Enzymatic redox isomerization of 1,6-disaccharides by pyranose oxidase and NADH-dependent aldose reductase

Pyranose 2-oxidase, a homotetrameric FAD-flavoprotein from the basidiomycete Trametes multicolor, catalyzes regioselectively the oxidation of the 1→6 disaccharides allolactose [β-d-Galp-(1→6)-d-Glc], gentiobiose [β-d-Glcp-(1→6)-d-Glc], melibiose [α-d-Galp-(1→6)-d-Glc], and isomaltose [α-d-Glcp-(1→6)-d-Glc] at position C-2 of their reducing moiety. The resulting glycosyl d-arabino-hexos-2-uloses can be reduced specifically at C-1 by NAD(P)H-dependent aldose reductase from the yeast Candida tenuis. By this novel, two-step redox isomerization process the four disaccharide substrates could be converted to the corresponding keto-disaccharides allolactulose [β-d-Galp-(1→6)-d-Fru], gentiobiulose [β-d-Glcp-(1→6)-d-Fru], melibiulose [α-d-Galp-(1→6)-d-Fru], and isomaltulose (palatinose, [α-d-Glcp-(1→6)-d-Fru]) in high yields. These products could find application in food technology as alternative sweeteners.