AAMC FL Practice Exam

What defines the Km of an enzyme?

1. The substrate concentration at half Vmax

2. The total enzyme concentration

3. The substrate concentration at full Vmax

4. The pH at which the enzyme operates best

The correct answer is: The substrate concentration at half Vmax

The Km, or Michaelis constant, is specifically defined as the substrate concentration at which the reaction rate of an enzyme-catalyzed process is half of its maximum velocity (Vmax). This value is crucial because it provides insight into the affinity of the enzyme for its substrate; a lower Km indicates higher affinity, meaning that the enzyme can achieve half of its maximum activity at a lower substrate concentration. In contrast, a higher Km suggests a lower affinity, requiring a higher substrate concentration to reach the same level of activity. Understanding Km is essential for characterizing the kinetics of enzyme reactions and helps in comparing how different enzymes perform or how they might respond to varying concentrations of substrates. The other choices provided do not accurately reflect what Km represents or relate to the enzyme kinetics in a standard manner.