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 kcat values 10(8)-10(9)-fold larger than the rates of spontaneous hydrolysis of the same compounds. Log(kcat) values correlate well with aglycon pK(a) [B1g(V) = -0.52, r = 0.99], whereas the correlation of log(kcat/Km) is poor [r = 0.77; beta 1g(V/K) = approximately -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.
Padmaperuma
B
B
Department of Chemistry (M/C 111), University of Illinois at Chicago 60607-7061.
Sinnott
M L
ML
D006026:Glycoside Hydrolases / Q000737:chemistry*; D006031:Glycosylation; D006868:Hydrolysis; D007700:Kinetics; D015394:Molecular Structure; D011726:Pyridinium Compounds / Q000737:chemistry*
D011726:Pyridinium Compounds; D006026:Glycoside Hydrolases