MCAT Chemistry Question #4: Answer and Rationale

4. In Michaelis-Menten enzyme kinetics, what is the velocity of the reaction when the substrate concentration is equal to KM?

A) ¼Vmax

B) ½Vmax

C) Vmax

D) 2Vmax

Answer Analysis

The correct answer is B.

Rationale: This question involves principles of chemical thermodynamics and kinetics. You must recognize the relationship between two variables in the context of an experiment. The Michaelis-Menten model relates reaction velocity to substrate concentration for a system where a substrate (S) binds reversibly to an enzyme (E) to form an enzyme-substrate complex (ES). The enzyme-substrate complex reacts further, and irreversibly, to generate a product (P) and to release the enzyme unaltered. In summary: E + S ⇆ ES  P + E

The Michaelis-Menten equation for this system is:

Let’s break down the variables within this equation:

• Vmax represents the maximum velocity achieved by the system, which occurs at saturating substrate concentrations.
• KM, the Michaelis constant, is the substrate concentration at which the reaction velocity is half of the Vmax.
• Square brackets represent concentration of a substance, therefore [S] refers to the concentration of the substrate.

The question states that the substrate concentration, or [S], is equal to KM. When [S] = KM, v = Vmax/2. Thus, when KM is equal to the substrate concentration, the reaction rate is half its maximal value, or ½Vmax.

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