ENZYME INHIBITORS, REVERSIBLE AND IRREVERSIBLE

BYANSARI MUHAMMAD NAJEEF

ABDUL HAQUEM. PHARM First Year

R.C. Patel Institute Of Pharmaceutical Education & Research, SHIRPUR

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IntroductionImportance of enzyme inhibitionTypes of enzyme inhibitors

Enzyme inhibitors

Reversible Irreversiblei. Competitiveii. Non competitiveiii. Uncompetitive

Michaelis-menten equationReference

i. Active site directedii. Suicide inhibitors

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Introduction• Any chemical entity (or compound) which either

critically retards or specially blocks the phenomenon of enzyme catalysis.

• Inhibitors are the chemicals that reduce the rate of enzymatic reactions.

• They block the enzyme but they do not usually destroy it.

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Importance of enzyme inhibition• For understanding the regulation of enzyme

activity within the living cells.

• To elucidate the kinetic mechanism of an enzyme catalyzing in a multi substrate reaction.

• Useful in elucidating the cellular metabolic pathways by causing accumulation of intermediates.

• Identification of the catalytic groups at the active site.

• Provide information about substrate specificity of the enzyme.

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Types of inhibitors1) Reversible inhibitor:• Inhibitor binds to Enzyme reversibly through non

covalent interactions.• An Equilibrium is established between the free

inhibitor & EI Complex and is defined by an equilibrium constant (Ki).

+• The activity of Enzyme Is fully restored on removing

the Inhibitor by dialysis.

E I E I

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a) Competitive inhibitors• A competitive inhibitor often has structural features

similar to those of the substrate whose reactions they inhibit.

• This means that a competitive inhibitor and enzyme’s substrate are in direct competition for the same binding active site on the enzyme.

EnzymeSubstrate

ES- complexProducts

Inactive enzyme

Competitive

inhibitor

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Example:

The antibiotic sulfanilamide is similar in structure to para-aminobenzoic acid (PABA), an intermediate in the biosynthetic pathway for folic acid. Sulfanilamide can competitively inhibit the enzyme that has PABA as it's normal substrate by competitively occupying the active site of the enzyme.

b) Non competitive inhibitor• These are not influenced by the concentration of the

substrate. It inhibits by binding irreversibly to the enzyme but not at the active site.

• They also bind with the same affinity to the free enzyme and form the Enzyme-Substrate complex.

• It change the shape of enzyme and active site.

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Example:• Silver ions (heavy metal) react with -SH groups in the

side groups of cysteine residues in the protein chain:

• If the cysteine residue is somewhere on the protein chain which affects the way it folds into its tertiary structure, then altering this group could have an effect on the shape of the active site, and so stop the enzyme from working.

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c) Uncompetitive inhibitor• Uncompetitive inhibitors do not bind to the free

enzyme. They bind only to the enzyme-substrate complex to yield an inactive E. S. I complex.

• Uncompetitive inhibitors frequently observed in multi substrate reaction.

• Inhibition can’t be reversed by increasing the [S] since I doesn't compete with S for the same binding site.

Enzyme Enzyme

S

Enzyme

I S

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2) Irreversible inhibitor:• Inhibitor binds at or near the active site of the enzyme

irreversibly, usually by covalent bonds, so it can’t dissociate from the enzyme.

• Irreversible inhibitors combine with the functional groups of the amino acids in the active site, irreversibly.

• Irreversible inhibitors occupy or destroy the active sites of the enzyme permanently and decrease the reaction rate.

• Enzyme activity is not regained on dialysis.

E I E I

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a) Active site directed inhibitor• Active site directed inhibitor is also called as affinity

label. It is a chemically reactive compound that is designed to resemble the substrate of an enzyme so that it binds at the active site and forms a stable covalent bond with a susceptible group of the nearby residue in the enzyme protein.

• Affinity labels are very useful for identifying catalytically important residues.

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b) Suicide inhibitor• A suicide inhibitor is a relatively inert molecule that

is transformed by an enzyme at its active site into a reactive compound that irreversibly inactivates the enzyme

• They are substrate analogs designed so that via normal catalytic action of the enzyme, a very reactive group is generated.

• The latter forms a covalent bond with a nearby functional group within the active site of the enzyme causing irreversible inhibition.

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Michaelis-menten equationThe michaelis-menten equation arises from the

general equation for an enzymatic reaction.E+S ES E+P

The michaelis menten equation is:

V˳=Where=

V˳= velocity of the reactionVmax= maximal rate of the reaction[substrate]= conc. Of the subcstrateKm= michaelis-menten constant

Vmax [S]km+[S]

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References• Ashutosh Kar. Medicinal Chemistry (Fifth revised &

Expandded edition), New Age International Publishers. 2010: 917-972

• Burger’s. Medicinal Chemistry & Drug Discovery. Sixth Edition (Volume 1). A john wiley and sond, Inc., Publiction, new jersey. 2003: 715-774

• Albert L. Lehninger. Biochemistry (Second edition), Worth Publishers, Inc. New York. 1979: 189-195

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