1. Chapter 5 (part 4)
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. 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

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 this allosteric inhibitor or this activator does not effect Vmax, but does alter Km
Allosteric enzyme do not follow M-M kinetics

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

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