This ppt describes the overview of enzyme regulation and Allosterism. Presented since October 23,2017GC at Addis Ababa University, School of Medicine, Department of medical biochemistry.

1. Addis Ababa University
School of Medicine
Department of Biochemistry
Regulatory and Allosteric enzymes and Allosterism
By
Ayetenew Abita
October 23,2017
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2. Contents
• Regulatory enzymes
• Reversible covalent modification of enzyme
• Reversible non-covalent modification of enzymes
Allosteric enzyme regulation
Feedback inhibition
• Allosteric inhibition
• Feedback regulation of enzymes
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3. Regulatory Enzyme
• Why we are curios about enzyme regulation???????
• Most metabolic reactions are multi-step cascade
processes.
• In each enzyme system there is at least one enzyme
that sets the rate of the overall sequence because it
catalyzes the rate limiting reaction.
Regulatory Enzyme
• These regulatory enzymes exhibit increased or
decreased catalytic activity in response to certain
signals.
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4. Cont’d
• In most multi-enzyme systems the first enzyme
that is specific for that sequence is a regulatory
enzyme.
• Catalyzing even the first few reactions of a
pathway that leads to an unneeded product,
diverts energy and metabolites from
more important processes.
• An excellent place to regulate a metabolic
pathway, therefore, is at the point of
commitment to the pathway.
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5. Cont’d
• The activity of regulatory enzymes is modulated
through various types of signal molecules, which
are generally small metabolites.
• There are two major classes of enzyme regulation
in metabolic pathways.
• These are reversible covalent modification and
reversible non-covalent modification.
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6. Cont’d
Regulation of Enzyme activity
Reversible non-covalent
modification:
Allosteric Regulation
1) Allostric Activator
2) Allostric Inhibitor
Reversible Covalent
Modification
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7. Reversible Covalent Modification
• Catalytic activity is modulated by reversible covalent
modification of enzymes.
• More than 500 molecules are known to modify
enzyme by this method.
• There will be separate enzyme for adding and
removing of modifying groups.
• Most common modifying group includes:
– Phosphoryl group - Acetyl group
– Adenylyl group - Uridylyl group
– Methyl group -Amid group
– Carboxyl group -Hydroxyl group
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8. Cont’d
 Common reversible covalent modification of
enzyme:
1. Phosphorylation: most common type, addition of
phosphate group to Tyr, Ser, Thr and His residue of
protein.
2. Adenylation: Addition of adenine to Tyr residue of
protein.
3. ADP-ribosylation: addition of ADP ribose to Arg, Gln,
Cys and Diphthamide residue of protein
( diphthamide is a modified Histidine residue)
4.Methylation: addition of methyl group to Gln residue of
protein.
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9. Cont’d
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10. Cont’d
• Examples of reversible covalent modifications of enzymes
• ADP ribose derived from Nicotinamide Adenine Dinucleotide
(NAD) is added to bacterial enzyme dinitrogenase reductase
resulting in the regulation of important process of biological
nitrogen fixation.
• Diphtheria toxin and Cholera toxin are enzymes that catalyze the
ADP ribosylation (and inactivation)of key cellular enzymes or
proteins.
• Diphtheria toxin inhibit elongation factor2(EF2) of protein
biosynthesis.
• Cholera toxin acts on G-protein that is part of a signaling pathway,
leading to several physiological responses including a massive loss
of body fluids and sometimes death.
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11. Phosphorylation Affect Structure and
Catalytic Activity of Proteins
• Phosphorylation: most common type of regulatory
covalent modification.
• Addition of phosphoryl groups to specific amino acid
residues of a protein is called Phosphorylation.
• Phosphorylation reaction catalyzed by the enzyme
protein kinase.
• ATP or GTP usually act as a phosphoryl group donor.
• Energy derived from the cleavage of phosphate group
donor is also utilized.
• One third to one half of all proteins in a eukaryotic cells
are phosphorylated.
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12. Cont’d
• Some proteins have only one phosphorylated
residue, other have several for phosphorylation.
• Phosphoryl groups are usually added to Ser, Thr or
Thy residues of the enzyme.
• Phosphorylation induces a bulky charged group
into a region that was only moderatly polar.
• Removal of phosphoryl group from the protein is
called dephosphorylation.
• Dephosphorylation reaction is catalyzed by protein
phosphatase.
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13. Regulation Glycogen phoshopylase by
Covalent Modification
• Phosphorylase exists in two forms:
More active phosphorylase a and
Les active phosphorylase b.
• specific phosphorylation of les active
phosphorylase b on Ser 14 on each
subunit by two molecule of ATP by
phosphorylase kinase enzyme produce
the more active phosphorylase a
enzyme .
• Similarly the dephosphorylation by the
enzyme phosphorylase phosphatase
produce the less active phosphorylase
b enzyme. 13

14. Allosteric Enzymes
• Allosteric enzymes are a class of Regulatory
enzymes.
• Definition: A type of enzyme regulation by the
reversible non-covalent binding of regulatory
molecules to the enzyme.
• Regulatory molecules are called allosteric
modulators or allosteric effectors .
• Allosteric enzyme has additional conformations
induced by the binding of modulators.
• Conformational changes induced by the allosteric
modulators can produce more active or less active
forms of enzyme. 14

15. Cont’d
• Allosteric modulators may be inhibitory or stimulatory.
• Two types of Allosteric enzymes based on the nature of
modulator:
– Homotropic Allosteric Enzymes
– Hetrotropic Allosteric Enzymes
• In most cases, the substrate itself act as the modulator
• Allosteric enzymes having the substrate and
modulators are the same are called homotropic
allosteric enzymes
• Binding of modulator causes conformational changes in
the enzyme.
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16. Cont’d
• Conformational change affect the subsequent
enzymatic activity.
• If modulator is any molecule other than substrate, the
enzyme is called hetrotropic allostric enzyme.
• Allosteric modulators are not to be considered as
competitive or non-competitive inhibitors.
• Allosteric enzymes possess one or more regulatory or
allosteric sites.
• Allosteric sites act as the binding site of modulator.
• Modulator and modulator binding site on enzyme are
very specific similar to substrate specific for its active
site.
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17. Allosteric Enzyme Regulation
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18. Cont’d
• Allosteric enzymes are larger & complex than non-
allosteric enzymes.
• Allosteric enzymes possess many sub-units
• Aspartate Transcarbomoylase ( an allosteric enzyme),
which catalyze an early reaction in the biosynthesis of
pyrimidine nucleotides has 12 polypeptide chain
organized into catalytic and regulatory subunits.
• Enzymes with several modulators have different and
specific binding sites for each.
• In most of the allosteric enzymes, substrate binding
site and modulator binding site are on different
subunits.
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19. Cont’d
• Substrate binding site is called catalytic subunit or C
subunit.
• Modulator binding site is called the regulatory
subunit or R subunit.
• Binding of a positive or stimulatory modulator (M)
to its specific site of the regulatory subunit is
communicated to the catalytic subunit through
conformational changes.
• This change render the activation of catalytic
subunit.
• Activation enables the binding of substrate (S) with
higher affinity.
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20. Cont’d
• Once the modulator is dissociate from the
regulatory subunit, the enzyme reverts to its
inactive or less active form.
• Allosteric enzymes shows variations in enzyme
kinetic parameters.
• Allosteric enzymes do not follow Michaelis- Menten
Kinetics.
• Allosteric enzyme do not show the usual hyper-
parabolic curve when the initial velocity Vo is
plotted against substrate concentration [S].
• They shows a sigmoid curve when the velocity is
plotted against substrate concentration.
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21. Lineweaver-Burk Plot & Allosteric
Enzyme
• The Lineweaver-Burk plot also shows difference from
usual enzymes.
• Lineweaver-Burk plot of an allosteric enzyme will be
upward concave shaped as showen below
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22. Feedback inhibition
• Feedback inhibition is a specific type of allosteric
enzymatic activity regulation mechanism in the
cells.
• Definition: in some multi enzyme pathways, the
regulatory enzyme is specifically inhibited by the end
product of the pathway whenever the concentration of
the end product exceeds the cells requirements.
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23. Feedback Regulation
• Feedback regulation is different from feedback
inhibition.
• A type of enzymatic activity regulation.
• Here the end product of an enzymatic pathway
directly inhibit the synthesis of concentrated enzyme
by interfering with the gene of that enzyme.
• Enzyme is not directly inhibited by the end product.
• End product reduces the concentration of enzyme by
inhibiting the synthesis new enzyme molecules.
• Best example is reduction of HMG CoA reductase by
dietary cholesterol.
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