Pyridoxal 5'-phosphate synthase

Pyridoxal 5'-phosphate synthase, EC 1.4.3.5, is an enzyme that oxidizes pyridoxamine phosphate or pyridoxine phosphate to pyridoxal phosphate. Despite the accepted synthase name, these reactions start with an already phosphorylated vitamin. The enzyme uses flavin mononucleotide as a cofactor.^[ref]

Both reactions consume oxygen and produce hydrogen peroxide. The pyridoxamine-phosphate reaction additionally consumes water and releases Ammonia, because its aminomethyl group becomes an aldehyde. The pyridoxine-phosphate reaction changes a hydroxymethyl group into an aldehyde without releasing that nitrogen-containing coproduct.^[ref]

This difference supplies an analytical example. If an ideal mixture undergoes complete conversion of $2\mu \mathrm{mol}$ of each substrate, it produces $4\mu \mathrm{mol}$ pyridoxal phosphate and $4\mu \mathrm{mol}$ peroxide, but only $2\mu \mathrm{mol}$ ammonia. A peroxide-only readout combines contributions from both substrates; the coproduct balance contains additional information.

Those values assume only the stated coupled reactions. In an actual enzyme assay, other peroxide- or ammonia-producing activities can interfere, so chemical product identification and appropriate controls remain important. Enzyme identity, substrate identity and the chosen reporter reaction each determine what the signal means.