In glycolysis and cellular energy metabolism, which shuttle transfers electrons to Coenzyme Q?

• A. Glycerol phosphate shuttle
• B. Malate-aspartate shuttle
• C. Both shuttles
• D. Neither shuttle

Answer: (A)

Cytosolic reducing equivalents from glycolysis must be moved into the mitochondria to join the respiratory chain. The glycerol phosphate shuttle does exactly that by transferring electrons to Coenzyme Q. In this shuttle, cytosolic glycerol-3-phosphate dehydrogenase uses NADH to reduce DHAP to glycerol-3-phosphate. In the inner mitochondrial membrane, mitochondrial glycerol-3-phosphate dehydrogenase then oxidizes glycerol-3-phosphate back to DHAP and passes the electrons to FAD, forming FADH2, which reduces Coenzyme Q to ubiquinol. So electrons enter the electron transport chain at CoQ.

In contrast, the malate-aspartate shuttle delivers electrons to NAD+, producing NADH in the mitochondrial matrix, where those electrons feed into the chain at Complex I rather than CoQ. Therefore, the shuttle that transfers electrons to Coenzyme Q is the glycerol phosphate shuttle.