The breakdown of sulphur glycosidic bonds in thioglycosides can produce isothiocyanate, a chemoprotective agent linked to the prevention of cancers; however, only a handful of enzymes have been identified that are k0nown to catalyse this reaction. Structural studies of the myrosinase enzyme, which is capable of hydrolysing the thioglycosidic bond, have identified residues that may play important roles in sulphur bond specific activity. Using rational design, two extremo-adapted beta-glycosidases from the species Thermus nonproteolyticus (TnoGH1) and Halothermothrix orenii (HorGH1) were engineered towards thioglycoside substrates. Twelve variants, six for TnoGH1and six for HorGH1, were assayed for activity. Remarkable enhancement of the specificity (k(cat)/K-M) of TnoGH1 and HorGH1 towards beta-thioglycoside was observed in the single mutants TnoGH1-V287R (2500 M-1 s(-1)) and HorGH1-M229R (13,260 M-1 s(-1)) which showed a 3-fold increase with no loss in turnover rate when compared with the wild-type enzymes. Thus, the role of arginine is key to induce beta-thioglycosidase activity. Thorough kinetic investigation of the different mutants has shed light on the mechanism of beta-glycosidases when acting on the native substrate.