NAD (Nicotinamide Adenine Dinucleotide) is a hydrogen acceptor, used as a coenzyme in many reactions especially in respiration to transfer (pairs of) hydrogen atoms to other molecules. Dehydrogenation - the removal of hydrogen - is effectively a form of oxidation. Accompanying this is the acceptance/gain and donation/loss of electrons.
NAD consists of two nucleotides, joined by their phosphate groups . On either side of the central
, attached to other rings (bases) containing nitrogen atoms (blue). On one end of the molecule is a double ring structure - the base
- and at the other end is a slightly simpler single ring structure -
- which is a derivative of vitamin B3.
The oxidised form
of the molecule carries a positive charge (so it may be called NAD+
) due to the arrangement of bonds on one of the
in the nicotinamide section.
The reduced form
of NAD (sometimes called NADH, NADH2
) has accepted an electron [cancelling the positive charge], and a hydrogen atom which can be seen on the nicotinamide section. The other hydrogen remains as an H+
In photosynthesis (and also in some reactions in animal cells), hydrogen is transferred using NADP
- which has an additional phosphate group attached to carbon 2 of the ribose of adenosine.