Lowest Km indicates highest enzyme-substrate affinity

Steps:

• Km is the Michaelis constant, the substrate concentration giving half maximal velocity; lower Km means stronger binding and higher affinity.
• Identify Km values: A (substrate 1): 4 × 10^{-6} M; B (substrate 2): 2.1 × 10^{-6} M; C (substrate 3): 2 × 10^{-6} M; D (substrate 4): 1 × 10^{-6} M.
• Compare values to find the minimum Km, which is 1 × 10^{-6} M for substrate 4.

Why D is correct:

• Substrate 4 has Km = 1 × 10^{-6} M, the lowest value, indicating highest affinity by the inverse relationship in Michaelis-Menten kinetics.

Why the others are wrong:

• A: Km = 4 × 10^{-6} M exceeds D's, showing weaker binding.
• B: Km = 2.1 × 10^{-6} M exceeds D's, showing weaker binding.
• C: Km = 2 × 10^{-6} M exceeds D's, showing weaker binding.

Final answer: D