Which one of the following statements is incorrect?

Options

A. The competitive inhibitor does not affect the rate of breakdown of the enzyme-substrate complex
B. The presence of the competitive inhibitor decreases the Km of the enzyme for the substrate
C. A competitive inhibitor reacts reversibly with the enzyme to form an enzyme-inhibitor complex
D. In competitive inhibition, the inhibitor molecule is not chemically changed by the enzyme

Answer

To determine which statement is incorrect regarding competitive inhibition, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Competitive Inhibition**:
- Competitive inhibition occurs when an inhibitor competes with the substrate for binding to the active site of an enzyme. The inhibitor has a similar structure to the substrate but is chemically different.

2. **Identify the Role of the Inhibitor**:
- When the inhibitor binds to the active site, it forms an enzyme-inhibitor complex (EI), preventing the substrate from binding and thus inhibiting product formation.

3. **Analyze the KM Value**:
- The KM value (Michaelis constant) is inversely proportional to the affinity between the enzyme and the substrate. If a competitive inhibitor is present, the affinity decreases, leading to an increase in the KM value.

4. **Evaluate Each Statement**:
- **Statement 1**: "The competitive inhibitor does not affect the rate of breakdown of the enzyme-substrate complex."
- This statement is true because the inhibitor does not affect the breakdown of the ES complex once it is formed.

- **Statement 2**: "The presence of competitive inhibitor decreases the KM value."
- This statement is false. The presence of a competitive inhibitor increases the KM value, not decreases it.

- **Statement 3**: "A competitive inhibitor reacts reversibly."
- This statement is true. Competitive inhibitors bind temporarily and can dissociate from the enzyme.

- **Statement 4**: "In competitive inhibition, the inhibitor molecule is not chemically changed."
- This statement is true. The inhibitor does not undergo any chemical change during the inhibition process.

5. **Conclusion**:
- The incorrect statement is **Statement 2**, which claims that the presence of a competitive inhibitor decreases the KM value.

### Final Answer:
The incorrect statement is: "The presence of competitive inhibitor decreases the KM value."

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