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 compounds. 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 poor [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-glucopyranosyl)isoquinolinium ion with a rate enhancement of 10(8). The amyloglucosidase II of Aspergillus niger hydrolyses three alpha-D-glucopyranosylpyridinium ions with rate enhancements of 10(5)-10(8). The efficient hydrolysis of glycosylpyridinium ions by these three inverting glycosidases, the catalytic mechanism of which is unlikely to involve a nucleophile from the enzyme, makes it improbable that the hydrolysis of glycosylpyridinium ions by retaining glycosidases, discovered some years ago, is initiated by addition of a catalytic nucleophilic carboxylate group of the enzyme to the pyridinium ring.