VBC-605 Unit II Enzyme Inhibition - PowerPoint Presentation

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VBC-605

Unit II

Enzyme

Inhibition

Inhibition-

The decrease in enzyme activity/loss of activityexert effect by decreasing affinity of the enzyme for the substratedecreasing the amount of active enzyme available for catalysisby a combination of these effectsInhibitors -substances that decrease the catalytic activity of enzymesMay be protein/ Non proteinAccording to mode of action- classify in 2 categories1. Reversible2. Irreversible

Enzyme Inhibition/ Inhibitor

Binds non covalently

Can be reversed if inhibitor is removedClassify into1. Competitive2.Non- competitive3.Un competitiveReversible Inhibition

structurally similar to that of substrate (Structural analogue)

competes with substrate to bind at active site but does not undergo any catalysisCompetitive Inhibition

Competitive Inhibition

V

max

- No change

K

m

INCREASES -

indicates a direct interaction

of the inhibitor in the active site

increasing the amount of substrate can overcome the effect of the inhibitor as number of enzyme molecules available for the inhibitor are far less, and restore the normal rate of the enzyme catalyzed reaction (Reversible)

e.g. succinate dehydrogenase- Malonate competitively inhibits the enzyme because it is structurally similar to the substrate succinate

in clinical situations, the competitive inhibitors are called as

antagonists or anti metabolites of the substrate with which they competeCompetitive inhibitors are useful chemotherapeutic agentsused as

1. Antibiotics

2. Anti-cancer drugs

3. In the treatment of metabolic diseases like gout, atherosclerosis and hypertension

Competitive Inhibitors as Chemotherapeutic Agents

Enzyme

Inhibitor

Substrate

significance

Dihydropteroate synthase

Sulfonamides

PABA

Antibiotic

Dihydrofolate reductase

methotrexate

folic acid

Anticancer

Xanthine oxidase

Allopurinol

Hypoxanthine

To reduce production of Uric Acid in Gout

HMG-CoA reductase

Lovastatin

HMG-CoA

atherosclerosis

Vit K epoxide reductase

Dicumarol

Vit K

anticoagulant

No competition occurs between substrate and inhibitor to bind at active site of enzyme

Not structurally related to substrateInhibitor binds to some other siteInhibition cannot be overcome by raising [S]Non-Competitive Inhibition

Pure Non competitive- no effect of Inhibitor on substrate binding

Mixed – influence binding of substrate because of conformational changes Types of Noncompetitive Inhibition

Pure Non competitive

V

max

DECREASES -

inhibitor affects rate of reaction

by binding to site other than substrate active-site

K

m

- No change

binds at a site distinct from the substrate binding site

inhibitor binds only to the ES complexUncompetitive Inhibition

Uncompetitive inhibitors are present only for enzymes catalyzing reactions of

two or more substrates

(with ordered substrate binding): analogs of S

2

will act as uncompetitive inhibitor for the enzyme (relative to S

1

)

The presence of uncompetitive inhibitors alter both the

Km and the Vmax of an enzyme

Both V

max

and

K

m

decreases

(but slope remains unchanged)

Effect of Inhibitor on V

max and Km

Inhibitor Type

Apparent V

max

Apparent K

m

None

V

max

K

m

Competitive

Unchange

Increase

Pure Non Competitive

Decrease

Unchange

Mixed non Competitive

Decrease

Increase

Uncompetitive

Decrease

Decrease

Chemically modify or form tight noncovalent interactions with functional groups in the active site of enzymes

Net effect is a loss of active enzymeDilution or dialysis of the enzyme-inhibitor solution does not dissociate the EI complex so no restoration of enzyme activityIrrversible Inhibitor

Covalent bonds take longer time to form so irreversible inhibition is a time- dependent process, with more enzyme being inactivated with increasing time

e.g. Iodoacetate- glyceraldehyde 3- phosphate dehydrogenaseCyclooxygenase catalyzes the first reaction in the biosynthesis of prostaglandins from arachidonate. By acelyating an active site serine, aspirin causes a stable modification that leads to irreversible inhibition.

Diisopropyl Phosphofluoridate: Irreversible Acetylcholinesterase Inhibitor (Example)

Mechanism-based inactivator

Inhibitory substrate analogs designed so that, via normal catalytic action of the enzyme, a very reactive group is generated and forms a covalent bond with a nearby functional group within the active site of the enzyme causing irreversible inhibitione.g. Penicillin Allopurinol

Suicide

inhibitor

Inhibition of activity of enzyme of a biosynthetic pathway by the end product of that pathway is called as feedback inhibition

e.g. 1.Inhibition of aspartate transcarbamoylase by CTP2. Inhibition of HMG-CoA reductase by cholesterol3. Inhibition of Amino Levulenic

Acid-synthase by

heme

Feedback Inhibition