A 52-year-old man with hypercholesterolaemia is started on atorvastatin. The drug works by competitively inhibiting HMG-CoA reductase. Which of the following is the most common mechanism by which competitive inhibitors exert their therapeutic effect in enzyme-catalysed reactions?

Explanation

## Mechanism of Competitive Enzyme Inhibition **Key Point:** Competitive inhibitors reversibly bind to the enzyme's active site, competing with the natural substrate. This is the most therapeutically exploited mechanism because it is reversible and substrate-concentration dependent. ### Kinetic Hallmarks of Competitive Inhibition ```mermaid flowchart TD A["Enzyme + Substrate<br/>(Competition for Active Site)"]:::outcome --> B{"Inhibitor<br/>Present?"}:::decision B -->|"No Inhibitor"| C["ES Complex<br/>Vmax unchanged<br/>Km normal"]:::action B -->|"Competitive Inhibitor"| D["E + I ⇌ EI<br/>E + S ⇌ ES"]:::action D --> E["Apparent Km ↑<br/>Vmax unchanged<br/>Reversible"]:::outcome C --> F["Normal kinetics"]:::outcome E --> G["Overcome by ↑[S]"]:::outcome ``` ### Why This Mechanism Is Most Common Clinically | Feature | Competitive | Non-competitive | Uncompetitive | Allosteric | |---|---|---|---|---| | **Binding site** | Active site | Non-active site | ES complex only | Regulatory site | | **Reversibility** | Reversible | Reversible | Reversible | Reversible | | **Vmax** | Unchanged | ↓ | ↓ | Variable | | **Km** | ↑ | Unchanged | ↓ | Variable | | **Overcome by ↑[S]** | **Yes** | No | No | No | | **Frequency in drugs** | **Most common** | Uncommon | Rare | Moderate | **High-Yield:** The ability to overcome competitive inhibition by increasing substrate concentration is why this mechanism is preferred therapeutically—it provides a safety margin and allows dose titration. ### Atorvastatin as a Clinical Example **Clinical Pearl:** Atorvastatin competitively inhibits HMG-CoA reductase: - Binds reversibly to the active site (competing with HMG-CoA) - Km for HMG-CoA increases (enzyme has lower affinity in presence of drug) - Vmax remains the same (maximum enzyme capacity unchanged) - Effect is dose-dependent and can be titrated - Reversible—discontinuation allows enzyme recovery ### Lineweaver-Burk Analysis In competitive inhibition, the Lineweaver-Burk plot (1/v vs 1/[S]) shows: - **X-intercept shifts** (Km increases) - **Y-intercept unchanged** (Vmax constant) - Lines converge on the y-axis **Mnemonic:** **CKVU** = Competitive: Km ↑, Vmax Unchanged **Warning:** Do not confuse competitive inhibition with non-competitive inhibition—non-competitive inhibitors bind outside the active site and CANNOT be overcome by increasing substrate concentration, making them less suitable for most therapeutic applications.