Which of the following statements about Michaelis-Menten kinetics is correct?

Options

A. `K_(m)`, the Michaelis constant, is defined as the concentration of substrate required for the reaction to reach maximum velocity.
B. `K_(m)`, the Michaelis constant, is defined as the dissociation constant of the enzyme-substrate complex.
C. `K_(m)`, the Michaelis constant, is expressed in terms of the reaction velocity.
D. `K_(m)`, the Michaelis constant, is a measure of the affinity the enzyme has for its substrate.

Answer

**Step-by-Step Solution:**

1. **Understanding Michaelis-Menten Kinetics**: Michaelis-Menten kinetics describes the rate of enzymatic reactions by relating reaction rate to substrate concentration. It is characterized by the Michaelis constant (KM), which is a key parameter in this model.

2. **Defining the Michaelis Constant (KM)**: The Michaelis constant (KM) is defined as the substrate concentration at which the reaction velocity is half of its maximum velocity (Vmax). This means that KM is not the concentration required to reach maximum velocity but rather the concentration at which the reaction rate is half-maximal.

3. **Evaluating the Statements**:
- **Statement 1**: "KM is defined as the concentration of substrate required for the reaction to reach maximum velocity." - This statement is incorrect because KM is not the concentration at Vmax; it is the concentration at half Vmax.
- **Statement 2**: "KM is defined as the dissociation constant of the enzyme-substrate complex." - This statement is also incorrect. KM is related to the formation of the enzyme-substrate complex but is not the dissociation constant.
- **Statement 3**: "KM is expressed in terms of reaction velocity." - This statement is incorrect. KM is expressed in terms of substrate concentration, not reaction velocity.
- **Statement 4**: "KM is a measure of affinity." - This statement is correct. A lower KM value indicates a higher affinity between the enzyme and substrate, meaning the enzyme binds the substrate more readily.

4. **Conclusion**: The correct statement regarding Michaelis-Menten kinetics is that KM is a measure of the affinity of the enzyme for its substrate. Higher KM values indicate a lower affinity, while lower KM values indicate a higher affinity.