Lecture 8-Kumar
Regulation of Enzyme Activity
• Regulation by modification
– Proteolytic cleavage
– Covalent modification
– Protein-protein interaction
• Allosteric regulation
– Properties of allosteric enzymes (important)
– Sigmoid kinetics (what does Km mean in this case)
(important)
– Positive and negative modulators (where do they act
and how do they modify activity at constant substrate
concentration) (most important)
– Models of allosteric transitions (important)
Regulation of Enzyme Activity
Normal metabolic control may be exerted in a variety of
ways. Examples are:
1. Proteolytic Cleavage of inactive Proenzymes to active
enzymes
Pepsinogen
pepsin + small peptide
in gastrointestinal tract for protein digestion
2.
Coagulation cascade—a series of proenzymes are
converted to active enzymes. The last step is
Fibrinogen
Fibrin
Coagulation Cascade
Covalent Modification as Control
Chemical modification can either increase or decrease activity. Some
examples are:
Glycogen Synthetase
Phosphatase
Kinase
Phosphorylated Glycogen
synthetase
Glycogen phosphorylase
Covalent Phosphorylated Glycogen phosphorylase
Protein-Protein Interaction
Example is activation of Protein Kinase A
R2C2 + 4 cAMP
R2C2(4cAMP)
Inactive
2R(cAMP)2 + 2C (active)
The catalytic unit (C) is able to phosphorylate and modulate the
activity of other enzymes
Allosteric Regulation
Properties of Allosteric enzymes
1.
2.
3.
4.
5.
6.
7.
Catalyze essentially irreversible reactions; are rate
limiting
Generally contain more than one polypeptide chain
Do not follow Michaelis-Menten Kinetics
Are regulated by allosteric activators or inhibitors
Can be up-regulated by allosteric activators at
constant [S]
Can be down regulated by allosteric inhibitors at
constant [S]
Activators and Inhibitors need not have any
structural resemblance to substrate structure
Sigmoid kinetics for allosteric enzymes
Effects of allosteric activators and allosteric
inhibitors on enzyme activity
Effect of allosteric activators and inhibitors
on rate at cellular concentration of the
substrate
Models of Allosteric Modulation
Symmetry model
Sigmoidal Curve Effect
vo
Sigmoidal curve
Noncooperative
(Hyperbolic)
ATP
Positive effector (ATP)
brings sigmoidal curve
back to hyperbolic
CTP
Cooperative
(Sigmoidal)
Negative effector (CTP)
keeps
vo
Exaggeration of
sigmoidal curve
yields a drastic
zigzag line that
shows the On/Off
point clearly
Consequently,
Allosteric enzyme
can sense the
concentration of
the environment and
adjust its activity
Off
[Substrate]
On
Learning objectives for lecture 8
• Learn various methodologies that enzymes employ to
control metabolism
• Know the properties of an allosteric enzyme.
• Understand the significance of sigmoid kinetics. Can one
determine Km and Vmax for these enzymes from the
sigmoid plot
• Understand how the activity of an allosteric enzyme is
regulated by allosteric activators and inhibitors
• Understand the mechanism of allosterism and negative
feedback inhibition