The modes of action of two endo-1,4-β- d-xylanases from Aspergillus sojae on various xylooligosaccharides

Abstract

The modes of action of two endo-1,4-β- d-xylanases (X-I and X-II-B) from Aspergillus sojae were investigated using xylooligosaccharides and their corresponding alditols. Due to the dependence of the molecular activity ( k 0) on the degree of polymerization of substrates, it was suggested that each xylanase had five subsites. With regard to the bond-cleavage frequencies of various xylooligosaccharide-alditols, the catalytic site of each xylanase was located between the third and fourth subsites, counting from the terminal site, which was attached to the non-reducing end of the substrates. The intrinsic rate constants ( k int) for the hydrolysis of glycosidic linkages of X-I and X-II-B were calculated to be 56 and 43 s −1, respectively. Evaluation of subsite affinities of each xylanase: the total subsite affinity of ( A 3 + A 4), adjacent to the catalytic site of X-I, had a positive value (0.10 kcal/mol), whereas that of X-II-B had a negative value (−0.39 kcal/mol). The other subsite affinities of X-I exhibited higher values than that of X-II-B, except for the subsite affinity of A 1.