A bifuncitonal enzyme is

Options

A. Phosphoglycerate kinase
B. PEPCO
C. RuBisCO
D. Phosphoglyceraldehyde dehydrogenase

Answer

To answer the question "A bifunctional enzyme is," we will analyze the options given and identify which enzyme fits the description of being bifunctional.

### Step-by-Step Solution:

1. **Understanding Bifunctional Enzymes**:
- A bifunctional enzyme is an enzyme that has two distinct catalytic activities. This means it can catalyze two different reactions or processes.

2. **Evaluating the Options**:
- **Phosphoglycerate Kinase**: This enzyme primarily catalyzes the conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate in glycolysis. It does not have a second distinct function.
- **PEPCO (Phosphoenolpyruvate Carboxylase)**: This enzyme catalyzes the carboxylation of phosphoenolpyruvate to oxaloacetate. It is not bifunctional as it primarily performs one reaction.
- **Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase)**: This enzyme catalyzes two reactions: the carboxylation of ribulose-1,5-bisphosphate (RuBP) to fix carbon dioxide during the Calvin cycle and the oxygenation of RuBP, which leads to photorespiration. Thus, it has two distinct functions.
- **Phosphoglycerate Dehydrogenase**: This enzyme is involved in the conversion of 3-phosphoglycerate to 1,3-bisphosphoglycerate in the Calvin cycle, and it does not exhibit bifunctionality.

3. **Conclusion**:
- Among the options provided, Rubisco is the only enzyme that exhibits bifunctionality by catalyzing both carboxylation and oxygenation reactions. Therefore, the correct answer is **Rubisco**.

### Final Answer:
Rubisco is a bifunctional enzyme.

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