The Crystal Structure of β-Glucosidase from Bacillus circulans sp. alkalophilus: Ability to Form Long Polymeric Assemblies

Family 1 of glycosyl hydrolases is a large and biologically important group of enzymes. A new three-dimensional structure of this family, β-glucosidase from Bacillus circulans sp. alkalophilus is reported here. This is the first structure of β-glucosidase from an alkaliphilic organism. The model was determined by the molecular replacement method and refined to a resolution of 2.7 Å. The quaternary structure of B. circulans sp. alkalophilus β-glucosidase is an octamer and subunits of the octamer show a similar (β/α)8 barrel fold to that previously reported for other family 1 enzymes. The crystal structure suggested that Cys169 in the active site is substituted. The Cys169 is located near the putative acid/base catalyst Glu166 and it may contribute to the high pH optimum of the enzyme. The crystal structure also revealed that the asymmetric unit contains two octamers which have a clear binding interaction with each other. The ability of the octamers to link with each other suggested that β-glucosidase from Bacillus circulans sp. alkalophilus is able to form long polymeric assemblies, at least in the crystalline state.