This document discusses various mechanisms of enzyme regulation, including regulation of enzyme quantity through gene expression and regulation of enzyme activity. Enzyme activity can be regulated rapidly through allosteric regulation, covalent modification, association/dissociation, or proteolytic cleavage of proenzymes. Specific examples discussed include allosteric regulation of phosphofructokinase and regulation of acetyl-CoA carboxylase by association/dissociation. Phosphorylation/dephosphorylation is highlighted as a common covalent modification that can regulate enzyme activity.

2. Enzyme Regulation

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

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

15. PROTEOLYTIC CLEAVAGE OF
PROENZYME(ZYMOGEN)

16. PROINSULIN TO INSULIN

17. BLOOD CLOTTING
•Clotting involves series of zymogen activations
•Seven clotting factors are serine proteases
involved in clotting cascade rxns

18. X
X
X
X
X
X