Chemical synthesis of methyl 6′-α-maltosyl-α-maltotrioside and its use for investigation of the action of starch synthase II

The branched pentasaccharide methyl 6′-α-maltosyl-α-maltotrioside was chemically synthesised and investigated as a primer for particulate starch synthase II (SSII) using starch granules prepared from the low-amylose pea mutant lam as the enzyme source. For chemical synthesis, the trichloroacetimidate activation method was used to synthesise methyl O-(2,3,4,6-tetra-O-benzyl-α-d-glucopyranosyl)-(1→4)-O-(2,3,6-tri-O-benzyl-α-d-glucopyranosyl)-(1→6)-O-[(2,3,4,6-tetra-O-benzyl-α-d-glucopyranosyl-(1→4)]-O-(2,3-di-O-benzyl-α-d-glucopyranosyl)-(1→4)-2,3,6-tri-O-benzyl-α-d-glucopyranoside, which was then debenzylated to provide the desired branched pentasaccharide methyl 6′-α-maltosyl-α-maltotrioside as documented by 1H and 13C NMR spectroscopy. Using a large excess of the maltoside, the pentasaccharide was tested as a substrate for starch synthase II (SSII). Both of the non-reducing ends of methyl 6′-α-maltosyl-α-maltotrioside were extended equally resulting in two hexasaccharide products in nearly equal amounts. Thus, SSII catalyses an equimolar and non-processive elongation reaction of this substrate. Accordingly, the presence of the α-1,6 linkages does not dictate a specific structure of the pentasaccharide in which only one of the two non-reducing ends are available for extension.