## Distinguishing Competitive from Non-Competitive Inhibition ### Kinetic Signature of Competitive Inhibition **Key Point:** In competitive inhibition, the inhibitor competes with substrate for the active site. The hallmark kinetic change is **increased Km with unchanged Vmax**. - The inhibitor reversibly binds to the free enzyme (E), blocking substrate access - Increasing substrate concentration can overcome inhibition (substrate outcompetes inhibitor) - On a Lineweaver-Burk plot: lines intersect on the y-axis (Vmax unchanged) - Km increases by a factor of (1 + [I]/Ki) ### Kinetic Signature of Non-Competitive Inhibition **Key Point:** In non-competitive inhibition, the inhibitor binds to both free enzyme (E) and enzyme-substrate complex (ES), but NOT to the active site. - Inhibitor binds to an allosteric site - **Both Km and Vmax decrease** (or Vmax decreases more than Km) - Increasing substrate concentration **cannot overcome** inhibition - On a Lineweaver-Burk plot: lines intersect to the left of the y-axis ### Comparison Table | Feature | Competitive | Non-Competitive | |---------|-------------|------------------| | **Binding site** | Active site | Allosteric site | | **Km** | Increases | Unchanged or decreases | | **Vmax** | Unchanged | Decreases | | **Reversibility by substrate** | Yes | No | | **Lineweaver-Burk pattern** | Y-axis intersection | Left-side intersection | ### High-Yield Mnemonic **"COMP-K"** = **COMP**etitive increases **K**m (Michaelis constant) **"NON-V"** = **NON**-competitive decreases **V**max **Clinical Pearl:** Statins (HMG-CoA reductase inhibitors) are competitive inhibitors of cholesterol synthesis — their effect can theoretically be overcome by very high substrate (acetyl-CoA) levels, which is why they are most effective when combined with dietary cholesterol restriction.
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