= 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.<Pyridoxal 5'-phosphate synthase in IUBMB Enzyme Nomenclature>[accepted name, reactions 1–2 and Comments]{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.<Pyridoxal 5'-phosphate synthase in IUBMB Enzyme Nomenclature>[reactions 1 and 2]{ref}
This difference supplies an analytical example. If an ideal mixture undergoes complete conversion of $2\,\mathrm{\mu mol}$ of each substrate, it produces $4\,\mathrm{\mu mol}$ pyridoxal phosphate and $4\,\mathrm{\mu mol}$ peroxide, but only $2\,\mathrm{\mu mol}$ 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.
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