A competitive inhibitor of an enzyme

Options

A. Decreases the km
B. Decreases the `V_("max")`
C. Decreases both the Km & `V_("max")`
D. Increases the km

Answer

To solve the question regarding the characteristics of a competitive inhibitor of an enzyme, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Competitive Inhibition**:
- A competitive inhibitor is a molecule that resembles the substrate of an enzyme and competes for binding at the active site of the enzyme. This means that the inhibitor can occupy the active site, preventing the substrate from binding.

**Hint**: Remember that competitive inhibitors mimic the substrate's structure.

2. **Effect on Enzyme Activity**:
- When a competitive inhibitor binds to the enzyme, it prevents the substrate from binding, which in turn inhibits the enzyme's activity. However, this inhibition can be overcome by increasing the concentration of the substrate.

**Hint**: Think about how increasing substrate concentration can outcompete the inhibitor.

3. **Analyzing Kinetic Parameters**:
- In terms of enzyme kinetics, the presence of a competitive inhibitor does not change the maximum velocity (Vmax) of the reaction, but it does increase the Michaelis constant (Km). This is because a higher concentration of substrate is required to reach half of Vmax when a competitive inhibitor is present.

**Hint**: Vmax remains constant, but Km increases in the presence of a competitive inhibitor.

4. **Graphical Representation**:
- When plotting the reaction rate against substrate concentration, the graph of a competitive inhibitor will show that while the Vmax remains the same, the Km will shift to a higher value. This indicates that more substrate is needed to achieve the same reaction rate.

**Hint**: Visualize the graph where the line for the competitive inhibitor is shifted to the right, indicating an increase in Km.

5. **Evaluating Options**:
- Based on the understanding of competitive inhibition:
- **Decrease Km**: Incorrect, as Km increases.
- **Decrease Vmax**: Incorrect, as Vmax remains the same.
- **Decrease both Km and Vmax**: Incorrect, as neither decreases.
- **Increase Km**: Correct, as we established that Km increases in the presence of a competitive inhibitor.

**Hint**: Eliminate options based on the effects of competitive inhibition on Km and Vmax.

### Conclusion:
The correct answer is that a competitive inhibitor of an enzyme **increases the Km value**.