Michaelis-Menten Kinetics: Defining Km

Steps:

• Recall the Michaelis-Menten equation: v = (Vmax [S]) / (Km + [S]), where v is reaction velocity, [S] is substrate concentration.
• Identify that Km represents the dissociation constant of the enzyme-substrate complex.
• Determine the velocity at [S] = Km: v = (Vmax Km) / (Km + Km) = Vmax / 2.
• Conclude Km is the [S] yielding half-maximal velocity.

Why D is correct:

• Km is defined as the substrate concentration ([S]) at which the enzyme's reaction rate reaches half of its maximum velocity (Vmax), per the Michaelis-Menten model.

Why the others are wrong:

• A: Vmax is the maximum reaction velocity, not Km.
• B: No finite [S] gives exactly Vmax; velocity approaches Vmax asymptotically as [S] increases.
• C: Identical to D, but listed separately—likely a redundancy; it matches the definition but is not the selected option.

Final answer: D