Pyridoxal phosphate participates in transaminations, decarboxylations, racemizations, and numerous modifications of amino acid side chains. All pyridoxal phosphate-requiring enzymes act via the formation of a Schiff base between the amino acid and coenzyme (Figure 20.15). A cation (a metal or a proton) is essential to bridge the phenolate ion of the coenzyme and the imino nitrogen of the amino acid. This bridging maintains the planarity of the structure, which is essential for catalysis. The most important catalytic feature of the coenzyme is the electrophilic nitrogen of the pyridine ring, which acts as an electron sink, drawing electrons away from the amino acid and stabilizing a carbanion intermediate.

All known reactions of PLP-containing enzymes can be described mechanistically in the same way-formation of a planar Schiff base or aldimine intermediate, followed by formation of a resonance-stabilized carbanion with a quinoid structure, as shown in Figure 20.15. Depending on the bond labilized, formation of the aldimine can lead to a transamination (as shown in Figure 20.15), to decarboxylation, to racemization, or to numerous side chain modifications.