The nicotinamide adenine dinucleotide participates in redox reactions as it brings electrons from one reaction to the next. The coenzyme is therefore found in two forms in cells: NAD is an oxidizing agent, accepting electrons from other molecules and becoming 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, notably as 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.
Because of the positive charge on the nitrogen atom in the nicotinamide ring (upper right), the oxidized forms of these important redox reagents are often depicted as NAD+ and NADP+ respectively. In cells, most oxidations are accomplished by the removal of hydrogen atoms. Both of these coenzymes play crucial roles in this. Each molecule of NAD+ (or NADP+) can acquire two electrons; that is, be reduced by two electrons. However, only one proton accompanies the reduction. The other proton produced as two hydrogen atoms are removed from the molecule being oxidized is liberated into the surrounding medium. For NAD, the reaction is thus: NAD+ + 2H -> NADH + H+