Question

The number of substrate level phosphorylations in one turn of citric acid cycle is :

• A. Zero
• B. One
• C. Two
• D. Three

Answer 1

The Correct Option is B

The citric acid cycle, also known as the Krebs cycle, is a crucial metabolic pathway that occurs in the mitochondria of eukaryotic cells. It is responsible for the oxidation of acetyl-CoA to carbon dioxide and water, generating high-energy electron carriers and a small amount of ATP, which is formed via substrate-level phosphorylation. 

Let us evaluate the question regarding substrate-level phosphorylation during one turn of the citric acid cycle:

1. Substrate-level phosphorylation is a type of chemical reaction that directly forms ATP (or GTP) by transferring a phosphate group from a substrate to ADP (or GDP). This differs from oxidative phosphorylation, where ATP is formed as a result of electron transfer through the electron transport chain.
2. In the citric acid cycle, only one such reaction occurs which involves substrate-level phosphorylation:
   1. In the step where succinyl-CoA is converted to succinate, the enzyme succinyl-CoA synthetase catalyzes the conversion. During this reaction, a phosphate group is transferred to GDP, forming GTP (which can readily be converted to ATP).
3. This reaction accounts for one substrate-level phosphorylation in the entire cycle.

Therefore, in each turn of the citric acid cycle, there is one substrate-level phosphorylation event.

Conclusion: The correct answer is "One". The options "Zero", "Two", and "Three" are incorrect because they do not match the specific number of substrate-level phosphorylations that occur in one complete turn of the citric acid cycle.