Km Indicates Enzyme-Substrate Affinity

Steps:

• Km is the substrate concentration [S] at which reaction velocity is half of Vmax.
• Low affinity means weak enzyme-substrate binding, requiring higher [S] to achieve half Vmax, so Km is high.
• Full saturation for Vmax demands even higher [S] when affinity is low.
• Thus, low-affinity enzymes have high Km and reach Vmax at high [S].

Why A is correct:

• Km inversely measures affinity per Michaelis-Menten equation; low affinity yields high Km, needing high [S] to approach Vmax asymptotically.

Why the others are wrong:

• B: Low Km indicates high affinity, reaching Vmax at low [S].
• C: High Km requires high [S] for Vmax, not low.
• D: Low Km and low [S] for Vmax describe high affinity.

Final answer: A