Hydrolysis of Aryl β‐ d ‐Glucopyranosides and β‐ d ‐Xylopyranosides by an Induced β‐ d ‐Glucosidase from Stachybotrys atra

The induced β‐ d ‐glucosidase from Stachybotrys atra hydrolyzes aryl β‐ d ‐glucopyranosides and aryl β‐ d ‐xylopyranosides by the same basic two‐step mechanism. In the first step the aglycon group is split of with simultaneous formation of an enzyme‐glycosyl complex. In the second step this intermediate complex reacts with water yielding β‐ d ‐glucose or β‐ d ‐xylose. For β‐ d ‐xyloside hydrolysis each of the two steps is partially rate‐controlling, whereas for β‐ d ‐glucoside hydrolysis the second step is rate‐limiting. The enzyme is inhibited by high concentrations of substrate and the exact rate‐concentration equation is a second‐order equation. 1‐Thio‐β‐ d ‐glycopyranosides with an aromatic aglycon inhibit the reaction in both a competitive and non‐competitive way. A tentative mechanism is proposed to explain all types of inhibition. In this mechanism substrates and inhibitors with an aromatic aglycon group bind through hydrophobic forces to the ‘aglycon subsite’ of the intermediate enzyme‐glycosyl complex. Binding of the second substrate molecule or of the inhibitor to this complex does not prevent the reaction of the glycosyl moiety with water, it only decreases the rate of the second step.