Lecture 7-KumarEnzyme Inhibition-Drug Discovery

• Aspirin (anti-inflammatory)• Penicillin (effective against gram positive

bacteria)• Sulfonamides (anti-diarrhea)• Anti HIV drugs

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ENZYME INHIBITORS AS DRUGS

• Competitive Inhibitors compete with the substrate for the active site

• Non-Competitive inhibitors bind near but not at the active site

• Irreversible inhibitors bind at the active site of the enzyme and form a covalent bond with a group at this site

Enzyme inhibition

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COMPETITIVE INHIBITORS

E + S ES E + P+I

EIKi= [E][I]/[EI]

V= Vmax S Km[1+(I/Ki)] +S

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Kinetics of competitive inhibitor bindingIncrease [S] toovercomeinhibition

Vmax attainable,Apparent Km is increased

Ki =dissociationconstant forinhibitor

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Vmax unaltered, Km increased Double reciprocal plot in the

presence of a competitive inhibitor

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Methotrexate in cancer chemotherapy

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COMPETITIVE INHIBITORS AS DRUGS

Adenine Thioadenine

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More competitive inhibitors as drugs

• Doxycycline– competitive inhibitor of collagenase- control of

periodontal disease

• AZT-competitive inhibitor of HIV RT• Mozenavir and Tipranavir

competitive inhibitors of HIV protease• Allopurinol (used in gout)—Oxypurinol—inhibits

xanthine oxidase

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ENZYME INHIBITORS IN CLINICAL USE • A clinical application in Periodontitis

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NONCOMPETITIVE INHIBITION

E + S ES E + P+ +I I

EI + S ESI

Vmax SV= [1+(I/Ki)] Km + S

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Kinetics of non-competitive inhibitionIncreasing [S] cannotovercome inhibition

Less E available,Vmax is lower,Km remains the samefor available E

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Noncompetitive inhibition

Km unaltered, Vmax decreased

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Non-competitive inhibitors as drugs

• Fluoride: Non-competitive inhibitor of enolase

• Non-nucleoside compounds as inhibitors of HIV RT

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Kinetics of irreversible inhibition

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EXAMPLES OF IRREVERSIBLE INHIBITORS AS DRUGS

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Amoxicillin and Clavulinic Acid

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Irreversible Inhibitors

• Emphysema: Alpha-1-Antitrypsin deficiency—This protein inhibitor irreversibly inactivates elastase

• Nerve Gas: Sarin—irreversibly inhibits acetylcholinesterase and conversion of acetylcholine to choline and acetic acid

• Anticancer Drug: 5-Fluorouracil—irreversibly inhibits thymidylate synthetase

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Lecture 7-Learning objectives

• Recognize the differences between competitive, non-competitive and irreversible inhibitors

• Know how each of the inhibitors affects Km and Vmax

• Know the importance of inhibition constant in drug design and discovery

• Be knowledgeable about some of the drugs currently on the market

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