Classify enzyme inhibition. Describe competitive, non-competitive and uncompetitive inhibition with their effects on Km and Vmax, and give examples.

Definition and Classification

• Reversible — binds non-covalently; can be removed by dialysis
• Irreversible — binds covalently; the enzyme is permanently inactivated

Competitive Inhibition

• The inhibitor is structurally similar to the substrate
• It competes for the same active site
• The effect can be overcome by increasing substrate concentration

Examples

• Malonate inhibits succinate dehydrogenase — the classical example
• Methanol poisoning treated with ethanol — ethanol competes for alcohol dehydrogenase and prevents formation of toxic formaldehyde and formate
• Sulphonamides compete with PABA for bacterial dihydropteroate synthase
• Statins compete with HMG-CoA for HMG-CoA reductase
• Allopurinol competes with hypoxanthine for xanthine oxidase
• Methotrexate competes with folate for dihydrofolate reductase

Non-competitive Inhibition

• The inhibitor binds at a site other than the active site (allosteric)
• It binds equally to E and ES
• Not overcome by increasing substrate

• Examples — heavy metals (lead, mercury, silver) binding sulphydryl groups; cyanide on cytochrome oxidase; fluoride on enolase

Uncompetitive Inhibition

• The inhibitor binds only to the ES complex, never to free enzyme
• Both Km and Vmax are decreased, in the same proportion
• Lineweaver–Burk gives parallel lines
• Example — lithium on inositol monophosphatase

Comparison Table

Irreversible and Suicide Inhibition

• Irreversible — covalent modification of the active site
• Organophosphates phosphorylate the serine of acetylcholinesterase
• Aspirin irreversibly acetylates cyclo-oxygenase
• Penicillin irreversibly inhibits bacterial transpeptidase
• Iodoacetate inhibits glyceraldehyde-3-phosphate dehydrogenase

Suicide (mechanism-based) inhibition

• The inhibitor is a substrate analogue that the enzyme itself converts into a reactive species, which then destroys the enzyme
• Examples — allopurinol on xanthine oxidase (also competitive), 5-fluorouracil on thymidylate synthase, α-methyldopa
• Advantage — very high specificity, since only the target enzyme activates it

Applied Aspects

• Antimetabolites in cancer therapy — methotrexate, 5-fluorouracil, 6-mercaptopurine all work as enzyme inhibitors
• Ethanol as an antidote in methanol and ethylene glycol poisoning is the classic clinical use of competitive inhibition; fomepizole is the modern alternative
• Heavy metal poisoning treated with chelating agents — BAL, EDTA, penicillamine — which remove the metal from the sulphydryl groups
• Feedback inhibition by the end product is usually allosteric and is the commonest physiological form of regulation