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Enzyme Regulation

Enzyme Regulation. Regulation of Enzyme Activity. Enzyme quantity – regulation of gene expression (Response time = minutes to hours) Transcription Translation Enzyme turnover Enzyme activity (rapid response time = fraction of seconds) Allosteric regulation Covalent modification

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Enzyme Regulation

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  1. Enzyme Regulation

  2. Regulation of Enzyme Activity Enzyme quantity – regulation of gene expression (Response time = minutes to hours) • Transcription • Translation • Enzyme turnover Enzyme activity (rapid response time = fraction of seconds) • Allosteric regulation • Covalent modification • Association-disassociation’ • Proteolytic cleavage of proenzyme

  3. Allosteric Regulation • End products are often inhibitors • Allosteric modulators bind to site other than the active site • Allosteric enzymes usually have 4o structure • Vo vs [S] plots give sigmoidal curve for at least one substrate • Can remove allosteric site without effecting enzymatic action

  4. 5” 4” H I J X 3” 2 1 B A C X 3’ 5’ 4’ E F G Regulation of Enzyme Activity(biochemical regulation) • 1st committed step of a biosynthetic pathway or enzymes at pathway branch points often regulated by feedback inhibition. • Efficient use of biosynthetic precursors and energy

  5. Phosphofructokinase( PFK) Fructose-6-P + ATP ----->Fructose-1,6-bisphosphate + ADP • PFK catalyzes 1st committed step in glycolysis (10 steps total) • (Glucose + 2ADP + 2 NAD+ + 2Pi  2pyruvate + 2ATP + 2NADH) • Phosphoenolpyruvate is an allosteric inhibitor of PFK • ADP is an allosteric activator of PFK

  6. Allosteric modulators bind to site other than the active site and allosteric enzymes have 4o structure Fructose-6-P + ATP ----->Fructose-1,6-bisphosphate + ADP ADP Allosteric Activator (ADP) binds distal to active site

  7. Vo vs [S] plots give sigmoidal curve for at least one substrate Binding of allosteric inhibitor or activator does not effect Vmax, but does alter Km Allosteric enzyme do not follow M-M kinetics

  8. I S ET-I ET ER ER-S I Concerted model Sequential model Allosteric T to R transition S

  9. Covalent modification • Regulation by covalent modification is slower than allosteric regulation • Reversible • Require one enzyme for activation and one enzyme for inactivation • Covalent modification freezes enzyme T or R conformation

  10. Phosphorylation /dephosphorylation • most common covalent modification • involve protein kinases/phosphatase • PDK inactivated by phosphorylation • Amino acids with –OH groups are targets for phosphorylation • Phosphates are bulky (-) charged groups which effect conformation

  11. Enzyme Regulation by Association/Disassociation • Acetyl-CoA Carboxylase • acetyl-CoA + CO2 + ATP  malonyl-CoA + ADP + Pi • 1St committed step in fatty acid biosynthesis • In presence of citrate activated • In presence of fatty acyl-CoA inactivated citrate polymerized unpolymerized Fatty acyl-CoA

  12. Proteolytic cleavage of proenzyme(zymogen)

  13. Proinsulin to Insulin

  14. Blood Clotting • Clotting involves series of zymogen activations • Seven clotting factors are serine proteases involved in clotting cascade rxns X X X X X X

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