Alternansucrase acceptor reactions with d-tagatose and l-glucose Original Research Article

Alternansucrase (EC 2.4.1.140) is a d-glucansucrase that synthesizes an alternating α-(1→3), (1→6)-linked d-glucan from sucrose. It also synthesizes oligosaccharides via d-glucopyranosyl transfer to various acceptor sugars. Two of the more efficient monosaccharide acceptors are d-tagatose and l-glucose. In the presence of d-tagatose, alternansucrase produced the disaccharide α-d-glucopyranosyl-(1→1)-β-d-tagatopyranose via glucosyl transfer. This disaccharide is analogous to trehalulose. We were unable to isolate a disaccharide product from l-glucose, but the trisaccharide α-d-glucopyranosyl-(1→6)-α-d-glucopyranosyl-(1→4)-l-glucose was isolated and identified. This is analogous to panose, one of the structural units of pullulan, in which the reducing-end d-glucose residue has been replaced by its l-enantiomer. The putative l-glucose disaccharide product, produced by glucoamylase hydrolysis of the trisaccharide, was found to be an acceptor for alternansucrase. The disaccharide, α-d-glucopyranosyl-(1→4)-l-glucose, was a better acceptor than maltose, previously the best known acceptor for alternansucrase. A structure comparison of α-d-glucopyranosyl-(1→4)-l-glucose and maltose was performed through computer modeling to identify common features, which may be important in acceptor affinity by alternansucrase.