The enzyme not involved in substrate level phosphorylation is:
**Core Concept**
Substrate-level phosphorylation is a type of ATP production in the cell, where a phosphate group is directly transferred from a high-energy compound to ADP, generating ATP. This process occurs in various metabolic pathways, including glycolysis, the citric acid cycle, and the pentose phosphate pathway.
**Why the Correct Answer is Right**
Phosphofructokinase is an enzyme involved in glycolysis, where it catalyzes the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate. However, this reaction is not a substrate-level phosphorylation event. Phosphofructokinase uses ATP to phosphorylate fructose-6-phosphate, but the phosphate group does not come directly from a high-energy compound. Instead, the energy is derived from the binding of fructose-6-phosphate to the enzyme, which facilitates the transfer of the phosphate group from ATP to the sugar molecule. In contrast, enzymes like Succinyl thiokinase, Pyruvate kinase, and Phosphoglycerate kinase are involved in substrate-level phosphorylation reactions, where a phosphate group is directly transferred from a high-energy compound to ADP.
**Why Each Wrong Option is Incorrect**
**Option A:** Succinyl thiokinase is involved in the citric acid cycle and catalyzes the conversion of succinyl-CoA to succinate, generating GTP (which is then converted to ATP) through substrate-level phosphorylation.
**Option C:** Pyruvate kinase is an enzyme in glycolysis that catalyzes the conversion of phosphoenolpyruvate to pyruvate, generating one ATP molecule through substrate-level phosphorylation.
**Option D:** Phosphoglycerate kinase is also an enzyme in glycolysis that catalyzes the conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate, generating one ATP molecule through substrate-level phosphorylation.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that substrate-level phosphorylation occurs in specific metabolic pathways, such as glycolysis, the citric acid cycle, and the pentose phosphate pathway. These pathways involve the direct transfer of a phosphate group from a high-energy compound to ADP, generating ATP.
**✓ Correct Answer: B. Phosphofructokinase**