Complex Reaction Pathway of Aryl β‐Xyloside Degradation by β‐Xylanase of Cryptococcus albidus

Abstract

The extracellular endo-1,4-β-xylanc se of the yeast Cryptococcus albidus catalyzes degradation of aryl β-xylosides by other reactions than simple hydrolytic cleavage. Liberation of phenol or β-nitrophenol from the corresponding β-xylosides is accompanied by formation of xylose oligosaccharides and only small amounts of xylose. With the aid of phenyl β-[U-¹⁴C]xyloside synthesized from [U-¹⁴C]xylose, it was established that the reaction followed a complex pattern with the rate of phenyl β-xyloside digestion and appearance of various products varying markedly with time. The reaction involves multiple transglycosylic reaction leading first to phenyl glycosides of xylooligosaccharides, which are subsequently hydrolyzed mainly to xylobiose and xylotriose. At concentrations of phenyl β-xyloside lower than 100 mM the reaction exhibited a significant lag phase, which was followed by period during which the rate of the degradation of the substrate could be determined. The rate showed a strong sigmoidal dependence on phcnyl-β-xyloside concentration. The lag phase could be eliminated and the initial rate accelerated by addition of xylose oligosaccharides, which are hydrolyzed by β-xylanase. After disappearance of the added oligosaccharides, the reaction transitioNatly ceased and then resumed again at a rate comparable to the control without added oligosaccharides. It is proposed that β-xylanase utilizes for degradation of phenyl β-xyloside two reaction pathways differing in the nature of glycosyl donors.