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 `K_m` of the enzyme for the substrate
C. A competitive inhibitor reacts 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 about competitive inhibitors is incorrect, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement A**: "The competitive inhibitor does not affect the rate of breakdown of the enzyme-substrate complex."
- In competitive inhibition, the inhibitor competes with the substrate for the active site of the enzyme. However, once the substrate binds to the enzyme, the rate of breakdown of the enzyme-substrate complex remains unchanged. Therefore, this statement is **correct**.

2. **Statement B**: "The presence of competitive inhibitor decreases the Km of the enzyme for the substrate."
- The Km (Michaelis constant) is a measure of the affinity of the enzyme for its substrate. In the presence of a competitive inhibitor, the effective concentration of substrate required to reach half of the maximum reaction velocity (Vmax) increases, which means that the Km actually **increases**. Thus, this statement is **incorrect**.

3. **Statement C**: "The competitive inhibitor reacts with the enzyme to form an enzyme-inhibitor complex."
- Competitive inhibitors bind to the active site of the enzyme, forming an enzyme-inhibitor complex. This statement is **correct**.

4. **Statement D**: "In competitive inhibition, the inhibitor molecule is not chemically changed by the enzyme."
- In competitive inhibition, the inhibitor does not undergo any chemical change when it binds to the enzyme. It can dissociate from the enzyme without being altered. This statement is **correct**.

### Conclusion:
The incorrect statement among the options provided is **Statement B**: "The presence of competitive inhibitor decreases the Km of the enzyme for the substrate."

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