Alternansucrase acceptor reactions with methyl hexopyranosides Original Research Article

Alternansucrase (EC 2.4.1.140, sucrose: (1→6), (1→3)-α-d-glucan 6(3)-α-d-glucosyltransferase) 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. We have studied the acceptor products arising from methyl glycosides as model compounds in order to better understand the specificity of alternansucrase acceptor reactions. The initial product arising from methyl β-d-glucopyranoside was methyl β-isomaltoside, which was subsequently glucosylated to yield methyl β-isomaltotrioside and methyl α-d-glucopyranosyl-(1→3)-α-d-glucopyranosyl-(1→6)-β-d-glucopyranoside. These products are analogous to those previously described from methyl α-d-glucopyranoside. The major initial acceptor product from methyl α-d-mannopyranoside was methyl α-d-glucopyranosyl-(1→6)-α-d-mannopyranoside, but several minor products were also isolated and characterized, including a 3,6-di-O-substituted mannopyranoside. Methyl α-d-galactopyranoside yielded two initial products, methyl α-d-glucopyranosyl-(1→3)-α-d-galactopyranoside and methyl α-d-glucopyranosyl-(1→4)-α-d-galactopyranoside, in a 2.5:1 molar ratio. Methyl d-allopyranosides were glucosylated primarily at position 6, yielding methyl α-d-glucopyranosyl-(1→6)-d-allopyranosides. The latter subsequently gave rise to methyl α-d-glucopyranosyl-(1→6)-α-d-glucopyranosyl-(1→6)-d-allopyranosides. In general, the methyl α-d-hexopyranosides were better acceptors than the corresponding β-glycosides.