Which cofactor is produced when succinate is oxidized to fumarate in the Krebs cycle?

Which cofactor is produced when succinate is oxidized to fumarate in the Krebs cycle?

• A. FADH2
• B. NADH
• C. NADPH
• D. CoA

Answer: (A)

The step where succinate becomes fumarate transfers electrons to the flavin adenine dinucleotide bound to the enzyme succinate dehydrogenase, reducing it to FADH2. This enzyme is part of the Krebs cycle and also acts as Complex II in the electron transport chain, so the reduced FADH2 carries electrons into the chain for further energy production. NADH, NADPH, and CoA don’t participate as the immediate electron acceptor in this particular oxidation, though NADH is generated at other steps in the cycle, NADPH is used in biosynthesis elsewhere, and CoA is involved in other reactions (being released from succinyl-CoA earlier).

The step where succinate becomes fumarate transfers electrons to the flavin adenine dinucleotide bound to the enzyme succinate dehydrogenase, reducing it to FADH2. This enzyme is part of the Krebs cycle and also acts as Complex II in the electron transport chain, so the reduced FADH2 carries electrons into the chain for further energy production. NADH, NADPH, and CoA don’t participate as the immediate electron acceptor in this particular oxidation, though NADH is generated at other steps in the cycle, NADPH is used in biosynthesis elsewhere, and CoA is involved in other reactions (being released from succinyl-CoA earlier).