Fig. 1. Proposed reaction mechanisms of the glycosylation of BsNagZ (A), SEA (B), and T82A-SEA (C).

　T82A-SEA and SEA¹⁵⁾ contain a Ser–His pair, and BsNagZ¹¹⁾ contains an Asp–His catalytic dyad at their active sites. In both cases, the histidine residue serves as an acid catalyst during the glycosylation step. E•S complex, Michaelis complex; E–I complex, covalent glycosyl–enzyme intermediate; E•P complex, enzyme–product complex.