β-D-Xylosidase from Penicillium wortmanni: binding and hydrolysis of alkyl and aryl 1-oxygen and 1-thio-β-D-xylopyranosides

Abstract

The influence of the chain length in n-alkyl .beta.-D-xylopyranosides and of the para substituents in aryl .beta.-D-xylopyranosides on the kinetic parameters (Ka, V) for hydrolysis of these substrates by a .beta.-D-xylosidase from P. wortmanni was investigated. Binding of the corresponding 1-thio derivatives as competitive inhibitors (Ki) was studied for comparative purposes. For the n-alkyl substrates, a slight dependency of V on the chain length is noted, whereas the aryl .beta.-D-xylopyranosides show nearly constant V values. The influence of the aglycone on Ka and Ki values is complex; for the n-alkyl derivatives the contribution of the hydrophobicity of the aliphatic chain seems predominant, although steric factors cannot be neglected. These results, together with previous observations, can tentatively be interpreted in terms of a double-displacement mechanism.