Structural characterization of extracellular polysaccharides produced by fungi from the genus Pleurotus.

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Citation abstract

The extracellular polysaccharides produced in liquid cultures of the ligninolytic fungi Pleurotus cornucopiae, P. eryngii, P. floridanus, P. ostreatus, P. pulmonarius and P. sajor-caju showed similar composition and structure. They included a minor water-soluble fraction, containing D-glucose, D-mannose, D-galactose and protein (rich in amide- and hydroxyl-containing residues which can be involved in protein-polysaccharide linkages). The structural analysis of this fraction mainly showed alpha-(1-->4)-linked glucose units. The structure of the major fraction (amounting to more than 80% of the total extracellular polysaccharide) was investigated in detail. Its low solubility was attributed to the high M(r). For its characterization a procedure of progressive dissolution and dialysis was set up. The methylation analysis showed that it was a (1-->3)-beta-D-glucan and that approximately 25% of the total units were C-6 branched. This result was confirmed by the intensities of the signals assigned to C-3 and C-6, free and substituted, in 13C-NMR spectra. Additional studies showed linear polysaccharides after one-cycle Smith degradation or after glucan acetolysis. Both results, as well as glucose release after the latter treatment, indicated that the C-6 linked side chains are constituted by single glucose residues, on every second or third unit of the main chain of the (1-->3)-beta-D-glucan.