Introduction to Enzyme Inhibition

Irreversible Inhibitors

• Covalent modifiers that permanently disable enzymes.
• Example: Aspirin
  • Aspirin acetylates a serine residue in cyclooxygenase enzymes, which leads to a reduction in inflammation by inhibiting the production pathways of prostaglandins, mediators of inflammation.
• Impact on kinetic parameters:
  • Lowers Vmax (maximum reaction rate) since fewer enzyme molecules are available for catalysis.
  • Does not affect the Km (Michaelis constant), which indicates that the affinity of the enzyme for the substrate remains unchanged.

Reversible Inhibitors

Competitive Inhibitors:

• Bind reversibly to the catalytic site of the enzyme.
• Compete with the substrate molecule for binding, creating an equilibrium between inhibitor and substrate.
• Characteristics:
  • Increase Km (concentration needed to reach half Vmax), indicating reduced affinity for the substrate.
  • No effect on Vmax; with sufficient substrate present, the reaction can still achieve maximum rate.

Non-competitive Inhibitors:

• Bind to an allosteric site (regulatory site) distinct from the catalytic site, which alters the enzyme's function without preventing substrate binding.
• Do not compete with the substrate; thus, Km remains unchanged as the binding affinity for the substrate is unaffected.
• Impact on kinetics:
  • Vmax is decreased due to a reduction in catalytic efficiency and the total enzyme activity.
  • Km remains the same since the binding of the substrate is not inhibited.

Uncompetitive Inhibitors

• Require substrate binding to the enzyme before the inhibitor can bind, making them dependent on substrate presence.
• Both Km and Vmax are decreased, resulting in higher affinity for the substrate, essentially enhancing the enzyme's efficiency under specific conditions.
• Note: Less common in nature and generally not essential for exam preparation, but important to acknowledge in the context of enzyme kinetics.

Comparing Inhibitor Types

• Irreversible: Cannot be overcome by increasing substrate concentration (e.g., sarin gas on acetylcholinesterase).
• Competitive: Can be overcome by increasing substrate concentration (e.g., methotrexate), demonstrating flexibility in enzyme regulation.
• Non-competitive: Cannot be overcome by increasing substrate concentration, reflecting a permanent modification of enzyme activity.

Sarin Gas Example

• Functions as an irreversible inhibitor by covalently modifying acetylcholinesterase, which leads to paralysis as acetylcholine remains elevated at neuromuscular junctions due to the inhibited breakdown.
• Notably used in chemical warfare due to its irreversible effects on catalytic activity and potential to cause severe neurological disruptions.

Methotrexate Example

• Functions as a competitive inhibitor, prevents the synthesis of nucleotides by inhibiting dihydrofolate reductase.
• Can be counteracted with increased levels of substrate (folate) to restore maximum activity, showcasing its relevance in cancer treatment and metabolic regulation.

Study Tips:

• Know key differences in mechanisms and effects of each inhibitor type, focusing on their binding sites and kinetic implications.
• Focus on examples discussed (aspirin for irreversible, methotrexate for competitive) and their clinical relevance.
• Recognize the clinical relevance of irreversible inhibitors like sarin gas in a biochemical context and their implications in toxicology, specifically addressing epidemics where such substances were used or encountered.
• Use visual aids, such as graphs of reaction rates against substrate concentration to visualize how different inhibitors interact with enzymes.