(nicotinamide adenine dinucleotide, coenzyme I) A pyridine-based nucleotide that functions as a coenzyme in many oxidation-reduction reactions, according to the general reaction: NAD+
+ reduced substrate ↔NADH + H+
+ oxidized substrate. An example of such a reaction is the oxidation of citrate to α-ketoglutarate in the TCA cycle.
Reduced NAD can act as a reducing agent or it may be reoxidized in the respiratory chain
with coupled ATP formation. NAD is not usually tightly bound to the enzyme with which it is acting but instead acts as a second substrate, binding only during the reaction and being released into the medium with the reaction products. NAD is sometimes called DPN, an abbreviation for the trivial name diphosphopyridine nucleotide.