Structural and functional characterization of galactooligosaccharides in Nostoc commune: β-d-galactofuranosyl-(1→6)-[β-d-galactofuranosyl-(1→6)]2-β-d-1,4-anhydrogalactitol and β-(1→6)-galactofuranosylated homologues

A new class of galactooligosaccharides has been identified from the terrestrial cyanobacterium Nostoc commune by MS and NMR techniques. These consist of β-d-galactofuranosyl-(1→6)-[β-d-galactofuranosyl-(1→6)]n-β-d-1,4-anhydrogalactitols with n ranging from 2 to 8, corresponding to compounds designated 1 through 7. In total these saccharides amounted to ∼0.35% of the dry thallus of N. commune, while in several other cyanobacteria they were not detected. Possibly they play some role in protection from damage by heat and desiccation as suggested by experiments with heterologous systems. For example, phosphoglucomutase (EC 2.7.5.1) from rabbit muscle was protected against heat inactivation by these oligosaccharides, and α-amylase (EC 3.2.1.1) from porcine pancreas by the oligosaccharides 6 and 7. The homologues of lower molecular mass, however, enhanced heat sensitivity of α-amylase. The viability of Escherichia coli was completely abolished by desiccation, whereas in the presence of 4 survival rates were ∼50% of controls not subjected to desiccation. The newly identified saccharides are compared with known galactofuranose-based oligo- and polysaccharides and possible biological functions of them are discussed.

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