General Biology - Energy Flow in Cell - Lecture Slides | BIOL 1001, Study notes of Biology

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Chapter 6 – Part II

Energy Flow in the

Cell

Biol 1001 Spring 2012

PATHWAY 1 enzyme 1 enzyme 2 enzyme 3 enzyme 4 PATHWAY 2 enzyme 5 enzyme 6 A (^) B D (^) E F C G

Metabolic Pathways

 Chemical reactions linked in a sequence

 Each reaction is modified by a specific enzyme

 Each reaction produces a specific product

 Product used in following reaction until end product is reached



Involve synthesis & breakdown of molecules

 Photosyntheis = produces high-energy molecules

 Glycolysis = breakdown glucose molecules

 All pathways are directly/indirectly connected

Fig 6-

Rate of Reactions

 Depends on amount of substrates or enzyme present  (^) More substrate/enzyme  faster reaction  Increases substrates chance of binding with enzyme active site  (^) Levels increase until enzyme active sites are continuously occupied by substrate  Enzymes regulated to ensure efficiency  (^) Regulation of synthesis  (^) Regulation of activity  (^) Inhibition of activity

Enzyme Synthesis Regulation

 Enzymes synthesized only when needed  Increased synthesis  more enzymes  (^) Occurs when lots of substrate available  (^) Ex: alcohol dehydrogenase enzyme  (^) Produced in liver cells to break down alcohol  (^) Found in large quantities in alcoholics  Lack of synthesis  health conditions  (^) Enzymes are not synthesized in adequate amounts or at all  Ex: lactose intolerance (lack of lactsae)  (^) Ex: phenlketonuria (lack of phenylalanine hydroxylase)

 Can cause mental retardation and seizures

Enzyme Activity Regulation



Enzymes become active when and where needed



Some synthesized only in an inactive form



Activated under specific conditions



Ex: Pepsin enzyme for protein-digestion

 Inactive form (= pepsinogen) prevents enzyme from

digesting cell that produced it

 Activated by stomach acid (HCl) which exposes active site

on enzyme

 Pepsinogen + HCl  Pepsin

http://www.78steps.com/plasma-membrane/chemical-digestion-begins-in-the-mouth-and-the-stomach.html

Enzyme Activity Inhibition

 Inhibition = inactivation of an enzyme  Inhibition prevents  (^) substrate from being used up  Overabundance of product  Types of inhibition  Competitive inhibition  (^) Non-competitive inhibition  Feedback inhibition

Competitive Inhibition

 Inhibitor molecule binds to active site instead of desired substrates  (^) Competition for active site  Depends on concentration of substrate vs. inhibitor  (^) Reversible once substrate outnumbers & displaces product http://www.elmhurst.edu/~chm/vchembook/573inhibit.html

Some Competitive Inhibitors

 Methanol = toxic alcohol in antifreeze  (^) Inhibitor of alcohol dehydrogenase  (^) Methanol + alcohol dehydrogenaseformaldehyde  (^) Can cause blindness & death  (^) Reversed by increasing ethanol concentration  (^) Ethanol = normal substrate for alcohol dehydrogenase  Ibuprofen (Advil)  (^) Inhibitor of enzyme that catalyzes reactions associated with swelling, pain, and fever

noncompetitive inhibitor molecule A noncompetitive inhibitor molecule causes the active site to change shape, so the substrate no longer fits

Non-competitive Inhibition

 Inhibitor molecule binds to a different site on enzyme & affects active site binding  (^) Not influenced by substrate concentration  Active site becomes distorted or blocked  (^) No competition for active site  Reversible or irreversible  AKA: Allosteric inhibition

Fig 6-

Some Non-competitive Inhibitors

 Nerve gases (e.g. sarin)  Permanantly inhibit acetylcholinesterase  Breaks down acetylcholine to activate mucles  Acetylcholine builds up & over-stimulates muscles  Can lead to paralysis, respitory failure, & death  Penicilian  Inhibits bacterial enzyme that produces cell wall  Bacteria eventually bursts without cell wall  Does not affect animal cells  Animal cells do not have a cell wall

Allosteric Regulation



Allosteric molecules bind to regulatory site on enzyme 

Allosteric activators– stabilize enzyme in active form

 Allosteric inhibitors– stabilizes enzyme in inactive

form



Regulate allosteric enzyme activity

 Allosteric enzyme = enzyme that can easily switch

from active form to inactive form



Example: ADP 

Allosteric activator for ATP synthesis

http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookenzym.html

Feedback Inhibition



Form of allosteric regulation



Cause metabolic pathways to stop when product concentration reaches optimal level



Enzyme at beginning of pathway is inhibited by end product of same pathway

 End product= allosteric inhibitor



Once product is used up, pathway restarts



Example: ATP

 Allosteric inhibitor of ATP synthesis

https://wikispaces.psu.edu/display/230/Enzyme+Kinetics+and+Catalysis

Influence of Environment on Enzymes

 Enzymes only function in optimal conditions  (^) Unfavorable conditions  denatured enzyme  (^) Loses specific 3-D configurations & ability to function  (^) pH  Most enzymes work best in pH of 7.  Protein-digesting pepsin prefers pH of -2.  Protein-digesting trypsin prefers pH of -8.  (^) Temperature  Higher temps  faster activity  (^) Too high  denaturation of enzyme  Colder temps halt reproduction & growth reactions in bacteria

Effect of pH

rate of reaction fast slow Most enzymes, maximum activity occurs at ~ pH 7- For trypsin, maximum activity occurs at ~pH 8 For pepsin, maximum activity occurs at ~pH 2

Effect of temperature

Human enzymes, maximum activity occurs at ~ 98.6°F

Fig 6-

Enzymes become denatured