Which of the following biochemical reaction is associated with substrate level phosphorylation?
**Core Concept**
Substrate level phosphorylation is a mechanism of generating ATP in cellular respiration where a phosphate group is directly transferred from a high-energy compound to ADP, resulting in the formation of ATP. This process occurs in conjunction with the electron transport chain and is an essential component of cellular energy metabolism.
**Why the Correct Answer is Right**
In the citric acid cycle (also known as the Krebs cycle or tricarboxylic acid cycle), succinyl CoA is converted to succinate by the enzyme succinyl-CoA synthetase (also known as succinic thiokinase). This reaction is a substrate-level phosphorylation reaction, where the energy released from the conversion of succinyl CoA to succinate is used to phosphorylate GDP to GTP, which in turn can be converted to ATP through substrate-level phosphorylation. This reaction is critical for the generation of ATP in cells, particularly in the mitochondria.
**Why Each Wrong Option is Incorrect**
**Option A:** Fumarate to malate is a step in the citric acid cycle catalyzed by the enzyme fumarase, but it does not involve substrate-level phosphorylation. This reaction involves the conversion of fumarate to malate through a non-phosphorylating reaction.
**Option B:** Succinate to fumarate is also a step in the citric acid cycle, catalyzed by the enzyme succinate dehydrogenase. This reaction involves the transfer of electrons from succinate to fumarate but does not involve substrate-level phosphorylation.
**Option D:** Acetoacetate to α-ketoglutarate is not a step in the citric acid cycle. Acetoacetate is a ketone body that can be converted to acetyl-CoA, but this conversion does not involve substrate-level phosphorylation.
**Clinical Pearl / High-Yield Fact**
It is essential to remember that substrate-level phosphorylation occurs in conjunction with the electron transport chain, and the energy released from the conversion of succinyl CoA to succinate is critical for the generation of ATP in cells.
**✓ Correct Answer: C. Succinyl CoA to Succinate**