HYDROLYSIS OF GLYCOSYLPYRIDINIUM IONS BY ANOMERIC-CONFIGURATION-INVERTING GLYCOSIDASES

The hydrolyses of five beta-D-xylopyranosylpyridinium ions by the beta -D-xylosidase of Bacillus pumilus proceed with k(cat) values 10(8)-10( 9)-fold larger than the rates of spontaneous hydrolysis of the same co mpounds. Log(k(cat)) values correlate well with aglycon pK(a) [beta(1g )(V)=-0.52, r=0.99], whereas the correlation of log(k(cat)/K-m) is poo r [r=0.77; beta 1g(V/K)=similar to-0.6]. The (1--> 3)-beta-D-glucanase of Sporotrichum dimorphosporum hydrolyses 4-bromo-2-(beta-D-glucopyra nosyl)isoquinolinium ion with a rate enhancement of 10(8). The amylogl ucosidase II of Aspergillus niger hydrolyses three alpha-D-glucopyrano sylpyridinium ions with rate enhancements of 10(5)-10(8). The efficien t hydrolysis of glycosylpyridinium ions by these three inverting glyco sidases, the catalytic mechanism of which is unlikely to involve a nuc leophile from the enzyme, makes it improbable that the hydrolysis of g lycosylpyridinium ions by retaining glycosidases, discovered some year s ago, is initiated by addition of a catalytic nucleophilic carboxylat e group of the enzyme to the pyridinium ring.