<p> This entry contains a small clade of dehydrogenases in gamma-proteobacteria which utilise NAD+ to oxidize erythrose-4-phosphate (E4P) to 4-phospho-erythronate, a precursor for the de novo synthesis of pyridoxine via 4-hydroxythreonine and D-1-deoxyxylulose [<cite idref="PUB00002270"/>]. This enzyme activity appears to have evolved from glyceraldehyde-3-phosphate dehydrogenase, whose substrate differs only in the lack of one carbon relative to E4P. It is possible that some of the GAPDH enzymes may prove to be bifunctional in certain species.</p>