beta-1,4-glycanases of Cellulomonas fimi: Families, mechanisms, and kinetics

Four endoglucanases, two cellobiohydrolases and a mixed function exoglucanase-xylanase from Cellulomonas fimi are modular proteins comprising from two to six domains. All of them contain a catalytic domain (CD) and at least one cellulose-binding domain (CBD). The CDs come from five of the families of glycoside hydrolases; the CBDs from three of the families of CBDs, although all but one of the enzymes has a CBD from family II. The two cellobiohydrolases attack cellulose molecules from opposite ends. The CDs and the CBDs function independently of each other when separated by proteolysis or genetic engineering. The enzymes interact with cellulose in two ways. The CDs have weak affinity for substrate, relative to the CBDs, and catalyze hydrolysis of glycosidic bonds with inversion or retention of anomeric configuration, depending on the CD. The CBDs have much greater affinities for cellulose, with K-a values of the order of 0.5-1.0 mu M for the family II CBDs. The family II CBDs adsorb to both crystalline and amorphous cellulose; the family IV CBD from endoglucanase CenC adsorbs to amorphous but not to crystalline CBDCex from the exoglucanase-xylanase Cex, is a beta-barrel in with extensive beta-sheet structure; three tryptophans, which participate in binding to cellulose, are adjacent in space and exposed on the surface of the beta-barrel. Adsorption of CBDCex to crystalline cellulose is entropically driven. Although CBDCex appears to bind irreversibly, the binding is dynamic and the polypeptide is mobile on the cellulose surface.