Which enzymes are utilized in both glycolysis and gluconeogenesis?

• A. Hexokinase
• B. Aldolase
• C. Pyruvate carboxylase
• D. Two of the above

Answer: (D)

The key idea is that some steps in glycolysis and gluconeogenesis are reversible and can be carried out by the same enzyme, while others are bypassed by different enzymes in gluconeogenesis. Aldolase is the enzyme that serves in both pathways: in glycolysis it splits fructose-1,6-bisphosphate into DHAP and glyceraldehyde-3-phosphate, and in the reverse direction it can catalyze the condensation of DHAP and glyceraldehyde-3-phosphate back to fructose-1,6-bisphosphate, enabling gluconeogenesis to form glucose precursors. Hexokinase starts glycolysis by phosphorylating glucose, but gluconeogenesis bypasses this step and uses glucose-6-phosphatase to produce glucose, so hexokinase isn’t used in gluconeogenesis. Pyruvate carboxylase is a gluconeogenic enzyme that converts pyruvate to oxaloacetate, not used in glycolysis. So, the enzyme common to both pathways is aldolase alone.