Abstract:
The hydrolyses of five ?-d-xylopyranosylpyridinium ions by the ?-d-xylosidase of Bacillus pumilus proceed with kcat values 108?109-fold larger than the rates of spontaneous hydrolysis of the same compounds. Log(kcat) values correlate well with aglycon pKa [?1g(V) = ?0.52, r = 0.99], whereas the correlation of log(kcat/Km) is poor [r = 0.77; ?1g(V/K) = ? ?0.6]. The (1 ? 3)-?-d-glucanase of Sporotrichum dimorphosporum hydrolyses 4-bromo-2-(?-d-glucopyranosyl)isoquinolinium ion with a rate enhancement of 108. The amyloglucosidase II of Aspergillus niger hydrolyses three ?-d-gluco-pyranosylpyridinium ions with rate enhancements of 105?108. 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 imporable 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.
