Coenzymatic activity of homologues of pyridoxal phosphate.

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Abstract
The coenzymatic activity of pyridoxal 5[image]-phosphate (PLP) for 4 highly purified enzymes (tryptophanase, aspartate aminotransferase, arginine decarboxylase, and D-serine dehydrase) was compared with that of 2 analogue coenzymes in which the 2-methyl group of PLP was replaced by hydrogen to yield norpyridoxal 5[image]-phosphate (norPLP, I) or by an ethyl group to yield [omega]-methylpyridoxal 5[image]-phosphate ([omega]-MePLP, III). Each of the 2 analogues replaced PLP as a coenzyme for each of the enzymes studied. The efficiency of replacement varied with the enzyme and with the criterion of activity chosen. No general pattern with respect to affinity of apoenzyme for coenzyme, apparent affinity of the reconstituted holoenzyme for its substrates, or the maximal velocity of the reaction catalyzed appeared as the coenzyme was changed from norPLP to PLP to [omega]-MePLP. By certain of these criteria, norPLP was superior to PLP as a coenzyme for aspartate aminotransferase and arginine decarboxylase. These differences are interpreted to mean that the 2-methyl group of PLP plays an important spatial role in interaction with the coenzyme binding site of the apoenzyme such that variations in this group lead to variations in the conformation of the substrate binding site and catalytic site of the holoenzyme, but that it does not play any catalytic role in the reactions catalyzed by pyridoxal phosphate proteins.