Pyridoxal and pyridoxal phosphate semicarbazone formation is subject to nucleophilic catalysis by primary and secondary amines. The reaction of pyridoxal phosphate with semicarbazide, over the pH range 7.3 to 14, proceeds through acidcatalyzed, base-catalyzed, and uncatalyzed reactions. In contrast, the reaction of the Schiff base formed from pyridoxal phosphate and methylamine, which serves as a model for "trans-Schiffization" reactions involved in pyridoxal phosphate-dependent enzymatic reactions, is nearly independent of pH over this range. The Schiff base is inherently thirtyfold more reactive than pyridoxal phosphate toward semicarbazide, but is much less reactive than protona ted Schiff bases, as judged from studies on morpholine-catalyzed pyridoxal phosphate semicarbazone formation. The latter reaction proceeds via a cationic imine intermediate, -N. , a model for protonated Schiff bases. Since the Schiff bases are more reactive than the free aldehyde, the existence of the coenzyme as the Schiff base is advantageous to pyridoxal phosphate-dependent enzymes. The
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