MOLECULAR AND ACTIVE-SITE STRUCTURE OF A BACILLUS 1,3-1,4-BETA-GLUCANASE

The three-dimensional structure of the hybrid Bacillus 1,3-1,4-beta-gl ucanase (beta-glucanase; 1,3-1,4-beta-D-glucan 4-glucanohydrolase, lic henase, EC 3.2.1.73) designated H(A16-M) was determined by x-ray cryst allography at a resolution of 2.0 angstrom and refined to an R value o f 16.4% using stereochemical restraints. The protein molecule consists mainly of two seven-stranded antiparallel beta-pleated sheets arrange d atop each other to form a compact, sandwich-like structure. A channe l crossing one side of the protein molecule accommodates an inhibitor, 3,4-epoxybutyl beta-D-cellobioside, which binds covalently to the sid e chain of Glu-105, as seen in a crystal structure analysis at 2.8-ang strom resolution of the protein-inhibitor complex (R = 16.8%). That Gl u-105 may be indispensible for enzyme catalysis by H(A16-M) is suggest ed by site-directed mutagenesis of this residue, which inevitably lead s to an inactive enzyme.