Sinh học - Chapter 5 (part 4): Enzyme regulation

Chapter 5 (part 4)Enzyme RegulationRegulation of Enzyme ActivityEnzyme quantity – regulation of gene expression (Response time = minutes to hours)TranscriptionTranslationEnzyme turnoverEnzyme activity (rapid response time = fraction of seconds)Allosteric regulationCovalent modificationAssociation-disassociation’Proteolytic cleavage of proenzyme Allosteric RegulationEnd products are often inhibitorsAllosteric modulators bind to site other than the active siteAllosteric enzymes usually have 4o structureVo vs [S] plots give sigmoidal curve for at least one substrateCan remove allosteric site without effecting enzymatic actionRegulation 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 energyB A C13”3’2E F G4’5’H I J4”5”XXPhosphofructokinase( PFK)Fructose-6-P + ATP -----> Fructose-1,6-bisphosphate + ADPPFK catalyzes 1st committed step in glycolysis (10 steps total)(Glucose + 2ADP + 2 NAD+ + 2Pi  2pyruvate + 2ATP + 2NADH)Phosphoenolpyruvate is an allosteric inhibitor of PFKADP is an allosteric activator of PFKAllosteric modulators bind to site other than the active site and allosteric enzymes have 4o structureFructose-6-P + ATP -----> Fructose-1,6-bisphosphate + ADPADPAllosteric Activator (ADP) binds distal to active siteVo vs [S] plots give sigmoidal curve for at least one substrateBinding of this allosteric inhibitor or this activator does not effect Vmax, but does alter KmAllosteric enzyme do not follow M-M kineticsAllosteric T to R transitionConcerted modelSequential modelET-I ET ER ER-SIISSCovalent modificationRegulation by covalent modification is slower than allosteric regulationReversibleRequire one enzyme for activation and one enzyme for inactivationCovalent modification freezes enzyme T or R conformationPhosphorylation /dephosphorylationmost common covalent modification involve protein kinases/phosphatasePDK inactivated by phosphorylationAmino acids with –OH groups are targets for phosphorylationPhosphates are bulky (-) charged groups which effect conformationEnzyme Regulation by Association/DisassociationAcetyl-CoA Carboxylaseacetyl-CoA + CO2 + ATP  malonyl-CoA + ADP + Pi1St committed step in fatty acid biosynthesisIn presence of citrate activatedIn presence of fatty acyl-CoA inactivatedpolymerizedunpolymerizedcitrateFatty acyl-CoAProteolytic cleavage of proenzyme(zymogen)Proinsulin to InsulinBlood ClottingClotting involves series of zymogen activationsSeven clotting factors are serine proteases involved in clotting cascade rxnsXXXXXX