium ion (H3O+) and a hydroxide ion (OH

−):

The transferred proton is actually associated with a cluster of water molecules. Protons exist in solution not only as H3O+, but also as multimers such as H5O2+ and

H O H O H O OH2 2 3 + + += −

D G A A G D− = + − +

H7O3+. The proton is nevertheless routinely repre- sented as H+, even though it is in fact highly hydrated.

Since hydronium and hydroxide ions continuously recombine to form water molecules, an individual hy- drogen or oxygen cannot be stated to be present as an ion or as part of a water molecule. At one instant it is an ion. An instant later it is part of a molecule. Individ- ual ions or molecules are therefore not considered. We refer instead to the probability that at any instant in time a hydrogen will be present as an ion or as part of a water molecule. Since 1 g of water contains 3.46 × 1022 molecules, the ionization of water can be described sta- tistically. To state that the probability that a hydrogen exists as an ion is 0.01 means that a hydrogen atom has one chance in 100 of being an ion and 99 chances out of 100 of being part of a water molecule. The actual probability of a hydrogen atom in pure water existing as a hydrogen ion is approximately 1.8 × 10−9. The proba- bility of its being part of a molecule thus is almost unity. Stated another way, for every hydrogen ion and hydroxyl ion in pure water there are 1.8 billion or 1.8 × 109 water molecules. Hydrogen ions and hydroxyl ions nevertheless contribute significantly to the properties of water.

For dissociation of water,

where brackets represent molar concentrations (strictly speaking, molar activities) and K is the dissociation constant. Since one mole (mol) of water weighs 18 g, one liter (L) (1000 g) of water contains 1000 × 18 = 55.56 mol. Pure water thus is 55.56 molar. Since the probability that a hydrogen in pure water will exist as a hydrogen ion is 1.8 × 10−9, the molar concentration of H+ ions (or of OH− ions) in pure water is the product of the probability, 1.8 × 10−9, times the molar concen- tration of water, 55.56 mol/L. The result is 1.0 × 10−7 mol/L.

We can now calculate K for water:

The molar concentration of water, 55.56 mol/L, is too great to be significantly affected by dissociation. It therefore is considered to be essentially constant. This constant may then be incorporated into the dissociation constant K to provide a useful new constant Kw termed the ion product for water. The relationship between Kw and K is shown below:

K = =

= × = ×

+[ ][ ]

[ ]

[ ][ ]

[ . ]

. . /

H OH

H O

mol L

− − −

− − 2

7 7

14 16

10 10

55 56

0 018 10 1 8 10

K = +[ ][

]

H OH

H O

− ] [ 2

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WATER & pH / 9

Note that the dimensions of K are moles per liter and those of Kw are moles

2 per liter2. As its name suggests, the ion product Kw is numerically equal to the product of the molar concentrations of H+ and OH−:

At 25 °C, Kw = (10 −7)2, or 10−14 (mol/L)2. At tempera-

tures below 25 °C, Kw is somewhat less than 10 −14; and

at temperatures above 25 °C it is somewhat greater than 10−14. Within the stated limitations of the effect of tem- perature, Kw equals 10

-14 (mol/L)2 for all aqueous so- lutions, even solutions of acids or bases. We shall use Kw to calculate the pH of acidic and basic solutions.

pH IS THE NEGATIVE LOG OF THE HYDROGEN ION CONCENTRATION

The term pH was introduced in 1909 by Sörensen, who defined pH as the negative log of the hydrogen ion concentration:

This definition, while not rigorous, suffices for many biochemical purposes. To calculate the pH of a solution:

1. Calculate hydrogen ion concentration [H+]. 2. Calculate the base 10 logarithm of [H+]. 3. pH is the negative of the value found in step 2.

For example, for pure water at 25°C,

Low pH values correspond to high concentrations of H+ and high pH values correspond to low concentra- tions of H+.

Acids are proton donors and bases are proton ac- ceptors. Strong acids (eg, HCl or H2SO4) completely dissociate into anions and cations even in strongly acidic solutions (low pH). Weak acids dissociate only partially in acidic solutions. Similarly, strong bases (eg, KOH or NaOH)—but not weak bases (eg, Ca[OH]2)—are completely dissociated at high pH. Many biochemicals are weak acids. Exceptions include phosphorylated in-

pH H= = =+− − − −−log [ ] ( log 10 7) = 7.07

pH H= +− log [ ]

K w H OH= +[ ][ ]−

K

K K

= = ×

= =

= ×

= ×

+

+

[ ][ ]

[ ] . /

( )[ ] [ ][ ]

( . / ) ( . / )

. ( / )

H OH

H O mol L

H O H OH

mol L mol L

mol L

w

− −

2

16

2

16

14 2

1 8 10

1 8 10 55 56

1 00 10

termediates, whose phosphoryl group contains two dis- sociable protons, the first of which is strongly acidic.

The following examples illustrate how to calculate the pH of acidic and basic solutions.

Example 1: What is the pH of a solution whose hy- drogen ion concentration is 3.2 × 10− 4 mol/L?

Example 2: What is the pH of a solution whose hy- droxide ion concentration is 4.0 × 10− 4 mol/L? We first define a quantity pOH that is equal to −log [OH−] and that may be derived from the definition of Kw:

Therefore:

or

To solve the problem by this approach:

Now:

Example 3: What are the pH values of (a) 2.0 × 10−2 mol/L KOH and of (b) 2.0 × 10−6 mol/L KOH? The OH− arises from two sources, KOH and water. Since pH is determined by the total [H+] (and pOH by the total [OH−]), both sources must be considered. In the first case (a), the contribution of water to the total [OH−] is negligible. The same cannot be said for the second case (b):

pH pOH= = =

14 14 3 4

10 6

− − . .

[ ] .

log [ ]

log ( . )

log ( . ) log )

OH

pOH OH

− −

− − ( − . + .

= .

= ×

=

= ×

= =

4 0 10

4 0 10

4 0 10

0 60 4 0

3 4

4

4

4

pH pOH+ = 14

log [ ] log [ ] log H OH+ −+ = 10 14−

K w H OH= = +[ ][ ]− −110 4

pH H=

= ×

= = + =

+−

− − −

log [ ]

log ( . )

log ( . ) log ( )

. .

.

3 2 10

3 2 10

0 5 4 0

3 5

4

4

ch02.qxd 2/13/2003 1:41 PM Page 9

10 / CHAPTER 2

Concentration (mol/L)

(a) (b)

Molarity of KOH 2.0 × 10−2 2.0 × 10−6 [OH−] from KOH 2.0 × 10−2 2.0 × 10−6 [OH−] from water 1.0 × 10−7 1.0 × 10−7 Total [OH−] 2.00001 × 10−2 2.1 × 10−6

Once a decision has been reached about the significance of the contribution by water, pH may be calculated as above.

The above examples assume that the strong base KOH is completely dissociated in solution and that the concentration of OH− ions was thus equal to that of the KOH. This assumption is valid for dilute solutions of strong bases or acids but not for weak bases or acids. Since weak electrolytes dissociate only slightly in solu- tion, we must use the dissociation constant to calcu- late the concentration of [H+] (or [OH−]) produced by a given molarity of a weak acid (or base) before calcu- lating total [H+] (or total [OH−]) and subsequently pH.

Functional Groups That Are Weak Acids Have Great Physiologic Significance

Many biochemicals possess functional groups that are weak acids or bases. Carboxyl groups, amino groups, and the second phosphate dissociation of phosphate es- ters are present in proteins and nucleic acids, most coenzymes, and most intermediary metabolites. Knowl- edge of the dissociation of weak acids and bases thus is basic to understanding the influence of intracellular pH on structure and biologic activity. Charge-based separa- tions such as electrophoresis and ion exchange chro- matography also are best understood in terms of the dissociation behavior of functional groups.

We term the protonated species (eg, HA or RNH3+) the acid and the unprotonated species (eg, A− or RNH2) its conjugate base. Similarly, we may refer to a base (eg, A− or RNH2) and its conjugate acid (eg, HA or RNH3+). Representative weak acids (left), their conjugate bases (center), and the pKa values (right) include the following:

We express the relative strengths of weak acids and bases in terms of their dissociation constants. Shown

R CH COOH COO

NH NH

H CO

H PO

a

a

a

a

— — —

— —

.

.

2

3 2

2 3

2 4

4 5

9 10

6 4

7 2

R — CH p

R — CH R — CH p

HCO p

HPO p

2

2 2

3

4

− −2

K

K

K

K

=

=

=

=

+

below are the expressions for the dissociation constant (Ka ) for two representative weak acids, RCOOH and RNH3+.

Since the numeric values of Ka for weak acids are nega- tive exponential numbers, we express Ka as pKa, where

Note that pKa is related to Ka as pH is to [H+]. The stronger the acid, the lower its pKa value.

pKa is used to express the relative strengths of both acids and bases. For any weak acid, its conjugate is a strong base. Similarly, the conjugate of a strong base is a weak acid. The relative strengths of bases are ex- pressed in terms of the pKa of their conjugate acids. For polyproteic compounds containing more than one dis- sociable proton, a numerical subscript is assigned to each in order of relative acidity. For a dissociation of the type

the pKa is the pH at which the concentration of the acid RNH3+ equals that of the base RNH2.

From the above equations that relate Ka to [H+] and to the concentrations of undissociated acid and its con- jugate base, when

or when

then

Thus, when the associated (protonated) and dissociated (conjugate base) species are present at equal concentra- tions, the prevailing hydrogen ion concentration [H+] is numerically equal to the dissociation constant, Ka. If the logarithms of both sides of the above equation are

Ka H = +[ ]

[ ] [ ]R NH R NH— —2 3= +

[ ] [R COO R COOH— —− ]=

R NH— 3 + → R — NH2

p aK K= − log

R COOH R COO H

R COO H

R COOH

R NH R NH H

R NH H

R NH

a

a

— —

[ — ][ ]

[ — ]

— —

[ — ][ ]

[ — ]

=

=

+

=

+

=

+

+

+ +

+

+

K

K

3 2

2

3

ch02.qxd 2/13/2003 1:41 PM Page 10

WATER & pH / 11

taken and both sides are multiplied by −1, the expres- sions would be as follows:

Since −log Ka is defined as pKa, and −log [H+] de- fines pH, the equation may be rewritten as

ie, the pKa of an acid group is the pH at which the pro- tonated and unprotonated species are present at equal concentrations. The pKa for an acid may be determined by adding 0.5 equivalent of alkali per equivalent of acid. The resulting pH will be the pKa of the acid.

The Henderson-Hasselbalch Equation Describes the Behavior of Weak Acids & Buffers

The Henderson-Hasselbalch equation is derived below. A weak acid, HA, ionizes as follows:

The equilibrium constant for this dissociation is

Cross-multiplication gives

Divide both sides by [A−]:

Take the log of both sides:

Multiply through by −1:

− − − −

log [ ] log log [ ]

[ ] H

HA

A a

+ = K

log [ ] log [ ]

[ ]

log log [ ]

[ ]

H HA

A

HA

A

a

a

+ = 

 



= +

K

K

[ ] [ ]

[ ] H

HA

A a

+ = K

[ ][ ] [ ]H A HAa + =− K

K a H A

HA =

+[ ][ ]

[ ]

HA H A = + + −

p pHaK =

K

K

a

a

H

H

=

=

+

+

[ ]

log [ ]− − log

Substitute pH and pKa for −log [H+] and −log Ka, re- spectively; then:

Inversion of the last term removes the minus sign and gives the Henderson-Hasselbalch equation:

The Henderson-Hasselbalch equation has great pre- dictive value in protonic equilibria. For example,

(1) When an acid is exactly half-neutralized, [A−] = [HA]. Under these conditions,

Therefore, at half-neutralization, pH = pKa.

(2) When the ratio [A−]/[HA] = 100:1,

(3) When the ratio [A−]/[HA] = 1:10,

If the equation is evaluated at ratios of [A−]/[HA] ranging from 103 to 10−3 and the calculated pH values are plotted, the resulting graph describes the titration curve for a weak acid (Figure 2–4).

Solutions of Weak Acids & Their Salts Buffer Changes in pH

Solutions of weak acids or bases and their conjugates exhibit buffering, the ability to resist a change in pH following addition of strong acid or base. Since many metabolic reactions are accompanied by the release or uptake of protons, most intracellular reactions are buffered. Oxidative metabolism produces CO2, the an- hydride of carbonic acid, which if not buffered would produce severe acidosis. Maintenance of a constant pH involves buffering by phosphate, bicarbonate, and pro- teins, which accept or release protons to resist a change

pH p pa a= + +K Klog ( 1/10 = 1)−

pH p A

HA

pH p p

a

a a

= +

= + +

K

K K

log [ ]

[ ]

log

100 /1=

2

pH p A

HA p pa a a= + = + = +K K Klog

[ ]

[ ] log

− 1

1 0

pH p A

HAa = +K log [ ]

[ ]

pH p HA

A a= K − −log

[ ]

[ ]

ch02.qxd 2/13/2003 1:41 PM Page 11

12 / CHAPTER 2

0

0.2

0.4

0.6

0.8

1.0

2 3 4 5 6 7 pH

8

0

0.2

0.4

0.6

0.8

1.0 m

eq o

f a lk

al i a

dd ed

p er

m eq

o f a

ci d

N et

c ha

rg e

Figure 2–4. Titration curve for an acid of the type HA. The heavy dot in the center of the curve indicates the pKa 5.0.

Table 2–2. Relative strengths of selected acids of biologic significance. Tabulated values are the pKa values (−log of the dissociation constant) of selected monoprotic, diprotic, and triprotic acids.

Monoprotic Acids

Formic pK 3.75 Lactic pK 3.86 Acetic pK 4.76 Ammonium ion pK 9.25

Diprotic Acids

Carbonic pK1 6.37 pK2 10.25

Succinic pK1 4.21 pK2 5.64

Glutaric pK1 4.34 pK2 5.41

Triprotic Acids

Phosphoric pK1 2.15 pK2 6.82 pK3 12.38

Citric pK1 3.08 pK2 4.74 pK3 5.40

Initial pH 5.00 5.37 5.60 5.86 [A−]initial 0.50 0.70 0.80 0.88 [HA]initial 0.50 0.30 0.20 0.12 ([A−]/[HA])initial 1.00 2.33 4.00 7.33

Addition of 0.1 meq of KOH produces [A−]final 0.60 0.80 0.90 0.98 [HA]final 0.40 0.20 0.10 0.02 ([A−]/[HA])final 1.50 4.00 9.00 49.0

log ([A−]/[HA])final 0.176 0.602 0.95 1.69 Final pH 5.18 5.60 5.95 6.69

∆pH 0.18 0.60 0.95 1.69

in pH. For experiments using tissue extracts or en- zymes, constant pH is maintained by the addition of buffers such as MES ([2-N-morpholino]ethanesulfonic acid, pKa 6.1), inorganic orthophosphate (pKa2 7.2), HEPES (N-hydroxyethylpiperazine-N9-2-ethanesulfonic acid, pKa 6.8), or Tris (tris[hydroxymethyl] amino- methane, pKa 8.3). The value of pKa relative to the de- sired pH is the major determinant of which buffer is se- lected.

Buffering can be observed by using a pH meter while titrating a weak acid or base (Figure 2–4). We can also calculate the pH shift that accompanies addi- tion of acid or base to a buffered solution. In the exam- ple, the buffered solution (a weak acid, pKa = 5.0, and its conjugate base) is initially at one of four pH values. We will calculate the pH shift that results when 0.1 meq of KOH is added to 1 meq of each solution:

Notice that the change in pH per milliequivalent of OH− added depends on the initial pH. The solution re- sists changes in pH most effectively at pH values close

to the pKa. A solution of a weak acid and its conjugate base buffers most effectively in the pH range pKa ± 1.0 pH unit.

Figure 2–4 also illustrates the net charge on one molecule of the acid as a function of pH. A fractional charge of −0.5 does not mean that an individual mole- cule bears a fractional charge, but the probability that a given molecule has a unit negative charge is 0.5. Con- sideration of the net charge on macromolecules as a function of pH provides the basis for separatory tech- niques such as ion exchange chromatography and elec- trophoresis.

Acid Strength Depends on Molecular Structure

Many acids of biologic interest possess more than one dissociating group. The presence of adjacent negative charge hinders the release of a proton from a nearby group, raising its pKa. This is apparent from the pKa values for the three dissociating groups of phosphoric acid and citric acid (Table 2–2). The effect of adjacent charge decreases with distance. The second pKa for suc- cinic acid, which has two methylene groups between its carboxyl groups, is 5.6, whereas the second pKa for glu-

ch02.qxd 2/13/2003 1:41 PM Page 12

WATER & pH / 13

taric acid, which has one additional methylene group, is 5.4.

pKa Values Depend on the Properties of the Medium

The pKa of a functional group is also profoundly influ- enced by the surrounding medium. The medium may either raise or lower the pKa depending on whether the undissociated acid or its conjugate base is the charged species. The effect of dielectric constant on pKa may be observed by adding ethanol to water. The pKa of a car- boxylic acid increases, whereas that of an amine decreases because ethanol decreases the ability of water to solvate a charged species. The pKa values of dissociating groups in the interiors of proteins thus are profoundly affected by their local environment, including the presence or absence of water.

SUMMARY

• Water forms hydrogen-bonded clusters with itself and with other proton donors or acceptors. Hydrogen bonds account for the surface tension, viscosity, liquid state at room temperature, and solvent power of water.

• Compounds that contain O, N, or S can serve as hy- drogen bond donors or acceptors.

• Macromolecules exchange internal surface hydrogen bonds for hydrogen bonds to water. Entropic forces dictate that macromolecules expose polar regions to an aqueous interface and bury nonpolar regions.

• Salt bonds, hydrophobic interactions, and van der Waals forces participate in maintaining molecular structure.

• pH is the negative log of [H+]. A low pH character- izes an acidic solution, and a high pH denotes a basic solution.

• The strength of weak acids is expressed by pKa, the negative log of the acid dissociation constant. Strong acids have low pKa values and weak acids have high pKa values.

• Buffers resist a change in pH when protons are pro- duced or consumed. Maximum buffering capacity occurs ± 1 pH unit on either side of pKa. Physiologic buffers include bicarbonate, orthophosphate, and proteins.

REFERENCES

Segel IM: Biochemical Calculations. Wiley, 1968. Wiggins PM: Role of water in some biological processes. Microbiol

Rev 1990;54:432.

ch02.qxd 2/13/2003 1:41 PM Page 13

Amino Acids & Peptides 3

14

Victor W. Rodwell, PhD, & Peter J. Kennelly, PhD

SECTION I Structures & Functions of Proteins & Enzymes

BIOMEDICAL IMPORTANCE

In addition to providing the monomer units from which the long polypeptide chains of proteins are synthesized, the L-α-amino acids and their derivatives participate in cellular functions as diverse as nerve transmission and the biosynthesis of porphyrins, purines, pyrimidines, and urea. Short polymers of amino acids called peptides perform prominent roles in the neuroendocrine system as hormones, hormone-releasing factors, neuromodula- tors, or neurotransmitters. While proteins contain only L-α-amino acids, microorganisms elaborate peptides that contain both D- and L-α-amino acids. Several of these peptides are of therapeutic value, including the an- tibiotics bacitracin and gramicidin A and the antitumor agent bleomycin. Certain other microbial peptides are toxic. The cyanobacterial peptides microcystin and nodularin are lethal in large doses, while small quantities promote the formation of hepatic tumors. Neither hu- mans nor any other higher animals can synthesize 10 of the 20 common L-α-amino acids in amounts adequate to support infant growth or to maintain health in adults. Consequently, the human diet must contain adequate quantities of these nutritionally essential amino acids.

PROPERTIES OF AMINO ACIDS

The Genetic Code Specifies 20 L--Amino Acids

Of the over 300 naturally occurring amino acids, 20 con- stitute the monomer units of proteins. While a nonre- dundant three-letter genetic code could accommodate

more than 20 amino acids, its redundancy limits the available codons to the 20 L-α-amino acids listed in Table 3–1, classified according to the polarity of their R groups. Both one- and three-letter abbreviations for each amino acid can be used to represent the amino acids in peptides (Table 3–1). Some proteins contain additional amino acids that arise by modification of an amino acid already present in a peptide. Examples include conver- sion of peptidyl proline and lysine to 4-hydroxyproline and 5-hydroxylysine; the conversion of peptidyl gluta- mate to γ-carboxyglutamate; and the methylation, formylation, acetylation, prenylation, and phosphoryla- tion of certain aminoacyl residues. These modifications extend the biologic diversity of proteins by altering their solubility, stability, and interaction with other proteins.

Only L--Amino Acids Occur in Proteins

With the sole exception of glycine, the α-carbon of amino acids is chiral. Although some protein amino acids are dextrorotatory and some levorotatory, all share the absolute configuration of L-glyceraldehyde and thus are L-α-amino acids. Several free L-α-amino acids fulfill important roles in metabolic processes. Examples in- clude ornithine, citrulline, and argininosuccinate that participate in urea synthesis; tyrosine in formation of thyroid hormones; and glutamate in neurotransmitter biosynthesis. D-Amino acids that occur naturally in- clude free D-serine and D-aspartate in brain tissue, D-alanine and D-glutamate in the cell walls of gram- positive bacteria, and D-amino acids in some nonmam- malian peptides and certain antibiotics.

ch03.qxd 2/13/2003 1:35 PM Page 14

Table 3–1. L- α-Amino acids present in proteins.

Name Symbol Structural Formula pK1 pK2 pK3

With Aliphatic Side Chains -COOH -NH3+ R Group Glycine Gly [G] 2.4 9.8

Alanine Ala [A] 2.4 9.9

Valine Val [V] 2.2 9.7

Leucine Leu [L] 2.3 9.7

Isoleucine Ile [I] 2.3 9.8

With Side Chains Containing Hydroxylic (OH) Groups Serine Ser [S] 2.2 9.2 about 13

Threonine Thr [T] 2.1 9.1 about 13

Tyrosine Tyr [Y] See below.

With Side Chains Containing Sulfur Atoms Cysteine Cys [C] 1.9 10.8 8.3

Methionine Met [M] 2.1 9.3

With Side Chains Containing Acidic Groups or Their Amides Aspartic acid Asp [D] 2.0 9.9 3.9

Asparagine Asn [N] 2.1 8.8

Glutamic acid Glu [E] 2.1 9.5 4.1

Glutamine Gln [Q] 2.2 9.1

(continued )

H CH

NH3 +

COO–

CH3 CH

NH3 +

COO–

CH

H3C

H3C

CH

NH3 +

COO–

CH

H3C

H3C

NH3 +

COO–CH2 CH

CH

CH2

CH3

CH

NH3 +

COO–

CH3

CH

NH3 +

COO–CH2

OH

CH

NH3 +

COO–CH

OH

CH3

CH

NH3 +

COO–CH2

S

CH2

CH3

CH

NH3 +

COO–CH2

SH

CH

NH3 +

COO–CH2 –OOC

CH

NH3 +

COO–CH2 CH2 –OOC

CH

NH3 +

COO–CH2C

O

H2N

CH

NH3 +

COO–CH2C

O

H2N CH2

15

ch03.qxd 2/13/2003 1:35 PM Page 15

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