Which of the statement is true regarding km?

Options

A. It is the measure of the stability of the ES complex
B. It is the measure of the stability of the affinity of an enzyme for its substrate
C. A high Km indicates week substrate binding
D. All of these

Answer

To solve the question regarding Km (Michaelis constant), we will analyze each option provided and determine which statements are true.

### Step-by-Step Solution:

1. **Understanding Km**:
- Km, or the Michaelis constant, is defined as the substrate concentration at which the reaction velocity is half of its maximum velocity (Vmax). It is a crucial parameter in enzyme kinetics.

2. **Analyzing Option A**:
- "It is a measure of the stability of the ES complex."
- This statement is partially correct. While Km does relate to the stability of the enzyme-substrate (ES) complex, it is more accurately described as a measure of the affinity of the enzyme for its substrate. A lower Km indicates a higher affinity, meaning the enzyme binds the substrate more effectively.

3. **Analyzing Option B**:
- "It is a measure of the affinity of an enzyme for its substrate."
- This statement is correct. Km is inversely related to the affinity of the enzyme for its substrate. A lower Km value indicates a higher affinity, while a higher Km indicates a lower affinity.

4. **Analyzing Option C**:
- "A high Km indicates a weak substrate binding."
- This statement is also correct. A high Km value means that a higher concentration of substrate is needed to reach half of Vmax, which implies weaker binding between the enzyme and substrate.

5. **Analyzing Option D**:
- "All of these."
- Since options A, B, and C have been determined to be correct, this option is also correct.

### Conclusion:
- All the statements regarding Km are true. Therefore, the correct answer is **Option D: all of these**.