Enz inhi 5 lec

Enzyme Inhibition
Dr.Geeta Jaiswal

Inhibitors
Enzymes are proteins. They
can be inactivated by agents
which denature them.
Chemical substances that
inactivate enzymes are
called Inhibitors

Inhibitors are organic
substances or inorganic
ions
Inhibitors decrease
enzyme activity without
altering the ionic strength
or pH of the medium

Inhibition may be reversible ,
where the inhibitor does not
react covalently with the
enzyme.
Some agents react covalently
with the functional groups of
enzymes resulting in non-
competitive irreversible
inhibition.

Inhibition
Reversible Irreversible
Competitive Non-Competitive Uncompetitive
Classification of Inhibition

Competitive Inhibition
It is called substrate analogue
inhibition
Inhibitors resembles substrate
and competes for the active
site of the enzyme.

Inhibitor binds with enzyme
to form EI complex .
Due to structural similarity
the substrate and inhibitor
molecule both compete for
the active site

This is a reversible
type of inhibition,
where both ES and EI
complexes are formed.

 The actual amount of ES and
EI will depend upon:-
(i) Affinity between enzyme
substrate/and inhibitor present.
(ii)Actual concentration of
substrate and inhibitor present
(iii) Time of pre-incubation with
substrate or inhibitor

Line weaver’s Burk plot representing

 Affinity Decreases
 I Decreases as Km increases
Km
 I Remains the same
Vmax
Efficiency Remains the same

Examples of Competitive Inhibitors
S.N Enzyme Substrate Inhibitor
1. LDH Lactate Oxamate
2. Aconitase Cis-Aconitase Trans-Aconitase
3. Succinate
Dehydrogenase
Succinate Malonate
4. H.M.G Co.A reductase HMG Co.A HMG
5. Di Hydrofolate
Reductase
7,8 dihydrofolate Amethopterin
6. Xanthine Oxidase Hypoxanthine Allopurinol used
in Gout
Treatment

Examples of Competitive Inhibitors
used as Drugs Clinically
Allopurinol ---- Used in the
treatment of Gout
Xanthine Oxidase
Hypoxanthine Uric Acid
Allopurinol

Sulphonamides:
Used as antibacterial agents.Similar in
structure to PABA
For Folate synthesis
PABA is essential
Sulphonamide
Needed for Bacterial Growth

Methotrexate
Methotrexate is 4-amino N10
methyl folic
acid.
Used in cancer therapy
Methotrexate resembles folic acid it
competitively inhibits “Folate Reductase”
Prevents the formation of FH4
DNA Synthesis is inhibited

MAO Inhibitors
 MAO inhibitors are Ephedrine and
Amphetamine
Enzyme Mono Amine Oxidase oxidizes
Epinephrine and Nor-epinephrine
MAO inhibitors competitively inhibit
MAO ,prolong action of presser amines.

Physotigmine
Physotigmine is Acetylcholine esterase
inhibitor
Acetylcholine Acetate + Choline
This drug prevents destruction of
Acetylcholine,
Continued presence of Acetylcholine in
post synaptic regions prolong neural
impulse.

Dicoumarol
• Dicoumarol is used as an
anticoagulant
• It competitively inhibits Vitamin
K

Clinically useful Competitive Inhibitors
Drug Enzyme Inhibited Clinical use
Penicillin Transpeptidase Bacteria
Sulphonamide Pteroid synthetase Bacteria
6-Mercapto Purine Adenylo succinate
Synthetase
Cancer
Neostigmine Acetyl Ch.esterase Myasthenia
Alpha methyl DOPA DOPA Decarboxylase Hypertension
Lovastatin HMG Co A-
reductase
Cholesterol
Lowering

Non-Competitive Inhibition
No competition between the Inhibitor and
substrate.
These inhibitors do not resemble the
substrate and bind to a site away from the
active site.
Enzyme inhibitor has normal affinity for
the substrate but produce products at a
decreased rate.

In Non-Competitive Inhibition
 (i) Affinity Remains the same
 (ii) Efficiency decreases
 (iii) 1/ Km remains the same as substrate
concentration has no effect on the inhibitor
 (iv) 1/ Vmax Increases as V has a decrease

Lineweaver Burk’s Plot for Non-
1/[s]1/ Km
1 / v
1 / Vmax
1 / Vmax
+ Inhibitor
No Inhibitor

 Non-competitive inhibition can be
reversed if the inhibitor can be removed
without affecting the enzyme activity.
Eg: Enzymes with –SH groups bind to
heavy metals like Hg , Pb, resulting in
non-competitive inhibition.
It can be reversed not by high levels of
substrate but by increasing –SH in the
medium.

Uncompetitive Inhibition
In this case the Inhibitor does
not bind to either enzyme or
substrate, but combines with
the ES complex to form ESI
complex.
 E + S  ES + I  ESI
(no product forms)

Degree of inhibition increases
with increase in substrate
concentration.
There is a change in Vmax .It is
lowered
This type of inhibition is very rare.

Uncompetitive Inhibition
+
1/V
1/V
1/V
1/ [s]
1/V
1/Km 1/Km 1/Km
More Inhibitor
+ Inhibitor

Suicide Inhibition
From all the discussions it can
be assumed that the inhibitor
binds to the enzyme reversibly.
 The inhibitor just sits there,
either in the active site or in a
second binding site, but it is
free to go back to solution.

There are another type of
inhibitors that undergo irreversible
chemistry with certain key amino
acid residues (or prosthetic
groups) in the active site, therefore
killing the enzyme's activity, and
preventing the inhibitor from being
released out of the active site.

Many of these molecules are very
effective drugs, because they are
targeted specifically for a certain
enzyme and kill the enzymes for
good.
 This inhibitors kill the enzyme for
good, but since they also 'die' in
the process, they are called suicide
or mechanism-based inhibitors.

Suicide inhibition
It is a type of irreversible inhibition
The inhibitor makes use of an enzyme’s
own reaction mechanism to inactivate it
In suicide inhibition, the structural
analogue is converted to a more effective
inhibitor with the help of the enzyme to be
inhibited.
This new product binds to the enzyme and
inhibits further reaction.

Eg : Of Suicide inhibition
1.Allopurinol a competitive inhibitor
for enzyme xanthine oxidase
When it comes in contact with the
enzyme it is oxidized by xanthine
oxidase to alloxanthine which is a
stronger irreversible inhibitor of the
enzyme

2. Anti-inflammatory Action of
Aspirin
Membrane bound phospholipids are
broken down first to Arichidonic acid (by
phospholipases)
Cyclooxygenase
Arichidonic Acid Prostaglandins
Aspirin acetylates a serine residue in the
active center of cyclo-oxygenase ,inhibiting
prostaglandin synthesis and reducing
inflammation

3. Difluromethyl ornithine against
sleeping sickness Trypanosomiasis
Ornithine Decarboxylase converts
Ornithine to putriscine a polyamine
When this enzyme ODC
inTrypanosoma(parasite) is inhibited,
multiplication of the parasite is arrested.
DFMO is initially inert. Binding with the
enzyme it forms an irreversible covalent
complex with co-enzyme PyPO4 and
amino acid residues

S.No ENZYME Allosteric
Inhibitor
Allosteric
activator
1 HMG Co A -reductase Cholesterol
2 Phosphofructokinase ATP,Citrate AMP,F2,6,P
3 Pyruvate Carboxylase ADP Acetyl Co A
4 Acetyl CoA Carboxylase AcylCoA Cirate
5 Citrate Synthase ATP
6 Carbamyl Phosphate Synthetase- I N-Acetyl Glut
7 Carbamyl PhosphateSynthetase-II UTP
8 Aspartate Transcarbamylase CTP ATP

Enz inhi 5 lec

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