Enzymes are biological catalysts. They are involved in all metabolic reactions inside the body. But we know that for the normal working of a body we do not require every metabolism to take place at a particular time. Thus, there must be a regulative mechanism for the enzymes. How is these enzymes regulated? Let's explore molecular details and the biochemistry behind it

1. Enzyme
Regulation
Adarsh P P
M.Sc. Bioinformatics
Union Christian College, Aluva

2. Modes of Regulation
• Allosteric Control
• Covalent modification
• Protein Processing

3. Levels of Enzyme Regulation
1. Enzyme level: The enzyme may be activated or inhibited by either
noncovalent or covalent interactions. This is the most rapid control
system. Eg. Allosteric control, covalent modification, proteolytic
cleavage, Isoezymes, Product inhibition (feedback inhibition)
2. Hormonal level: A hormone is secreted and carries a message to the
cell and in turn an enzyme is activated or inhibited. The speed of
this control system is intermediate.
3. Gene level: A message is sent to the nucleus either express or
repress a gene. This determines the amount of enzyme produced
and is the slowest control measure

4. Allosteric mode of regulation
The enzyme has two binding sites, one for the substrate (the active site) and the other
for the allosteric modulator (the regulatory site). When the allosteric activator is not
bound to the regulatory site, the active site of the enzyme is not able to bind substrate
and catalyze the production of product. However, when the allosteric activator binds to
the enzyme at the regulatory site, the shape of the active site changes so that it can bind
its substrate and catalyze the production of products

5. Homotropic
A homotropic allosteric modulator is a molecule which binds to the
regulatory as well as active site of an enzyme, which is the substrate
for the enzyme. It is typically an activator of the enzyme. For example,
O2 is a homotropic allosteric modulator of hemoglobin.
Heterotropic
A heterotropic allosteric modulator is a molecule which binds to the
regulatory site. It may be either an activator or an inhibitor of the
enzyme. For example, H+, CO2, and 2,3-bisphosphoglycerate are
heterotropic allosteric modulators of hemoglobin.
Some allosteric proteins can be regulated by both their substrates and
other molecules. Such proteins are capable of both homotropic and
heterotropic interaction

6.  A positive modulator activates the enzyme
(an activator).
 A negative modulator inhibits the enzyme
(an inhibitor

7. Feedback Inhibition
Negative modulators (inhibitors)- Inhibits the
enzymatic reaction.
Eg. PFK (phospho fructo kinase)
PFK

8. Allosteric Regulation and
Conformational Changes in
Subunits.
• Monod, Wyman, Changeux (MWC) Model: allosteric
proteins can exist in two states: R (relaxed) and T (taut
or tight).
• In this two-state model, all the subunits of an oligomer
must be in the same state (they all change together) and
is therefore termed the concerted model.
• T state predominates in the absence of substrate S.
• S binds much tighter to R than to T.
• Cooperativity is achieved because S binding increases
the population of R, which increases the sites available
to S

10. The Sequential Model for Allosteric
Regulation (KNF)
• An alternative model – proposed by Koshland, Nemethy,
and Filmer (the KNF model) relies on the idea that
ligand binding triggers a conformation change in a
protein.
• In this one-state model, ligand-induced conformation
changes in one subunit may lead to conformation
changes in adjacent subunits.
• This model explains positive and negative
cooperativity
• The KNF model is termed the sequential model.

11. S S S S S
S
S
S
S
S
S
S
S
S
S S
S
S
SS
MWC: Two state concerted
KNF: One state sequential

12. Covalent Modification
• Enzymes can be regulated by transfer of a
molecule or atom from a donor to an amino acid
side chain that serves as the acceptor of the
transferred molecule.
• Another way of regulating an enzyme is by
altering the amino acid sequence itself by
proteolytic cleavage.

13. Phosphorylation
Enzyme regulation by reversible covalent modification.
Depending on the enzyme, phosphorylation may activate
or inactivate its catalytic function.

14. Phosphorylation-
dephosphorylation of glycogen
Phosphorylase

15. •Cyclic AMP is the intracellular agent of
extracellular hormones - thus a ‘second
messenger’
• Hormone binding stimulates a GTP-
binding protein (G protein), releasing
G(GTP)
• Binding of G(GTP) stimulates adenylyl
cyclase to make cAMP

16. Proteolytic cleavage
Regulating an enzyme is by altering the amino acid
sequence itself by proteolytic cleavage.

17. Chymotrypsin
bond fission(Hydrolysis)
Removal of two peptide
fragments
The final form of the enzyme contains 3 polypeptides
linked by disulfide linkages

18. Trypsin-

19. Genetic level control of
enzyme regulation
• Constitutive Enzymes
• Inducible enzymes/Adaptive Enzymes

20. • Constitutive enzymes are produced in constant amounts without regard to
the physiological demand or the concentration of the substrate.
• They are continuously synthesized because their role in maintaining cell
processes or structure is indispensable.eg. Enzymes of glycolytic pathway.
• The genes that code for constitutive enzymes are always on (always
expressed, the gene product is constantly synthesized.
• Constitutive enzyme activity is regulated by feed back mechanisms or
second messenger systems.

21. • An adaptive enzyme or inducible enzyme is an
enzyme that is expressed only under conditions in which
it is clear of adaptive value
• Inducible enzyme is used for the breaking-down of
things in the cell.
• It is also a part of the Operon Model, which illustrates a
way for genes to turn "on" and "off“
• Eg COX-2 which is synthesized in macrophages to
produce Prostaglandin E2

22. Protein Processing
• Post translation modification and folding
Trancription and Translation
(cell)
Folding (ER /golgi or
cytosol)
chaperones
Active enzyme (send to the
targets)
Degradation
by
proteasome
s

24. THANK YOU