Pyridoxal phosphate, spectral properties

The chromophoric pyridoxal phosphate coenzyme provides a useful spectrophotometric probe of catalytic events and of conformational changes that occur at the pyridoxal phosphate site of the P subunit and of the aiPi complex. Tryptophan synthase belongs to a class of pyridoxal phosphate enzymes that catalyze /3-replacement and / -elimination reactions.3 The reactions proceed through a series of pyridoxal phosphate-substrate intermediates (Fig. 7.6) that have characteristic spectral properties. Steady-state and rapid kinetic studies of the P subunit and of the aiPi complex in solution have demonstrated the formation and disappearance of these intermediates.73-90 Fig. 7.7 illustrates the use of rapid-scanning stopped-flow UV-visible spectroscopy to investigate the effects of single amino acid substitutions in the a subunit on the rate of reactions of L-serine at the active site of the P subunit.89 Formation of enzyme-substrate intermediates has also been observed with the 012P2 complex in the crystalline state.91 ... 

Yost and Harrison (118) have reported that m-MDH is irreversibly inactivated by pyridoxal 5 -phosphate (PLP). In view of the usual reversibility of this reagent when reacted with the -amino group of lysine, and the unusual spectral properties of the derivative formed, it was concluded that the reaction of m-MDH with PLP proceeds via modification of lysine followed by the formation of a secondary stable complex. Such a complex might be a thiazolidine-like compound formed by the reaction of the Schiff base with a neighboring cysteinyl residue. The role of the lysine residue in the active site of m-MDH remains to be clarified. 

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