Which of the following statements about enzyme inhibition is NOT true?

Explanation

## Enzyme Inhibition: Classification and Characteristics ### Overview of Inhibition Types Enzyme inhibition is classified based on the mechanism of action and the kinetic parameters affected. Understanding the distinction between these types is critical for NEET PG. ### Analysis of Each Statement | Inhibition Type | Km | Vmax | Mechanism | Reversible? | |---|---|---|---|---| | **Competitive** | ↑ | → | Competes with substrate for active site | Usually yes | | **Non-competitive** | → | ↓ | Binds to enzyme-substrate complex or free enzyme at allosteric site | Usually yes | | **Uncompetitive** | ↓ | ↓ | Binds only to ES complex; proportional decrease | Yes | | **Allosteric** | ↑ or ↓ | ↓ | Binds to regulatory site; causes conformational change | Usually yes | ### Statement-by-Statement Evaluation **Statement 1: Competitive inhibition — Overcome by substrate ✓ CORRECT** - Competitive inhibitors compete with substrate for the active site - Increasing substrate concentration shifts equilibrium in favor of substrate binding - Km increases (appears as if affinity decreases), but Vmax remains unchanged - This is the hallmark of competitive inhibition **Statement 2: Non-competitive inhibition — Vmax ↓, Km → ✓ CORRECT** - Non-competitive inhibitors bind to the enzyme at a site OTHER than the active site - They can bind to free enzyme (E) or enzyme-substrate complex (ES) - Vmax decreases because some enzyme molecules are inactivated - Km remains unchanged because substrate affinity is unaffected - Cannot be overcome by increasing substrate concentration **Statement 3: Uncompetitive inhibition — Proportional decrease ✓ CORRECT** - Uncompetitive inhibitors bind ONLY to the ES complex, not to free enzyme - Both Vmax and Km decrease proportionally (same ratio) - The inhibition constant (Ki) is the same for both parameters - Rare in practice; more common in two-substrate enzymes **Statement 4: Allosteric inhibition — Direct active site binding ✗ INCORRECT** - **Allosteric inhibitors do NOT bind to the active site** - They bind to a regulatory (allosteric) site on the enzyme - This binding causes a conformational change that reduces catalytic activity - The inhibition is non-competitive in kinetic terms - Example: Phosphofructokinase inhibition by ATP in glycolysis ### Key Point: **Allosteric regulation is fundamentally different from active-site directed inhibition.** The allosteric site is spatially and functionally distinct from the active site. Binding at the allosteric site induces a conformational change (often via cooperative binding in multi-subunit enzymes) that reduces activity. ### High-Yield: **Mnemonic: "ACUA"** - **A**llosteric = Alternate site (not active site) - **C**ompetitive = Competes with substrate - **U**ncompetitive = Unique to ES complex - **A**ffects both = Affects both Vmax and Km proportionally ### Clinical Pearl: In clinical practice, allosteric inhibition is often more elegant and physiologically relevant than competitive inhibition. Examples include: - Feedback inhibition of phosphofructokinase by ATP and citrate - Inhibition of aspartate transcarbamoylase by CTP - These allow cells to fine-tune metabolism without needing massive substrate concentration changes ### Lineweaver-Burk Plot Recognition - **Competitive**: Lines intersect on y-axis (same Vmax) - **Non-competitive**: Lines intersect on x-axis (same Km) - **Uncompetitive**: Parallel lines (proportional decrease) - **Allosteric**: Non-linear kinetics; not well-represented by Lineweaver-Burk (sigmoidal curve) [cite:Lehninger Principles of Biochemistry Ch 6]