The crystal structure of a 2-fluorocellotriosyl complex of the Streptomyces lividans endoglucanase CelB2 at 1.2 angstrom resolution

Glycoside hydrolases have been classified into over 66 families on the basi s of amino acid sequence. Recently a number of these families have been gro uped into "clans" which share a common fold and catalytic mechanism [Henris sat, B., and Bairoch, A. (1996) Biochem. J, 316, 695-696]. Glycoside hydrol ase Clan GH-C groups family 11 xylanases and family 12 cellulases, which sh are the same jellyroll topology, with two predominantly antiparallel beta-s heets forming a long substrate-binding cleft, and act with net retention of anomeric configuration. Here we present the three-dimensional structure of a family 12 endoglucanase, Streptomyces lividans CelB2, in complex with a 2-deoxy-2-fluorocellotrioside. Atomic resolution (1.2 Angstrom) data allow clear identification of two distinct species in the crystal. One is the gly cosyl-enzyme intermediate, with the mechanism-based inhibitor covalently li nked to the nucleophile Glu 120, and the other a complex with the reaction product, 2-deoxy-2-fluoro-beta-D-cellotriose. The active site architecture of the complex provides insight into the double-displacement mechanism of r etaining glycoside hydrolases and also sheds light on the basis of the diff erences in specificity between family 12 cellulases and family 11 xylanases .