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.
1 A 2 C
B X
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3”
3’X
4’
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F
5”
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G
• Efficient use of biosynthetic precursors and
energy
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
I
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ET
Concerted
model
ER
S
Sequential
model
S
ER-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
citrate
unpolymerized
Fatty acyl-CoA
polymerized
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
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