3. REGULATION OF ENZYME
ACTIVITY
Enzyme quantity – regulation of gene
expression (Response time = minutes to
hours)
a) Transcription
b) Translation
c) Enzyme turnover
4. Enzyme activity
(rapid response time = fraction of seconds)
Allosteric regulation
Covalent modification
Association-disassociation’
Proteolytic cleavage of proenzyme
5. ALLOSTERIC REGULATION
End products are often inhibitors
Allosteric modulators bind to site other than
the active site
Allosteric enzymes usually have 4 o structure
Vo vs [S] plots give sigmoidal curve for at least
one substrate
Can remove allosteric site without effecting
enzymatic action
6. REGULATION OF ENZYME ACTIVITY
(BIOCHEMICAL REGULATION)
1 st committed step of a biosynthetic pathway or
enzymes at pathway branch points often
regulated by feedback inhibition.
1 A 2 C
B X
H 4” I
3”
3’X
E 4’ F
5”
5’
J
G
Efficient use of biosynthetic precursors and
energy
7. 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
8. ALLOSTERIC MODULATORS BIND TO SITE
OTHER THAN THE ACTIVE SITE AND
ALLOSTERIC ENZYMES HAVE 4 O STRUCTURE
Fructose-6-P + ATP -----> Fructose-1,6-bisphosphate + ADP
ADP
Allosteric Activator (ADP)
binds distal to active site
9. 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
10. ALLOSTERIC T TO R
TRANSITION
I
ET-I
I
S
ET
Concerted
model
ER
Sequential
model
S
ER-S
11. 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
12. 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
13.
14. ENZYME REGULATION BY
ASSOCIATION/DISASSOCIAT
ION Carboxylase
•Acetyl-CoA
•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