Assignment of 13C-n.m.r. signals for reduced isomaltooligosaccharides

Co-ordinated synthesis of gentiobiitol and sorbitol, evidence of sorbitol glycosylation in transgenic sugarcane

Sugarcane (a Saccharum spp. interspecific hybrid) was previously engineered to synthesize sorbitol (designated as sorbitolcane). Motivated by the atypical development of the leaves in some sorbitolcane, the polar metabolite profiles in the leaves of those plants were compared against a group of control sugarcane plants. Eighty-six polar metabolites were detected in leaf extracts by GC-MS. Principal component analysis of the metabolites indicated that three compounds were strongly associated with sorbitolcane. Two were identified as sorbitol and gentiobiose and the third was unknown. Gentiobiose and the unknown compound were positively correlated with sorbitol accumulation. The unknown compound was only abundant in sorbitolcane. This compound was structurally characterized and found to be a sorbitol-glucose conjugate. (13)C NMR analysis indicated that the glucopyranose and glucitol moieties were 1,6-linked. Ligand exchange chromatography confirmed that the compound was a beta-anomer, thus identifying the compound as 6-O-beta-d-glucopyranosyl-D-glucitol, or gentiobiitol.

Dextran acceptor reaction of Streptococcus sobrinus glucosyltransferase GTF-I as revealed by using uniformly 13C-labeled sucrose

A sucrose glucosyltransferase GTF-I from cariogenic Streptococcus sobrinus transferred the uniformly 13C-labeled glucosyl residue ([U-(13)C]Glc) from [U-(13)C]sucrose to exogenous dextran T500 at the non-reducing-end, mostly by alpha-(1-->6) linkages and partially by alpha-(1-->3) linkages, as revealed by the 13C-(13)C NMR coupling pattern. With increasing amounts of [U-(13)C]sucrose, transfer of [U-(13)C]Glc to the alpha-(1-->3)-linked chain became predominant without increase in the number of chains. The transfer of [U-(13)C]Glc to an isomaltopentaose acceptor occurred similarly to its transfer to T500. alpha-(1-->3)-branches in the [U-(13)C]dextran, specifically synthesized from [U-(13)C]sucrose by a Streptococcus bovis dextransucrase, were not formed by GTF-I, as judged by the observation that a newly-formed alpha-1,3,6-branched [U-(13)C]Glc was not detected, which could have been formed by transferring the unlabeled Glc from sucrose to the internal alpha-(1-->6)-linked [U-(13)C]Glc at C-3. The 13C-(13)C one-bond coupling constants (1J) were also recorded for the C-1--C-6 bond of the internal alpha-(1-->6)-linked [U-(13)C]Glc and of the non-reducing-end [U-(13)C]Glc.

Nigerooligosaccharide acceptor reaction of Streptococcus sobrinus glucosyltransferase GTF-I

Nigerose and nigerooligosaccharides served as acceptors for a glucosyltransferase GTF-I from cariogenic Streptococcus sobrinus to give a series of homologous acceptor products. The soluble oligosaccharides (dp 5-9) strongly activated the acceptor reaction, resulting in the accumulation of water-insoluble (1-->3)-alpha-D-glucan. The enzyme transferred the labeled glucosyl residue from D-[U-13C]sucrose to the 3-hydroxyl group at the non-reducing end of the (1-->3)-alpha-D-oligosaccharides, as unequivocally shown by NMR 13C-13C coupling patterns. The values of the 13C-13C one-bond coupling constant (1J) are also presented for the C-1-C-6 of the 13C-labeled alpha-(1-->3)-linked glucosyl residue and of the non-reducing-end residue.