Differences in Active Site Structure in a Family of β-glucan Endohydrolases Deduced from the Kinetics of Inactivation by Epoxyalkyl β-Oligoglucosides

Abstract

The active sites of a spectrum of beta-glucan endohydrolases with distinct, but related substrate specificities have been probed using a series of epoxyalkyl beta-glycosides of glucose, cellobiose, cellotriose, laminaribiose, laminaritriose, 3O-beta-D-glucosyl-cellobiose and 4O-beta-D-glucosyl-laminaribiose with different aglycon chain lengths. The inactivation of each of the endohydrolases by these compounds results from active site-directed inhibitor action, as indicated by the dependence of the inactivation rate on pH, glycosyl chain length and linkage position, aglycon length, and the protective effect of disaccharides derived from the natural substrates. Comparisons of inhibitor specificity between a Bacillus subtilis 1,3;1,4-beta-D-glucan 4-glucanohydrolase (EC 3.2.1.73), a Streptomyces cellulase (EC 3.2.1.4), a Schizophyllum commune cellulase (EC 3.2.1.4), a Rhizopus arrhizus 1,3-(1,3;1,4)-beta-D-glucan 3(4)-glucanohydrolase (EC 3.2.1.6), and a Nicotiana glutinosa 1,3-beta-D-glucan 3-glucanohydrolase (EC 3.2.1.39) demonstrated different tolerances for glycosyl linkage positions in the inactivation process and a critical role of aglycon length reflecting differences in the active site geometry of the enzymes. For the B. subtilis endohydrolase it was concluded that the aglycon residue of the inhibitor spans the glycosyl binding subsite occupied by the 3-substituted glucosyl residue involved in the glucosidic linkage cleaved in the natural substrate. Appropriate positioning of the inhibitor epoxide group with respect to the catalytic amino acids in the active site is crucial to the inactivation step and the number of glucosyl residues in the inhibitor affects aglycon chain length specificity. The importance of this effect differs between the glucanases tested and may be related to the number of glycosyl binding subsites in the active site.