Which of the following is NOT a key enzyme of the citric acid cycle?

The correct answer is that hexokinase is not a key enzyme of the citric acid cycle. Hexokinase plays a crucial role in the glycolytic pathway, where it catalyzes the phosphorylation of glucose to form glucose-6-phosphate, facilitating its entry into glycolysis. While important for energy metabolism, hexokinase does not directly participate in the reactions of the citric acid cycle.

In contrast, citrate synthase, isocitrate dehydrogenase, and succinyl CoA synthetase are all essential enzymes within the citric acid cycle. Citrate synthase catalyzes the condensation of acetyl-CoA with oxaloacetate to form citrate, thereby initiating the cycle. Isocitrate dehydrogenase plays a significant role in the oxidative decarboxylation of isocitrate to α-ketoglutarate, producing NADH and releasing carbon dioxide. Succinyl CoA synthetase is involved in the conversion of succinyl-CoA to succinate, coupled with the generation of GTP (or ATP), another important step in the cycle. These enzymes are integral to the overall function and progression of the citric acid cycle, which is critical for cellular respiration and energy production in