Kinetics and specificities of two closely related β‐glucosidases secreted by Schizophyllum commune

Two β‐glucosidases (I and II) were isolated from Schizophyllum commune , and their physical and chemical properties studied. The two enzymes have very similar sequences, as shown by HPLC analysis of tryptic digests and partial amino acid sequencing. As judged by their circular dichroism spectra, they have almost identical secondary structure. The estimates for α‐helix, β‐sheet, and other structures were 21%, 40% and 39%, respectively, for β‐glucosidase I and 27%, 32% and 41% for β‐glucosidase II. Their near‐ultraviolet spectra were identical. β‐Glucosidase I was more highly glycosylated than β‐glucosidase II, having 2 mol N ‐acetylglucosamine/mol enzyme 36, mol mannose/mol enzyme and 1.2 mol glucose/mol enzyme vs 1.2, 17 and 3 mol/mol, respectively, in β‐glucosidase II. The native glycosylated form of β‐glucosidase I had a molecular mass of 102 kDa, and that of β‐glucosidase II, 96 kDa. As estimated from sensitivity to N ‐glycanase, β‐glucosidase II sugars were mainly asparagine linked, but much of the sugar in β‐glucosidase I was not removed by this treatment and was apparently serine or threonine linked. Kinetic analysis showed that both forms had similar K m values (0.3–2.1 mM) for oligosaccharides of 2–6 residues, but the k cat values of β‐glucosidase II were lower by 30–75% than those of β‐glucosidase I. The substrate dependence of k cat / K m indicated that both enzymes had binding sites for three glucose residues. The pH optimum of β‐glucosidase I was higher than that of β‐glucosidase II (5.8 vs 5.1). Both had similar specificities for several ( R )‐β‐D‐glucosides tested. Both enzymes were competitively inhibited by their glucose product, but β‐glucosidase II was consistently less inhibited than β‐glucosidase I. Cellobiase activity was much more markedly inhibited than the activity with higher oligosaccharides, and the result of this, plus the lower hydrolytic rate with cellobiose, resulted in an accumulation of cellobiose as higher oligosaccharides were digested. Glucono‐δ‐lactone inhibited both enzymes and the hydrolysis of all oligosaccharide substrates similarly ( K i = 4 μM). We conclude that the catalytic site is identical in both enzymes, but subtle structural differences are reflected in a differential activity on the higher oligosaccharides and in the differential effects of the glucose product as an inhibitor. Furthermore, ethanol had a stimulatory effect on β‐glucosidase I but inhibited β‐glucosidase II, which presumably reflects differential effects of ethanol on the conformations of the two species.