= Pyridoxal phosphate
<Pyridoxal phosphate> is the 5′-phosphate derivative of <pyridoxal>, a <Vitamin B6> form whose ring carries a reactive <aldehyde> group. It serves as a <coenzyme> in <amino acid> chemistry. The phosphate-bearing substituent and the <aldehyde> have different roles: identifying one does not explain the reactivity of the other.<Pyridoxal phosphate in ChEBI>[CHEBI:18405; definition, structure and classification]{ref}
An example is <1-aminocyclopropane-1-carboxylate synthase>. In a 1990 experiment, chemical reduction with tritium-labelled reagent trapped the <coenzyme> attached to the <enzyme>. Subsequent <protein> digestion and sequencing localized the labelled active-site segment; acid <hydrolysis> released a modified <lysine> bearing the pyridoxyl group. This provided chemical evidence for the linkage, beyond observing that adding <pyridoxal phosphate> improves activity.<Characterization and sequencing of the active site of 1-aminocyclopropane-1-carboxylate synthase by W. K. Yip and others>[PNAS 87, pp. 7930–7934; Abstract, tritiated borohydride labelling and pyridoxyllysine identification]{ref}
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