IRREVERSIBLE ENZYME

INHIBITORS

Sazan J. Ali

Pharmacutical chemistry

department1

Irreversible enzyme inhibitors : Are

binding covalently or non covalently to the

enzyme and permanently inhibit it. there is

no reversal of inhibition on decreasing the

inhibitor concentration.

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Types of irreversible inhibition

Group-specific covalent modifying

agents

Affinity labels

Suicide inhibitors

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1.Group specific covalent modifying agents: react with

specific type of enzyme functional group (e.g., Ser-OH, or

Cys-SH, or His imidazole) on any enzyme/protein

React with amino acid side chains.

Lead to inhibition by interfering with the catalysis (e.g. by reacting with side-chains important for the catalysis)

E.g. diisopropylfluorophosphate(DFP),Nerve gas.

Inhibits acetylcholine esterase (and many other proteases with Ser at the active site).

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2.Affinity labeling agents(reactive

based analog)

Active-site directed irreversible inhibitors.

Recognized by the enzyme (reversible,

specific binding) followed by covalent

bond formation.

structurally similar to substrate, transition

state or product allowing for specific

interaction between the compound and

target enzyme.

E+I E I E-IKD Kinactivation

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Affinity labels(reactive based

analog)

Structural similarity to substrate

guides reagent to the active site of

enzyme.

It is a common and powerful tool to

isolate and characterize the enzymes

More specific than group specific

reagents.

Tosyl phenylalanin chloromethylketon(TPCK)

irreversibly in activate chymotrypsin

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3.Mechanism-based enzyme inactivators(Suicide Inhibitors)

• Active-site directed reagent (unreactive) binds to the enzyme active site transformed to a reactive form. Once activated, a covalent bond between the inhibitor and the enzyme forms

• This approach minimizes side reactions (non specific covalent bond formation) which may occur with an affinity reagent

E+I E..I E..I E-I

E+P

• 1) inhibitor binds to active site 2) converted to reactive compound via enzyme's catalytic capabilities 3) covalently reacts with the enzyme

• Inactivation (covalent bond formation, k4) must occur prior to dissociation (k3) otherwise the now reactive inhibitor is released into the environment

KD K2K4

K

3

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Bind like normal substrate

Begine, but unable to complete the

reaction

Get stuck in active site

TSInactive

enzyme

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• Allopurinol - the anti-gout drug - is a suicidal irreversible

mechanism-based inhibitor of the enzyme xanthine oxidase that

works as oxidase or dehydrogenase. The enzyme commits suicide

by initial activating allopurinol into a transition state analog -

oxypurinol - that bind very tightly to molybdenum-sulfide (Mo-S)

complex at the active site . The molybdenum-sulfide (Mo-S)

complex binds the substrates

and transfers the electrons required for the oxidation reactions.

(Mo-S)

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Suicide inhibitors - penicillin

Interfere with the synthesis of the

bacterial cell wall.

Makes bacteria much less resistant to

stress.

Penicillin blocks the formation of the

link between the tetra peptide and the

pentaGly bridge.

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13Inactive trans peptidase enzyme

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