What does Km represent in the Michaelis-Menten equation?

Km, or the Michaelis constant, is a crucial parameter in the Michaelis-Menten equation, which describes the kinetics of enzyme-catalyzed reactions. This constant specifically represents the substrate concentration at which the reaction rate is half of its maximum velocity, or Vmax. This means that when the substrate concentration equals Km, the reaction is occurring at a rate that is 50% of its maximum speed.

Km provides insight into the affinity of the enzyme for its substrate; a low Km value indicates high affinity, meaning that the enzyme can achieve half of its maximum activity at a low substrate concentration. Conversely, a high Km suggests a lower affinity for the substrate. This is essential for understanding enzyme behavior in various biochemical contexts, as it can influence how enzymes perform under different physiological conditions.

In this case, the other options do not accurately describe Km. The first option incorrectly associates Km with Vmax, while the concentration of the enzyme is irrelevant to Km, making the third option inaccurate. Lastly, the maximum rate of reaction itself is represented by Vmax rather than Km. Thus, focusing on the concept that Km is the substrate concentration at which the reaction rate is 50% of Vmax clearly identifies why the second answer is the most accurate representation of