THE MODES OF ACTION OF 2 ENDO-1,4-BETA-D-XYLANASES FROM ASPERGILLUS-SOJAE ON VARIOUS XYLOOLIGOSACCHARIDES

The modes of action of two endo-1,4-beta-D-xylanases (X-I and X-II-B) from Aspergillus sojae were investigated using xylooligosaccharides an d 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 bo nd-cleavage frequencies of various xylooligosaccharide-alditols, the c atalytic site of each xylanase was located between the third and fourt h subsites, counting from the terminal site, which was attached to the non-reducing end of the substrates. The intrinsic rate constants (k(i nt)) 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 k cal/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 tha t of X-II-B, except for the subsite affinity of A(1).