STRUCTURE OF THE BACILLUS-AGARADHERANS FAMILY-5 ENDOGLUCANASE AT 1.6 ANGSTROM AND ITS CELLOBIOSE COMPLEX AT 2.0 ANGSTROM RESOLUTION

The enzymatic degradation of cellulose, by cellulases, is not only ind ustrially important in the food, paper, and textile industries but als o a potentially useful method for the environmentally friendly recycli ng of municipal waste, An understanding of the structural and mechanis tic requirements for the hydrolysis of the beta-1,4 glycosidic bonds o f cellulose is an essential prerequisite for beneficial engineering of cellulases for these processes, Cellulases have been classified into 13 of the 62 glycoside hydrolase families [Henrissat, B., and Bairoch, A. (1996) Biochem J. 316, 695-696]. The structure of the catalytic co re of the family 5 endoglucanase, Ce15A, from the alkalophilic Bacillu s agaradherans has been solved by multiple isomorphous replacement at 1.6 Angstrom resolution. Cel5A has the (alpha/beta)(8) barrel structur e and signature structural features typical of the grouping of glycosi de hydrolase families known as dan GH-A, with the catalytic acid/base Glu 139 and nudeophile Glu 228 on barrel strands beta 4 and beta 7 as expected, In addition to the native enzyme, the 2.0 Angstrom resolutio n structure of the cellobiose-bound form of the enzyme has also been d etermined, Cellobiose binds preferentially in the -2 and -3 subsites o f the enzyme. Kinetic studies on the isolated catalytic core domain of Cel5A, using a series of reduced cellodextrins as substrates, suggest approximately five to six binding sites, consistent with the shape an d size of the cleft observed by crystallography.