Which one of the following statements is incorrect ?

Options

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

Answer

To determine which statement is incorrect regarding competitive inhibition, let's analyze each statement step by step.

### Step-by-Step Solution:

1. **Understanding Competitive Inhibition**:
- Competitive inhibition occurs when an inhibitor molecule competes with the substrate for binding to the active site of the enzyme. When the inhibitor binds to the active site, it prevents the substrate from binding, thus inhibiting the enzyme's activity.

2. **Analyzing the First Statement**:
- The first statement claims that "in competitive inhibition, the inhibitor molecule is not chemically changed by the enzyme."
- This statement is **true**. In competitive inhibition, the inhibitor does not undergo any chemical change; it simply occupies the active site.

3. **Analyzing the Second Statement**:
- The second statement states that "the competitive inhibitor does not affect the rate of breakdown of the enzyme-substrate complex."
- This statement is **incorrect**. The presence of a competitive inhibitor does affect the rate of breakdown because it prevents the substrate from binding to the enzyme, thus reducing the formation of the enzyme-substrate complex.

4. **Analyzing the Third Statement**:
- The third statement mentions that "the presence of a competitive inhibitor decreases the Km of the enzyme for the substrate."
- This statement is **true**. In competitive inhibition, the Km (Michaelis constant) increases, indicating that a higher concentration of substrate is needed to reach half of the maximum reaction velocity (Vmax). Therefore, this statement is misleading as it implies a decrease in Km, which is incorrect.

5. **Analyzing the Fourth Statement**:
- The fourth statement says, "the competitive inhibitor reacts reversibly with the enzyme to form an enzyme-inhibitor complex."
- This statement is **true**. Competitive inhibitors typically bind reversibly to the enzyme, forming a complex that can dissociate, allowing the substrate to bind again.

### Conclusion:
Based on the analysis, the **incorrect statement** is the second one: "the competitive inhibitor does not affect the rate of breakdown of the enzyme-substrate complex."

### Final Answer:
The incorrect statement is option number 2.

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