Sucrose Isomerase and Its Mutants from Erwinia rhapontici Can Synthesise α-Arbutin

Sucrose isomerase (SI) from Erwinia rhapontici is an intramolecular isomerase that is normally used to synthesise isomaltulose from sucrose by a mechanism of intramolecular transglycosylation. In this study, it was found that SI could synthesise α-arbutin using hydroquinone and sucrose as substrates, via an intermolecular transglycosylation reaction. Five phenylalanine residues (F185, F186, F205, F297, and F321) in the catalytic pocket of SI were chosen for sitedirected mutagenesis. Mutants F185I, F321I, and F321W, whose hydrolytic activities were enhanced after the mutation, could synthesise α-arbutin through intermolecular transglycosylation with a more than two-fold increase in the molar transfer ratio compared with wild type SI. The F297A mutant showed a strong ability to synthesise a novel α-arbutin derivative and a four-fold increase in its specific activity for intermolecular transglycosylation over the wild type. Our findings may lead to a new way to synthesise novel glucoside products such as α-arbutin derivatives by simply manipulating the Phe residues in the catalytic pocket. From the structure superposition, our strategy of manipulating these Phe residues may be applicable to other similar transglycosylating enzymes.