Pyruvate Dehydrogenase - Biochemistry - Lecture Slides, Slides of Biochemistry

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CITRIC ACID CYCLE

- Pyruvate Dehydrogenase

Reading:

 Harper’s Biochemistry Chapter 18

 Lehninger Principles of Biochemistry

3rd Ed. pp. 567- 583

OBJECTIVES

 To understand how the pyruvate dehydrogenase complex acts as a mediator of flow of carbon compounds between the glycolytic pathway and the citric acid cycle.  To understand the central role of the citric acid cycle in mitochondrial energy metabolism.  The structures of the intermediates in the cycle.  The names of the enzymes that catalyze each step.  The cofactors and products involved in each step.

Catabolism of

proteins, fats,

and

carbohydrates

in the three

steps of

cellular

respiration

Biomedical Importance

 The citric acid cycle acts as a final common pathway for the oxidation of carbohydrate, lipids, and protein, because glucose, fatty acids, and many amino acids can all be metabolized to acetyl-CoA or intermediates in the cycle.  The citric acid cycle also plays a major role in gluconeogenesis, transamination, deamination, and lipogenesis. The liver is the only tissue where all of these occur to a significant extent.  When large numbers of liver cells are damaged or destroyed, in acute hepatitis or cirrhosis, this can have major repercussions on metabolism.  Very few genetic abnormalities exist for enzymes of the citric acid cycle, suggesting such abnormalities are incompatible with normal development and highlighting the vital nature of the process.  Pyruvate hydrogenase is a key enzyme needed to convert pyruvate to acetyl-CoA. A variety of disorders in pyruvate metabolism are due to defects in this enzyme.

Pyruvate transport into mitochondria

 Pyruvate generated in the cytoplasm by glycolysis must be transported across the inner mitochondrial membrane via a pyruvate/H

symport.  This transport uses some of the energy stored in the mitochondrial inner membrane electrical potential gradient.

Coenzyme A (CoA)

 Coenzyme A (CoA). A hydroxyl group of pantothenic acid is joined to a modified ADP moiety by a phosphate ester bond, and its carboxyl group is attached to -mercaptoethylamine in amide linkage. The hydroxyl group at the 3´position of the ADP moiety has a phosphoryl group not present in ADP itself. The - SH group of the mercaptoethylamine moiety forms a thioester with acetate in acetyl-coenzyme A (acetyl-CoA)

Pyruvate Dehydrogenase Complex

 Contains three enzymes each present in multiple copies:

1. pyruvate dehydrogenase (E1)
2. dihydrolipoyl transacetylase (E2)
3. dihydrolipoyl dehydrogenase (E3)  The complex from mammals has 60 copies of E2, which contains lipoate, and constitutes the core of the complex.  In addition, five different coenzymes or prosthetic groups are needed:
4. thiamine pyrophosphate (TPP)
5. flavin adenine dinucleotide (FAD)
6. coenzyme A (CoA)
7. nicotinamide adenine dinucleotide (NAD)
8. lipoate  All are clustered for efficient handling of intermediates

Oxidation decarboxylation of pyruvate to

acetyl-CoA by the pyruvate dehydrogenase

complex

 Pyruvate reacts with thiamine pyrophosphate bound to E1. Pyruvate undergoes decarboxylation to the hydroxylethyl derivative, with a loss of CO 2

 The acetyl group and 2 electrons from TPP are transferred to the oxidized form of the lipoyllysyl group of the core enzyme (E2).  A transestenification occurs where the - SH group of CoA replaces the - SH group of E2 to yield acetyl-CoA and the reduced form of the lipoyl group.  E3 transfers two hydrogens from the reduced lipoyl groups to E2 to FAD.  The reduced FADH 2 of E3 transfers a hydride ion to NAD

, forming NADH.

Reactions of the citric acid cycle

Overview of citric acid cycle

 Eight successive reaction steps.  The six carbon citrate is formed from two carbon acetyl-CoA and four carbon oxaloacetate.  Oxidation of citrate yields CO 2 and regenerates oxaloacetate, which plays essentially a catalytic role.  The energy released is captured in the reduced coenzymes NADH and FADH 2

Step 1: Formation of citrate

 Citrate synthase catalyzes the condensation of acetyl-CoA with oxaloacetate to form citrate  Oxaloacetate binds first and induces a conformational change, creating a binding site for acetyl-CoA  CoA is liberated and recycled Docsity.com