Km Measures Enzyme-Substrate Affinity and Half-Max Velocity

Steps:

• Recall Km from Michaelis-Menten equation: v = (Vmax [S]) / (Km + [S]), where Km = (k-1 + k2)/k1, dissociation constant analog.
• Evaluate statement 1: Higher Km means lower affinity, as more substrate needed for half Vmax, so false.
• Evaluate statement 2: Km inversely reflects affinity; lower Km indicates tighter binding, so true.
• Evaluate statement 3: By definition, Km is [S] at v = Vmax/2, so true.

Why D is correct:

• D selects statements 2 and 3, matching Km's role as affinity measure and [S] for half-maximal velocity per Michaelis-Menten kinetics.

Why the others are wrong:

• A includes false statement 1 on affinity direction.
• B includes false statement 1.
• C includes false statement 1.

Final answer: D