Which of the following enzymes fixes nitrogen?

Options

A. Nitrate reductase
B. Nitrogenase
C. PEP carboxylase
D. RuBisCo

Answer

To answer the question "Which of the following enzymes fixes nitrogen?", we will analyze each option provided and determine which one is responsible for nitrogen fixation.

### Step-by-Step Solution:

1. **Identify the Options**: The options given are:
- A) Nitrate reductase
- B) Nitrogenase
- C) PEP carboxylase
- D) RuBisCO

2. **Analyze Nitrate Reductase**:
- Nitrate reductase is an enzyme that reduces nitrate (NO3-) to nitrite (NO2-).
- This process is important for the production of proteins in plants, but it does not fix atmospheric nitrogen (N2) into a usable form.
- **Conclusion**: This option is incorrect.

3. **Analyze Nitrogenase**:
- Nitrogenase is an enzyme produced by certain bacteria that is capable of fixing atmospheric nitrogen (N2) into ammonia (NH3).
- This process is essential for making nitrogen available to plants in a form they can use.
- **Conclusion**: This option is correct.

4. **Analyze PEP Carboxylase**:
- PEP carboxylase is an enzyme that catalyzes the formation of oxaloacetic acid from phosphoenolpyruvate and carbon dioxide.
- This enzyme is involved in the process of carbon fixation, not nitrogen fixation.
- **Conclusion**: This option is incorrect.

5. **Analyze RuBisCO**:
- RuBisCO, or ribulose bisphosphate carboxylase, is an enzyme involved in the first step of carbon fixation in photosynthesis.
- It catalyzes the reaction of carbon dioxide with ribulose bisphosphate to form 3-phosphoglycerate.
- Like PEP carboxylase, it does not fix nitrogen.
- **Conclusion**: This option is incorrect.

### Final Answer:
The enzyme that fixes nitrogen is **B) Nitrogenase**.