NAD⁺ / NADH

NAD⁺ and NADH are the oxidized and reduced forms of nicotinamide adenine dinucleotide, a coenzyme that carries electrons in cellular oxidation-reduction reactions. NAD consists of two nucleotide units, one containing adenine and the other nicotinamide. Its nicotinamide portion undergoes the chemical change that allows the coenzyme to alternate between the NAD⁺ and NADH forms .^1,2

During many catabolic reactions, NAD⁺ acts as an electron acceptor. It accepts a hydride ion, equivalent to two electrons and one proton, and is reduced to NADH. The molecule donating the hydride is oxidized in the same redox reaction. NADH can later donate reducing equivalents in another reaction and return to the oxidized NAD⁺ form, allowing the coenzyme to be repeatedly recycled .^1,2

NAD⁺/NADH is particularly important in energy metabolism. Glycolysis, conversion of pyruvate to acetyl-CoA, the citric acid cycle, and other oxidative pathways generate NADH as fuels are oxidized. Under aerobic conditions, NADH in the mitochondrial matrix can transfer electrons to complex I of the respiratory chain. Their subsequent passage through the electron-transport chain contributes to formation of the proton gradient that drives ATP synthesis .^2,3

NAD⁺/NADH is closely related to, but functionally distinct from, NADP⁺/NADPH. NAD⁺ and NADH are used extensively in energy-yielding oxidative metabolism, whereas NADPH commonly supplies reducing power for biosynthetic and other reductive reactions. The two pairs are chemically similar but participate in differently regulated cellular reaction networks .²