<Tr> <Td_colspan="2"> Infobox references </Td> </Tr> <Tr> <Td> </Td> <Td> </Td> </Tr> <P> Nicotinamide adenine dinucleotide (NAD) is a coenzyme found in all living cells . The compound is a dinucleotide, because it consists of two nucleotides joined through their phosphate groups . One nucleotide contains an adenine base and the other nicotinamide . Nicotinamide adenine dinucleotide exists in two forms: an oxidized and reduced form abbreviated as NAD and NADH respectively . </P> <P> In metabolism, nicotinamide adenine dinucleotide is involved in redox reactions, carrying electrons from one reaction to another . The coenzyme is, therefore, found in two forms in cells: NAD is an oxidizing agent--it accepts electrons from other molecules and becomes reduced . This reaction forms NADH, which can then be used as a reducing agent to donate electrons . These electron transfer reactions are the main function of NAD . However, it is also used in other cellular processes, most notably a substrate of enzymes that add or remove chemical groups from proteins, in posttranslational modifications . Because of the importance of these functions, the enzymes involved in NAD metabolism are targets for drug discovery . </P>

The reaction nadh → nad+ is an example of