Fine structural analysis of the fungal polysaccharide pullulan elaborated by Aureobasidium pullulans, CH-1 strain

The structure of pullulan, the extracellular alpha -D-glucan elaborated by the yrast-like fungus Aureobasidium pullulans, may be described as a linear alpha -D-glucan consisting of maltotriosyl repeal units connected terminally by (1 -->6)-alpha -D-glucosidic bonds. Occasionally some of maltotriosyl residues are replaced by higher oligosaccharide units, most frequently with maltotetraosyl residues. Using the susceptibility of pullulan CH-I (obtained from strain CH-I of Aureobasidium pullulans) to hydrolysis catalysed by porcine alpha-amylase. the polysaccharide was cleaved and the fragments obtained fractionated by gel-permeation chromatography. The heterogenous size of the fragments indicates that there is no apparent regular distribution of tetrasaccharide units in the pullulan chain. Enzymatic digestion of pullulan CH-I using pullulanase, followed by gel-permeation chromatography of the resulting digest confirmed these results as did preparative paper chromatography and CI mass spectrometry of the separated components, i.e. that maltotetraosyl units (about 7 %) are building units of pullulan CH-I.