Gap2 has a major role in carbon fixation as a component of the Calvin cycle. Catalyzes the oxidative phosphorylation of glyceraldehyde 3-phosphate (G3P) to 1,3-bisphosphoglycerate (BPG) using the cofactor NADP. The first reaction step involves the formation of a hemiacetal intermediate between G3P and a cysteine residue, and this hemiacetal intermediate is then oxidized to a thioester, with concomitant reduction of NADP to NADPH. The reduced NADPH is then exchanged with the second NAD, and the thioester is attacked by a nucleophilic inorganic phosphate to produce BPG. D-glyceraldehyde 3-phosphate + NADP(+) + phosphate = (2R)-3-phospho-glyceroyl phosphate + H(+) + NADPH D-glyceraldehyde 3-phosphate + NAD(+) + phosphate = (2R)-3-phospho-glyceroyl phosphate + H(+) + NADH Carbohydrate biosynthesis; Calvin cycle. Homotetramer. Expressed in vegetative cells and heterocysts, however the expression is higher in vegetative cells than in heterocysts. Belongs to the glyceraldehyde-3-phosphate dehydrogenase family.