Enzyme Inhibition: Competitive and Non-competitive

Enzyme Inhibition - Enzyme Slowdown

• Enzyme inhibitors are molecules that bind to enzymes and ↓ their activity.
• Crucial for metabolic regulation and drug action.
• Classified based on reversibility:
  • Reversible Inhibition:
    • Inhibitor binds non-covalently.
    • Enzyme activity can be restored by removing inhibitor.
    • Includes competitive, non-competitive, and uncompetitive types.
  • Irreversible Inhibition:
    • Inhibitor binds covalently (usually).
    • Causes permanent enzyme inactivation.
    • Examples: Aspirin (COX), organophosphates (AChE), heavy metals. ⭐ > Many drugs act as enzyme inhibitors; e.g., statins inhibit HMG-CoA reductase.

Competitive Inhibition - Active Site Battle

• Mechanism: Inhibitor (I) structurally resembles substrate (S).
  • Competes directly with S for binding to the enzyme's active site (E).
  • Reversible: Effect overcome by ↑ [S].
• Kinetic Impact:
  • $K_m$: ↑ (Apparent $K_m$ increases; more S needed to reach 1/2 $V_{max}$ as enzyme affinity for S appears to decrease).
  • $V_{max}$: Unchanged (With sufficient [S], I is outcompeted, and the original $V_{max}$ is reached).
• Lineweaver-Burk Plot: Lines intersect on the Y-axis (1/$V_{max}$ is unchanged).
• Clinical Examples:
  • Statins (e.g., Atorvastatin) inhibit HMG-CoA reductase (hypercholesterolemia treatment).
  • Methotrexate inhibits Dihydrofolate reductase (cancer chemotherapy, rheumatoid arthritis).
  • Ethanol for methanol poisoning (competes for alcohol dehydrogenase).

⭐ Competitive inhibitors increase the apparent $K_m$ of the enzyme for the substrate but do not alter the $V_{max}$ achievable at saturating substrate concentrations.

Non-competitive Inhibition - Allosteric Attack

• Inhibitor (I) binds to an allosteric site (distinct from the active site).
  • Binds to Enzyme (E) or Enzyme-Substrate (ES) complex with equal affinity.
  • Reduces catalytic efficiency; prevents product (P) formation at normal rate.
• Kinetic Effects:
  • ↓$V_{max}$ (effectively ↓ concentration of functional enzyme).
  • $K_m$ unchanged (substrate affinity to active site is not affected).
• Cannot be overcome by increasing substrate concentration [S].
• Lineweaver-Burk Plot:
  • Lines (inhibited vs. uninhibited) intersect on the x-axis.
  • Y-intercept (1/$V_{max}$) increases.
  • X-intercept (-1/$K_m$) is unchanged.
• Examples: Lead (e.g., on ferrochelatase, ALA dehydratase), Alanine (on pyruvate kinase), Pepstatin (on aspartic proteases).
• 📌 Mnemonic: "Non-Compete? $K_m$ No Change, $V_{max}$ Vanishes (↓)."

⭐ Non-competitive inhibitors effectively reduce the number of functional enzyme molecules, thus lowering $V_{max}$, but do not interfere with substrate binding to the active site ($K_m$ unchanged).

Inhibitor Comparison - Key Differences

High‑Yield Points - ⚡ Biggest Takeaways

• Competitive inhibitors bind active site, ↑ Km (↓ affinity), Vmax unchanged; overcome by ↑ [S].
• Non-competitive inhibitors bind allosteric site, Km unchanged, ↓ Vmax; not overcome by ↑ [S].
• Lineweaver-Burk: Competitive lines intersect on Y-axis; Pure non-competitive lines intersect on X-axis.
• Km is [S] at ½ Vmax; reflects substrate affinity.
• Vmax is maximum reaction velocity at saturating [S].
• Example (Competitive): Statins (vs HMG-CoA reductase).
• Example (Non-competitive): Lead (vs ferrochelatase).