Enzymes Pt 3:
Regulation of enzyme activity
Relicardo M. Coloso, Ph. D.
College of Medicine
Central Philippine University
Regulate – to control or direct according
to a rule, principle or law
Enzyme regulationis the control of the rate of a reaction
catalyzed by an enzyme by some effector
(e.g., inhibitors or activators) or by alteration
of some condition (e.g., pH or ionic
strength).
Five ways by which enzyme activity in the cell can be changed:
1. Enzyme production – synthesis or degradation
2. Compartmentation – different metabolic pathways occur in different cell
compartments
3. Activation and inhibition – by activators or inhibitors, for example
feed back inhibition by one of products
of the reaction
4. Post-translational modification – for example by phosphorylation,
methylation, glycosylation
5. Localization to a different environment – from a reducing (cytoplasm) to
an oxidizing (periplasm) environment,
of high pH to a low pH, or low salinity to
high salinity, high to low energy charge
Enzyme Regulation
• Constitutive enzymes
– Enzymes needed at the same level all of the time
• Regulated enzymes
– Enzymes needed under some conditions but not
others
• For example enzymes of the Lac Operon
– Enzymes are made to break down lactose only if
lactose is present
2 Types of Regulation
• Regulation of amount of enzyme made
– At the level of transcription = is RNA made?
– At the level of translation = is protein made?
– Slower process (minutes)
• Regulation of enzyme activity
– After the protein is synthesized
– Posttranslational modification
– Very rapid process (seconds or less than a
second)
Mechanisms of Regulation
Cooperativity shown by an allosteric enzyme
Postranslational regulation of activity by
feed back inhibition
• Feedback inhibition
• Pathways with many
intermediates
• The end product of a
pathway feeds back and
inhibits the activity of the
first step in a pathway
• If there are enough end
products available, more
does not need to be made
• By inhibiting the enzymes in
the pathway, the end product
will not be made
Allosteric release of catalytic subunits
Activation of cAMP-dependent protein kinase (cAPK) by cyclic AMP
(b) At low concentrations of cAMP, the enzyme exists as an inactive tetramer composed of two
regulatory (R) and two catalytic (C) subunits. The tetrameric protein is inactive because the
pseudosubstrate sequences on the R subunits block the active sites on the C subunits. Binding
of cAMP to the regulatory subunits causes release of the active monomeric catalytic subunits.
(b) Structure of cAMP. This unusual nucleotide, which acts as a “second messenger” in many
intracellular signaling pathways, controls the activity of many proteins.
Copyright © 2000, W. H. Freeman and Company
Allosteric transition between
active and inactive states
Allosteric regulation of aspartate transcarbamoylase (ATCase), the initial enzyme
in synthesis of pyrimidines
This enzyme comprises a pair of trimeric catalytic subunits (orange) connected by three
pairs of dimeric regulatory subunits (green). Binding of cytidine triphosphate (CTP; the
blue dot) to the regulatory subunits causes a conformational transition from the active R
state to the inactive T state. The more open conformation of the R state permits
substrate binding. Thus an increase in the concentration of CTP, an end product in the
pyrimidine pathway, shuts off ATCase, an example of feedback inhibition.
Copyright © 2000, W. H. Freeman and Company
Binding of ligands
Cooperativity shown by an allosteric enzyme
•binding of one ligand molecule affects the binding of
subsequent ligand molecules
•positive cooperativity, sequential binding is enhanced; in
•negative cooperativity, sequential binding is inhibited.
Phosphorylation and dephosphorylation
Cyclic phosphorylation and dephosphorylation is a common cellular mechanism
for regulating protein activity
In this example, the target protein R (orange) is inactive when phosphorylated and active
when dephosphorylated; the opposite pattern occurs in some proteins.
Copyright © 2000, W. H. Freeman and Company
Proteolytic activation
A linear representation of the
conversion of chymotrypsinogen
into chymotrypsin by the excision
of two dipeptides
These reactions yield three separate
chains (A, B, and C), which are
covalently linked by disulfide bonds
(yellow) in the active enzyme. In the
folded, native conformation of
chymotrypsin, histidine 57, aspartate
102, and serine 195 are located in the
active site.
Copyright © 2000, W. H. Freeman
and Company