Mechanisms of substrate digestion by endo-1,4-beta-xylanase of Cryptococcus albidus. Lysozyme-type pattern of action.
The action pattern and reaction mechanism of the endo-1,4-beta-xylanase of the yeast Cryptococcus albidus were investigated using reducing-end (1-3H)-labelled and uniformly 14C-labelled beta-1,4-xylooligosaccharides up to xylopentaose. The enzyme was found to catalyze degradation of oligosaccharides also by other pathways than a simple hydrolytic cleavage. Bond-cleavage frequency of xylotriose, xylotetraose and xylopentaose were found to be concentration dependent. At high substrate concentration reactions such as xylosyl, xylobiosyl and xylotriosyl transfer occur and result in the formation of products larger than the starting substrate. Xylose and xylobiose to significant extent enter the reaction pathways as glycosyl acceptors. None of the transglycosylic reactions observed with reducing-end-labelled substrates or acceptors were accompanied by a significant label redistribution from the reducing-end unit, suggesting that the enzyme-glycosyl intermediates effective in the transfer reactions can be formed from the non-reducing-end units of oligosaccharides. Evidence for the formation of a termomolecular shifted complex of beta-xylanase with xylotriose has also been obtained. All features of the degradation of oligosaccharides by beta-xylanase are consistent with the lysozyme-type reaction mechanism.