GAPDS is a 40-44kD member of the GAPDH family of enzymes. Due to a high incidence of proline residues, it may migrate anomalously at 56kD in SDS-PAGE. GAPDH-2 is expressed only in testis as a homotetramer, where it is found in the cytoplasm bound to the sperm fibrous sheath. Unlike GAPDH that has multiple functions, GAPDH-2 appears to participate only in carbohydrate metabolism, generating energy via glycolysis for sperm motility and fertility. Human GAPDH-2 is 408aa in length. There are two NAD binding sites (Asp106 and Asn388) with a catalytic region between aa223-226. One splice variant shows a 9aa insertion after Ser220. Over aa75-222, human GAPDH-2 shares 84% aa identity with mouse GAPDH-2.