redox carriers

Meaning: Oxidizing a fuel molecule in glycolysis, pyruvate dehydrogenase, the citric acid cycle or β-oxidation moves electrons onto an oxidized carrier. NAD⁺ accepts a hydride (two electrons plus one proton) to become NADH. FAD accepts two hydrogen atoms to become FADH₂. NADH and FADH₂ then donate electrons to the electron transport chain, which powers proton pumping and ATP synthesis. NADPH, made largely in the pentose phosphate pathway, mostly serves as the reductant in biosynthesis such as fatty acid synthesis. Cells hold only small pools of carriers, so the oxidized form must be regenerated, for example by the chain with O₂ or by fermentation.

Equation: NAD⁺ + H⁻ → NADH, and FAD + 2H → FADH₂. NAD⁺ and FAD are the oxidized carriers, NADH and FADH₂ the reduced carriers, H⁻ is a hydride (2 e⁻ plus H⁺), and H is a hydrogen atom. Amounts are in mol.

On the MCAT: Expect bookkeeping per glucose: 2 NADH from glycolysis, 2 from pyruvate dehydrogenase, and 6 NADH plus 2 FADH₂ from the citric acid cycle. NADH enters at complex I and FADH₂ at complex II, so FADH₂ yields less ATP. If the chain is inhibited, NADH accumulates, NAD⁺ falls and the citric acid cycle slows.

Common confusion: NADH is the reduced form and the electron donor; NAD⁺ is the oxidized form and the oxidizing agent. Carriers are recycled, not consumed. NADH is mainly catabolic, whereas NADPH is mainly anabolic.