Substrate‐Binding Site of Endo‐1,4‐β‐Xylanase of the Yeast Cryptococcus albidus

Abstract

The substrat‐binding site of endo‐1,4‐β‐xylanase of the yeast Cryptococcus albidus was investigated using, 1,4‐β‐xylooligosaccharides (1‐3H)‐ labeled at the reducing end. Evaluation of the affinities of ten imaginary subsites by the method of suganuma et al.[1978,j Biochem.(Tokyo) 84, 293‐316] pointed out that the substrate‐binding site of the enzyme is composed of four subsites and that the catalytic groups are localized in the centre.The imaginary subsites on the left‐hand side of the binding site (‘non‐reducing‐end’ side) showed little or no affinity to bind xylosylresidues. For the subsites on the right‐hand side of the binding site (‘reducing end’ side) negaive values of affinity were obtained, which means this region of the enzyme is unfavourable for complexing with xylosyl residues. As a consequence of the asymmetric distribution of negative values of affinity around the binding site, the enzyme displays a strong preference for attacking near the reducing end of the substrate.Regardless of the length of[1‐3H] xylooligosaccharides, [1‐3H] xylobiose was the prevailing reaction product at an early stage of hydrolysis, and frequency distribution of bond cleavage decreased from the second glycosidic bond towards the non‐reducing end. Additional information on the substrate‐binding site of C albidus β‐xylanase was obtained by evaluating the efficiency of xylose, xylobiose, methyl β‐d‐xyloside and phenyl β‐d‐xyloside to serve as glycosyl acceptors in the transglycosylic reactions proceeding at high concentrations of xylotriose. Copyright © 1981, Wiley Blackwell. All rights reserved