In citric acid cycle, the step which is not mediated by dehydrogenase enzyme is :

Options

A. Oxaloacetic acid to citric acid
B. Citric acid to `alpha` - ketoglutaric acid
C. Succinic acid to fumaric acid
D. Malic acid to oxaloacetic acid

Answer

To determine which step in the citric acid cycle is not mediated by a dehydrogenase enzyme, we will analyze each step of the cycle based on the provided information.

**Step 1: Identify the steps of the citric acid cycle.**
- The citric acid cycle (also known as the TCA cycle or Krebs cycle) consists of several key steps where different enzymes catalyze the conversion of substrates.

**Step 2: Analyze the options given in the question.**
1. **Oxaloacetic acid to citric acid**: This step involves the condensation of acetyl-CoA with oxaloacetate to form citric acid (citrate). The enzyme responsible for this reaction is citrate synthase, which is not a dehydrogenase.

2. **Citric acid to alpha-ketoglutaric acid**: This step is catalyzed by isocitrate dehydrogenase, which is a dehydrogenase enzyme.

3. **Succinic acid to fumaric acid**: This step is catalyzed by succinate dehydrogenase, which is also a dehydrogenase enzyme.

4. **Malic acid to oxaloacetic acid**: This step is catalyzed by malate dehydrogenase, which is again a dehydrogenase enzyme.

**Step 3: Conclusion**
From the analysis, we can conclude that the step **oxaloacetic acid to citric acid** is the only step that is not mediated by a dehydrogenase enzyme; instead, it is mediated by citrate synthase.

**Final Answer:** The step in the citric acid cycle that is not mediated by a dehydrogenase enzyme is **oxaloacetic acid to citric acid**.

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