The pH dependence and group modification of beta-D-xylosidase from Bacillus pumilus: evidence for sulfhydryl and histidyl groups

The pH dependence of the kinetic parameters of beta-D-xylosidase (EC. 3.2.1.37) from Bacillus pumilus reveals that an acidic functional group with pK 8.0 is involved in the catalysis. The fast inactivation of the dimeric enzyme by near equivalent amounts of methylmethanethiolsulfonate indicates that one thiol group per monomer is essential for catalysis, consistent with previously reported results. From the reactivity of the thiol groups with respect to 5,5'-dithiobis(2-nitrobenzoic acid), the absence of subunit cooperativity was indicated. The present study also reports on the inactivation of the enzyme by diethylpyrocarbonate, and provides evidence of the importance of a histidine residue. A mechanism of catalysis is presented, in which the thiol group interacts with the substrate via partial proton transfer. The mode of participation of the histidine group is difficult to specify, but may be associated with the maintenance of the active conformation of the enzyme.