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Cleavage of S/O‐glucosides by β‐glucosidase from sweet almond

Sweet almond β‐glucosidase (EC ) is a highly efficient (GH family 1) exo‐glycohydrolase. The activity of this enzyme depends on a deprotonated carboxylate and a protonated carboxylic acid for the optimal activity. In this study we report on the hydrolysis of thioglycoside catalyzed by β‐glucosidase. While the Km values for the S‐ and O‐glycosides are similar, the kcat values are about 1,000‐times lower for the S‐glycosides. Linear free energy relationships between the binding and catalytic parameters and the aryl substituents on the phenyl glucosides reveal a striking similarity between the S‐ and O‐glycosides. For both substrates, catalysis (kcat/Km) shows a strong dependence on the pKa of leaving group (βlg= −1) suggesting that in the transition state there is nearly full development of a negative charge on the (thio)phenolic oxygen or sulfur. Substrate binding shows a dependence on both hydrophobicity and the electronic properties of aryls substituents. The brφnsted coefficient for substrate binding is β ≈ −0.45, suggesting a weakening of glucosidic bond when the E · S complex is formed.