Chapter 5 (part 4) 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 Association-disassociation’ Proteolytic cleavage of proenzyme

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

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 B A C 1 3” 3’ 2 E F G 4’ 5’ H I J 4” 5” X

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

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

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

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

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

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

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

Proteolytic cleavage of proenzyme(zymogen)

Proinsulin to Insulin

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