A biochemistry student is studying enzyme inhibition mechanisms. Which of the following statements about enzyme inhibition is NOT true?

## Distinguishing Enzyme Inhibition Types ### Key Characteristics of Each Type | Inhibition Type | Binding Site | K_m | V_max | Reversibility | Overcome by ↑[S]? | | --- | --- | --- | --- | --- | --- | | **Competitive** | Active site | ↑ | — | Reversible | Yes | | **Non-competitive** | Allosteric site | — | ↓ | Reversible | No | | **Uncompetitive** | ES complex | ↓ | ↓ | Reversible | No | | **Allosteric** | Regulatory site | ↓ or ↑ | ↓ or ↑ | Reversible | Variable | **Key Point:** Allosteric inhibition occurs when an inhibitor binds to a site OTHER than the active site (the allosteric site). This is fundamentally different from competitive inhibition, which targets the active site directly. **High-Yield:** The defining feature of allosteric regulation is that the inhibitor does NOT compete for the active site. Instead, it causes a conformational change that reduces enzyme activity. This is why allosteric inhibition cannot be overcome by increasing substrate concentration. **Clinical Pearl:** Feedback inhibition in metabolic pathways (e.g., CTP inhibiting aspartate transcarbamoylase in pyrimidine synthesis) is typically allosteric, not competitive. ### Why Option 3 Is Incorrect Allosteric inhibition binds to a **regulatory site distinct from the active site**, not the active site itself. The statement "always involves direct binding to the active site" is the defining error — it describes competitive inhibition, not allosteric inhibition. ### Verification of Other Options 1. **Competitive inhibition + ↑[S]:** True. Increasing substrate concentration displaces the competitive inhibitor from the active site (Lineweaver-Burk: K_m increases, V_max unchanged). 2. **Non-competitive inhibition:** True. Binds to allosteric site; V_max decreases, K_m unchanged (Lineweaver-Burk: parallel lines). 3. **Uncompetitive inhibition:** True. Inhibitor binds only to ES complex; both V_max and K_m decrease proportionally (Lineweaver-Burk: parallel lines with different slope).