Which one of the following statements in incorrect?

Options

A. The competitive inhibitor does not affect the rate of breakdown of he enzyme substrate complex
B. The presence of the competitive inhibitor decreases the km 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 chemcially changed by the enzyme

Answer

To determine which statement is incorrect regarding competitive inhibition, we will analyze each option step by step.

### Step 1: Analyze Statement A
**Statement A:** Competitive inhibitor does not affect the rate of breakdown of enzyme-substrate complex.

- **Explanation:** This statement is true. A competitive inhibitor competes with the substrate for the active site of the enzyme. However, it does not affect the breakdown of the enzyme-substrate complex once it has formed. The rate of breakdown is determined by the concentration of the substrate and the enzyme's catalytic activity.

**Hint:** Remember that competitive inhibitors compete for the active site but do not alter the enzyme's ability to break down the substrate once bound.

### Step 2: Analyze Statement B
**Statement B:** The presence of a competitive inhibitor decreases the Km of the enzyme for the substrate.

- **Explanation:** This statement is incorrect. In the presence of a competitive inhibitor, the Km (Michaelis constant) actually increases. Km is a measure of the affinity of the enzyme for its substrate; a higher Km indicates a lower affinity. Therefore, the presence of a competitive inhibitor makes it harder for the substrate to bind, resulting in an increased Km.

**Hint:** Recall that an increase in Km means a decrease in enzyme affinity for the substrate, especially when a competitive inhibitor is present.

### Step 3: Analyze Statement C
**Statement C:** Competitive inhibitor reacts with enzyme to form an enzyme-inhibitor complex.

- **Explanation:** This statement is true. Competitive inhibitors bind to the active site of the enzyme, forming an enzyme-inhibitor complex, which prevents the substrate from binding.

**Hint:** Focus on the mechanism of competitive inhibition where the inhibitor occupies the active site.

### Step 4: Analyze Statement D
**Statement D:** In competitive inhibition, the inhibitor molecule is not chemically changed by the enzyme.

- **Explanation:** This statement is also true. In competitive inhibition, the inhibitor does not undergo any chemical change when it binds to the enzyme. It simply competes with the substrate for the active site.

**Hint:** Remember that competitive inhibitors do not alter their chemical structure during the inhibition process.

### Conclusion
After analyzing all statements, we find that the incorrect statement is **Statement B**, which claims that the presence of a competitive inhibitor decreases the Km of the enzyme for the substrate.

**Final Answer:** The incorrect statement is **B**.