Mechanisms of Substrate Digestion by Endo-1,4-beta-Xylanase of Cryptococcus albidus. Lysozyme-Type Pattern of Action

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Abstract
The action pattern and reaction mechanism of th endo-1,4-β-xylanase of the yeast Cryptococcus albidus were investigated using reducing-end(13H)-labelled and uniformly 14C-labelled β-1-4-xylooligosaccharides up to xylopentaose. The enzyme was found to catalyze degradation of oligosaxxharides also by other pathways than a simple hydrolytic cleavage. Bond-cleavage frequency of xylotriose, xylotertraose 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 product 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 β-xylanase with xylotriose has also been obtained. All features of the degradation of oligosaccharides by β-xylanase are consistent with the lysozyme-type reaction mechanism.