Lower Km Means Higher Enzyme-Substrate Affinity

Steps:

• Km is the substrate concentration at half Vmax in Michaelis-Menten kinetics.
• A lower Km value indicates the enzyme reaches half Vmax at lower substrate levels.
• This reflects stronger binding between enzyme and substrate.
• Thus, the modified enzyme binds substrate more tightly, matching choice B.

Why B is correct:

• Km inversely measures affinity; lower Km means higher affinity per the Michaelis-Menten equation, where Km = (k_off + k_cat)/k_on.

Why the others are wrong:

• A: Specificity relates to k_cat/Km ratio, not Km alone.
• C: Vmax depends on enzyme concentration and turnover, unaffected by Km.
• D: Lower Km means saturation at lower substrate, but this restates the effect without defining the cause.

Final answer: B