Chapter 5 (part 4)

Enzyme Regulation

Regulation of Enzyme Activity

Enzyme quantity – regulation of gene expression (Response time = minutes to hours)

a) Transcription

b) Translation

c) Enzyme turnover

Enzyme activity (rapid response time = fraction of seconds)

a) Allosteric regulation

b) Covalent modification

c) Association-disassociation’

d) 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 C1 3”

3’2

E F G4’ 5’

H I J4” 5”

XX

Phosphofructokinase( PFK)Fructose-6-P + ATP -----> Fructose-1,6-bisphosphateFructose-1,6-bisphosphate + ADPADP

•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-bisphosphateFructose-1,6-bisphosphate + ADPADP

ADPADP

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

Concerted model

Sequential model

ET-I ET ER ER-SI

I S

S

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

polymerizedunpolymerized

citrate

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

XX X

X

X