The first reaction of kreb's cycle i.e. condensation of actyl group with OAA and water is catalysed by -

Options

A. Citrate synthetase
B. Succinate dehydrogenase
C. RuBisCo
D. PEPCase

Answer

**Step-by-Step Solution:**

1. **Understanding the Krebs Cycle**: The Krebs cycle, also known as the citric acid cycle, is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetate derived from carbohydrates, fats, and proteins.

2. **Identifying the First Reaction**: The first reaction of the Krebs cycle involves the condensation of an acetyl group (from acetyl CoA) with oxaloacetic acid (OAA) and water.

3. **Components of the Reaction**:
- **Acetyl CoA**: A 2-carbon compound that provides the acetyl group.
- **Oxaloacetic Acid (OAA)**: A 4-carbon compound that combines with acetyl CoA.
- **Water**: Participates in the reaction.

4. **Formation of Citric Acid**: When acetyl CoA combines with oxaloacetic acid, a 6-carbon compound called citric acid is formed. This is the first stable product of the Krebs cycle.

5. **Identifying the Enzyme**: The enzyme that catalyzes this reaction (the condensation of acetyl CoA and OAA) is known as citrate synthase.

6. **Conclusion**: Therefore, the enzyme that catalyzes the first reaction of the Krebs cycle is citrate synthase.

**Final Answer**: The first reaction of the Krebs cycle is catalyzed by citrate synthase.

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