Reaction pathways of substrate degradation by an acidic endo-1,4-beta-xylanase of Aspergillus niger.

An acidic endo-1,4-beta-xylanase (1,4-beta-D-xylan xylanohydrolase, EC 3.2.1.8) of Aspergillus niger catalyzes degradation of linear 1,4-beta-xylooligosaccharides by multiple reaction pathways analogous to those catalyzed by lysozyme and alpha-amylases. Quantitative product analysis of enzyme-substrate mixtures using 1-3H-reducing end-labeled xylooligosaccharides and [U-14C]xylotriose led to the following conclusions: (1) bond cleavage frequencies of xylotriose, xylotetraose and xylopentaose are strongly dependent on substrate concentration; (2) at relatively low concentration of the oligosaccharides the enzyme catalyzes transglycosylic reactions leading to products larger than the substrates; (3) xylobiose and to a low extent also xylose, are utilized as glycosyl acceptors in the transfer reactions; (4) the enzyme-glycosyl intermediates effective in the transfer reactions are formed only from the non-reducing part of oligosaccharides, since no evidence was obtained for condensation of two molecules of oligosaccharides; (5) the enzyme does not catalyze degradation of xylobiose and aryl beta-xylosides at an appreciable rate.