Catalytic properties of a mutant β-galactosidase from Xanthomonas manihotis engineered to synthesize galactosyl-thio-β-1,3 and -β-1,4-glycosides

The identity of the acid/base catalyst of the Family 35 β-galactosidases from Xanthomonas manihotis (BgaX) has been confirmed as Glu184 by kinetic analysis of mutants modified at that position. The Glu184Ala mutant of BgaX is shown to function as an efficient thioglycoligase, which synthesises thiogalactosides with linkages to the 3 and 4 positions of glucosides and galactosides in high (>80%) yields. Kinetic analysis of the thioglycoligase reveals glycosyl donor Km values of 1.5–21 μM and glycosyl acceptor Km values from 180 to 500 μM. This mutant should be a valuable catalyst for the synthesis of metabolically stable analogues of this important glycosidic linkage.