Question

Why does oxidation of FADH₂ provide less energy for the synthesis of ATP than oxidation of NADH in oxidative phosphorylation?

Answer 1

ATP yield varies slightly depending on the type of shuttle used to transport electrons from the cytosol into the mitochondrion.The mitochondrial inner membrane is impermeable to NADH, so NADH in the cytosol is segregated from the machinery of oxidative phosphorylation. The 2 electrons of NADH captured in glycolysis must be conveyed into the mitochondrion by one of several electron shuttle systems. Depending on the kind of shuttle in a particular cell type, the electrons are passed either to NAD+ or to FAD in the mitochondria matrix. If the electrons are passed to FAD, as in brain cells, only about 1.5 ATP can result from each NADH that was originally generated in the cytosol. If the electrons are passed to mitochondrial NAD+, as in liver cells and heart cells, the yield is about 2.5 ATP per NADH.A single molecule of NADH generates enough proton-motive force for the synthesis of 2.5 ATP. The citric acid cycle also supplies electrons to the electron transport chain via FADH2, but since its electrons enter later in the chain, each molecule of this electron carrier is responsible for transport of only enough H+ for the synthesis of 1.5 ATP. So, oxidation of FADH2 provide less energy for the synthesis of ATP than oxidation of NADH in oxidative phosphorylation.