Presence of competitive inhibitor

Options

A. Increases km
B. Decrease km
C. 1st increase then decrease km
D. 1st decreases then increases km

Answer

### Step-by-Step Solution:

1. **Understanding Km**:
- Km, or the Michaelis constant, is a measure of the substrate concentration required for an enzyme to reach half of its maximum velocity (Vmax). It indicates the affinity of the enzyme for its substrate; a lower Km means higher affinity, while a higher Km means lower affinity.

2. **Effect of Competitive Inhibitors**:
- Competitive inhibitors are molecules that bind to the active site of an enzyme, competing with the substrate for binding. This means that in the presence of a competitive inhibitor, more substrate is required to achieve the same level of enzyme activity.

3. **Impact on Km**:
- When a competitive inhibitor is present, the effective concentration of substrate needed to reach half Vmax increases. This is because the inhibitor reduces the likelihood of the substrate binding to the enzyme, thus requiring a higher concentration of substrate to achieve the same reaction rate.

4. **Conclusion**:
- Therefore, the presence of a competitive inhibitor increases the Km value. The correct answer to the question is that the presence of a competitive inhibitor increases Km.

### Final Answer:
The presence of a competitive inhibitor increases Km.

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