Which enzyme is involved in both gluconeogenesis and the citric acid cycle?

• A. Citrate synthase
• B. Pyrvate carboxylase
• C. Aconitase
• D. Isocitrate dehydrogenase

Answer: (B)

Pyruvate carboxylase plays a crucial role in both gluconeogenesis and the citric acid cycle. In gluconeogenesis, pyruvate carboxylase converts pyruvate into oxaloacetate, which is a key step in synthesizing glucose from non-carbohydrate precursors. This reaction occurs in the mitochondria and is the first step in the gluconeogenesis pathway.

In the citric acid cycle, pyruvate carboxylase is involved indirectly by facilitating the replenishment of oxaloacetate, which is necessary for the cycle to continue processing acetyl-CoA into carbon dioxide and high-energy electron carriers. This connection highlights how pyruvate carboxylase links energy production and glucose synthesis, serving critical functions in metabolic pathways.

The other enzymes listed each have distinct roles that do not overlap with both pathways. Citrate synthase is specific to the citric acid cycle, where it catalyzes the first step by combining acetyl-CoA and oxaloacetate to form citrate. Aconitase is also specific to the citric acid cycle, involved in transforming citrate into isocitrate. Isocitrate dehydrogenase catalyzes the decarboxylation of is