THE CRYSTAL-STRUCTURE OF A CYANOGENIC BETA-GLUCOSIDASE FROM WHITE CLOVER, A FAMILY-1 GLYCOSYL HYDROLASE

Background: beta-glucosidases occur in a variety of organisms and catalyze the hydrolysis of aryl and alkyl-beta-D-glucosides as well as glucosides with only a carbohydrate moiety (such as cellobiose). The cyanogenic beta-glucosidase from white clover (subsequently referred to as CBG) is responsible for the cleavage of cyanoglucosides. Both CBG and the cyanoglucosides occur within the plant cell wall where they are found in separate compartments and only come into contact when the leaf tissue experiences mechanical damage. This results in the eventual production of hydrogen cyanide which acts as a deterrent to grazing animals. beta-glucosidases have been assigned to particular glycosyl hydrolase families on the basis of sequence similarity; this classification has placed CBG in family 1 (there are a total of over 40 families) for which a three-dimensional structure has so far not been determined. This is the first report of the three-dimensional structure of a glycosyl hydrolase from family 1. Results: The crystal structure of CBG has been determined using multiple isomorphous replacement. The final model has been refined at 2.15 Angstrom resolution to an R factor of 18.9%. The overall fold of the molecule is a (beta/alpha)(8) [Or (alpha/beta)(8)] barrel (in common with a number of glycosyl hydrolases) with all residues located in a single domain. Conclusions: Sequence comparisons between beta-glucosidases of the same family show that residues Glu183 and Glu397 are highly conserved. Both residues are positioned at the end of a pocket located at the C terminus of the barrel and have been assigned the respective roles of proton donor and nucleophile on the basis of inhibitor-binding and mutagenesis experiments. These roles are consistent with the environments of the two residues. The pocket itself is typical of a sugar-binding site as it contains a number of charged, aromatic and polar groups. in support of this role, we present crystallographic data on a possible product complex between CBG and glucose, resulting from co-crystallization of the native enzyme with its natural substrate, linamarin.