Candida albicans cell wall comprises a branched β- d-(1→6)-glucan with β- d-(1→3)-side chains

Abstract

The structure of immunogenic and immunomodulatory cell wall glucans of Candida albicans is commonly interpreted in terms of a basic polysaccharide consisting of a β- d-(1→3)-linked glucopyranosyl backbone possessing β- d-(1→6)-linked side chains of varying distribution and length. This proposed molecular architecture has been re-evaluated by the present study on the products of selective enzymolysis of insoluble C. albicans glucan particles (GG). High resolution 1H (400 and 700 MHz) and 13C (100 and 175 MHz) NMR analyses were performed on a soluble β-glucan preparation (GG-Zym) obtained by GG digestion with endo-β- d-(1→3)-glucanase and on its high- (Pool 1) and low-molecular weight (Pool 2) sub-fractions. The resonances typical of uniformly β- d-(1→6)- and β- d-(1→3)-linked linear glucans, together with additional multiplets assigned to short-chain oligoglucosides, were detected in GG-Zym. Pool 1 (46.3 ± 6.4% of GG-Zym content) consisted of β- d-(1→6)-linked glucopyranosyl polymers, with short β- d-(1→3)-branched side chains of 2.20 ± 0.02 units (branching degree (DB) = 0.14 ± 0.03). Pool 2 was a mixture of glucose and linear short-chain β- d-(1→3)-oligoglucosides. Further digestion of Pool 1 by β- d-(1→6)-glucanase yielded a mixture of glucose and short β- d-(1→6)-linked, either linear or β- d-(1→3,6) branched, oligomers. These endoglucanase digestion patterns were consistent with the presence in C. albicans cell wall glucans of β- d-(1→6)-linked glucopyranosyl backbones possessing β- d-(1→3)-linked side chains, a structure very close to that of β- d-(1→6)-glucan from Saccharomyces cerevisiae yeast. This finding may provide the grounds for further elucidation of the cell wall structure and a better understanding of the biological properties of C. albicans β-glucans.