Nickel in an essential part of which of the following enzymes:

Options

A. Urease
B. Nitrogenase
C. Nitrate reductase
D. PEP carboxy las

Answer

To determine which enzyme contains nickel as an essential part, we will analyze each of the given options:

1. **Urease**: Urease is an enzyme that catalyzes the hydrolysis of urea into carbon dioxide (CO2) and ammonia (NH3). It is known to contain nickel as a crucial component for its activity. The presence of nickel in urease is essential for the enzyme's function in breaking down urea, which is important for nitrogen metabolism in plants.

2. **Nitrogenase**: Nitrogenase is an enzyme involved in the fixation of atmospheric nitrogen into ammonia. It primarily contains molybdenum or vanadium as its active site metal, not nickel.

3. **Nitrate Reductase**: This enzyme is responsible for the reduction of nitrate to nitrite in the nitrogen assimilation pathway. It typically contains molybdenum or iron, but not nickel.

4. **PEP Carboxylase**: This enzyme catalyzes the carboxylation of phosphoenolpyruvate (PEP) to form oxaloacetate. It is usually associated with magnesium or manganese, but nickel is not a component of this enzyme.

Based on the analysis above, the enzyme that contains nickel as an essential part is **urease**.

### Final Answer:
Nickel is an essential part of the enzyme **urease**.