## Analysis of Factors Affecting Michaelis-Menten Kinetics ### The INCORRECT Statement **Option A: Non-competitive inhibitor decreases Vmax AND increases apparent Km** This statement is **FALSE** and is the correct answer to this EXCEPT question. - A **non-competitive inhibitor** binds to an allosteric site on the enzyme (or enzyme-substrate complex) that is distinct from the active site - It **decreases Vmax** — because a fraction of enzyme molecules are rendered catalytically inactive regardless of substrate concentration - It does **NOT change the apparent Km** — because the inhibitor does not affect substrate binding affinity; the enzyme-substrate complex still forms normally - The Lineweaver-Burk plot shows lines with the **same x-intercept** (same Km) but different y-intercepts (different Vmax) > **Note:** It is the **uncompetitive inhibitor** that decreases *both* Vmax *and* apparent Km (both decrease proportionally, so Km/Vmax ratio is unchanged). **High-Yield:** Non-competitive inhibition → Vmax ↓, Km unchanged. Uncompetitive inhibition → Vmax ↓, Km ↓. Competitive inhibition → Vmax unchanged, Km ↑. --- ### Correct Statements (TRUE — not the answer) **Option B: Increasing temperature increases both Km and Vmax up to denaturation** - Temperature increases molecular motion and collision frequency → **Vmax increases** - Km is a thermodynamic ratio of rate constants (k₋₁ + k₂)/k₁; it may remain relatively stable or show slight changes with temperature, but the *primary* effect of temperature below denaturation is increased Vmax - While the statement slightly oversimplifies Km behavior, it is closer to correct than Option A's outright misattribution of non-competitive inhibitor effects - This statement is considered **TRUE** in the context of this question **Option C: pH change decreases Vmax without necessarily changing Km** - pH affects ionizable groups in the active site, altering catalytic efficiency → **Vmax decreases** - Km may or may not change depending on whether substrate-binding residues are affected - This is a **TRUE** statement (Lehninger Principles of Biochemistry, 8e, Ch. 6) **Option D: Competitive inhibitor increases apparent Km without changing Vmax** - Competitive inhibitors compete with substrate at the active site → apparent Km ↑ - At very high [S], inhibitor is outcompeted → Vmax is unchanged - This is a **TRUE** statement ### Summary Table | Factor | Vmax | Km | |--------|------|----| | Competitive inhibitor | Unchanged | ↑ (apparent) | | Non-competitive inhibitor | ↓ | **Unchanged** | | Uncompetitive inhibitor | ↓ | ↓ | | Temperature ↑ (below denaturation) | ↑ | Relatively unchanged | | pH change from optimum | ↓ | Variable | **Key Point:** The classic error is confusing non-competitive inhibition (Km unchanged) with uncompetitive inhibition (Km decreased). Option A incorrectly states that non-competitive inhibitors increase Km, making it the FALSE statement. [cite: Lehninger Principles of Biochemistry 8e Ch 6; Stryer Biochemistry 9e Ch 8]
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