KINETIC COMPARISON OF FUNGAL BETA-XYLOSID ASES IN THE CONDENSATION REACTION OF XYLOSE

The kinetic characterization of two fungal (Aspergillus niger and Malb ranchea pulchella) beta-xylosidases reveals ''product specificity'' in the condensation reaction of xylose. The specificity of xylodisacchar ide production in the condensation reaction yields more variation for the M. pulchella enzyme than for the A. niger one. Of the products, be ta-1,4-xylodisaccharide is produced most in the reaction of both enzym es. A simple mechanism with forward (condensation) and reverse (hydrol ysis) reactions is assumed and the rate constants for the forward reac tions of the respective A-linked xylodisaccharides are experimentally determined as well as the equilibrium constants. The rate constants fo r the reverse reactions are estimated from these equilibrium and rate constants, and found to have smaller values than those observed in the initial velocities of the hydrolysis, inferring inhibition of the rea ction with a high concentration of xylose. Comparison of the actual ti me course in xylodisaccharide production with that obtained by a compu ter simulation for this simple mechanism implies that the reaction mec hanism of the M. pulchella enzyme comprises the condensation and hydro lysis reactions, while the mechanism of the A. niger enzyme further in cludes the reaction producing xylotrisaccharide.