Nicotinamide adenine dinucleotide is a coenzyme found in all living cells. The compound is a dinucleotide, because it consists of two nucleotides joined through their phosphate groups. In metabolism, nicotinamide adenine dinucleotide is involved in redox reactions, carrying electrons from one reaction to another. In an alternative fashion, more complex components of the coenzymes are taken up from food as niacin.
Similar compounds are released by reactions that break down the structure of NAD. These preformed components then pass through a salvage pathway that recycles them back into the active form. Although NAD+ is written with a superscript plus sign because of the formal charge on a particular nitrogen atom, at physiological pH for the most part it is actually a singly charged anion , while NADH is a doubly charged anion.
Further information: Redox
Nicotinamide adenine dinucleotide, like all dinucleotides, consists of two nucleosides joined by a pair of bridging phosphate groups. The nucleosides each contain a ribose ring, one with adenine attached to the first carbon atom and the other with nicotinamide at this position. The nicotinamide moiety can be attached in two orientations to this anomeric carbon atom.These nucleotides are joined together by a bridge of two phosphate groups through the 5′ carbons.
The proton is released into solution, while the reductant RH2 is oxidized and NAD+ reduced to NADH by transfer of the hydride to the nicotinamide ring.
From the hydride electron pair, one electron is transferred to the positively charged nitrogen of the nicotinamide ring of NAD+, and the second hydrogen atom transferred to the C4 carbon atom opposite this nitrogen. The midpoint potential of the NAD+/NADH redox pair is −0.32 volts, which makes NADH a strong reducing agent. This means the coenzyme can continuously cycle between the NAD+ and NADH forms without being consumed. In appearance, all forms of this coenzyme are white amorphous powders that are hygroscopic and highly water-soluble.
The solids are stable if stored dry and in the dark. Solutions of NAD+ are colorless and stable for about a week at 4 °C and neutral pH, but decompose rapidly in acids or alkalis. NADH in solution has an emission peak at 460 nm and a fluorescence lifetime of 0.4 nanoseconds, while the oxidized form of the coenzyme does not fluoresce. The properties of the fluorescence signal changes when NADH binds to proteins, so these changes can be used to measure dissociation constants, which are useful in the study of enzyme kinetics.
These changes in fluorescence are also used to measure changes in the redox state of living cells, through fluorescence microscopy«.