Km Measures Enzyme-Substrate Affinity

Steps:

• Recall Km definition: substrate concentration [S] where reaction velocity v = ½ Vmax, indicating enzyme-substrate binding affinity.
• Evaluate statement 1: Higher affinity means enzyme binds substrate tightly at lower [S], so Km decreases—correct.
• Evaluate statement 2: Lower affinity means weaker binding, so Km increases (higher [S] needed for ½ Vmax), but "slower Km" is invalid phrasing as Km is a concentration, not a rate—incorrect.
• Evaluate statement 3: By Michaelis-Menten equation, at [S] = Km, enzyme is 50% saturated (half active sites occupied)—correct.

Why C is correct:

• C selects statements 1 and 3, matching Km's definition as [S] for ½ Vmax saturation and inverse relation to affinity.

Why the others are wrong:

• A includes incorrect statement 2.
• B includes incorrect statement 2, excludes correct 3.
• D excludes correct 1, includes incorrect 2.

Final answer: C