## **Core Concept**
The Michaelis constant, denoted as Km, is a measure of the affinity of an enzyme for its substrate. It is a key parameter in enzyme kinetics. The unit of Km is directly related to the units of substrate concentration.

## **Why the Correct Answer is Right**
Km (Michaelis constant) is defined as the substrate concentration at which the reaction rate is half of the maximum rate (Vmax/2). Since it represents a substrate concentration, its unit is the same as that of substrate concentration, which is typically measured in moles per liter (M) or millimoles per liter (mM). Therefore, the unit of Km is **mM**.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because Km is not dimensionless; it has units related to concentration.
- **Option B:** This option suggests a unit of 1/mole, which does not make sense for Km as Km is a concentration, not a reciprocal concentration.
- **Option D:** This option suggests the unit is sec^-1, which is incorrect because Km is a measure of concentration, not a rate or time.

## **Clinical Pearl / High-Yield Fact**
A low Km value indicates high affinity of the enzyme for the substrate, meaning the enzyme can effectively bind and act on the substrate at lower concentrations. Conversely, a high Km value indicates low affinity, requiring higher substrate concentrations to achieve the same rate of reaction.

## **Correct Answer:** B. .