Crystal structure and biological implications of a glycoside hydrolase family 55 β-1,3-glucanase from Chaetomium thermophilum

•Crystal structures of a GH55 β-1,3-glucanase (CtLam55) were determined at high resolution.•CtLam55 was found to act as an exo-β-1,3-glucanase.•Glu654 was identified as the catalytic acid residue.•Lack of stacking interactions in the substrate binding site was observed.•A potential sugar-binding pocket for branched substrates was found.

Crystal structures of a β-1,3-glucanase from the thermophilic fungus Chaetomium thermophilum were determined at 1.20 and 1.42 Å resolution in the free and glucose-bound form, respectively. This is the third structure of a family 55 glycoside hydrolase (GH55) member and the second from a fungus. Based on comparative structural studies and site-directed mutagenesis, Glu654 is proposed as the catalytic acid residue. The substrate binding cleft exhibits restricted access on one side, rendering the enzyme as an exo-β-1,3-glucanase as confirmed also by thin layer chromatography experiments. A lack of stacking interactions was found at the substrate binding cleft, suggesting that interactions at positions − 1, + 1 and + 2 are sufficient to orientate the substrate. A binding pocket was identified that could explain binding of branched laminarin and accumulation of laminaritriose.