Enzymic alpha-galactosylation of a cyclic glucotetrasaccharide derived from alternan

Alternanase catalyzes the hydrolysis of alternan, an alpha-(1-->3)-alpha-(1-->6)-D-glucan produced by Leuconostoc mesenteroides, resulting in the formation of a cyclic tetramer cyclo -->3)-alpha-D-Glcp-(1-->6)-alpha-D-Glcp-(1-->(2) (cGlc(4)). Two alpha-galactosidases, one from coffee bean and the other produced by a fungus, currently described as Thermomyces lanuginosus, were found to catalyze an efficient 6-O-alpha-D-galactopyranosylation of cGlc(4). The attachment of a nonreducing alpha-D-galactopyranosyl residue to the cGlc(4) molecule opens new possibilities for future applications of the cyclic tetramer, since the D-galactopyranosyl residue can be easily modified by D-galactose oxidase to introduce a reactive aldehyde group. The results also extend our knowledge about the synthetic potential of T. lanuginosus alpha-galactosidase.