A class of organic compounds whose molecular structure comprises a nitrogen-containing base linked to a sugar and a phosphate group. The nucleotide of interest for fish technology is adenosine triphosphate (ATP) which is central to many metabolic processes in cells including the reactions that result in contraction of muscles resulting in movement. On the death of the fish, ATP is degraded by a series of reactions (see K-value), and the first step, loss of ATP to adenosine diphosphate, is linked to the onset of rigour mortis.