Structure and function of Humicola insolens family 6 cellulases: structureof the endoglucanase, Cel6B, at 1.6 angstrom resolution

Cellulases are traditionally classified as either endoglucanases or cellobi ohydrolases on the basis of their respective catalytic activities on crysta lline cellulose, which is generally hydrolysed more efficiently only by the cellobiohydrolases. On the basis of the Trichonerma reesei cellobiohydrola se II structure, it was proposed that the active-site tunnel of cellobiohyd rolases permitted the processive hydrolysis of cellulose, whereas the corre sponding endoglucanases would display open active-site clefts [Rouvinen, Be rgfors, Teeri, Knowles and Jones (1990) Science 249, 380-386]. Glycoside hy drolase family 6 contains both cellobiohydrolases and endoglucanases. The s tructure of the catalytic core of the family 6 endoglucanase Cel6B from Hum icola insolens has been solved by molecular replacement with the known T. r eesei cellobiohydrolase II as the search model. Strangely, at the sequence level, this enzyme exhibits the highest sequence similarity to family 6 cel lobiohydrolases and displays just one of the loop deletions traditionally a ssociated with endoglucanases in this family. However, this enzyme shows no activity on crystalline substrates but a high activity on soluble substrat es, which is typical of an endoglucanase. The three-dimensional structure r eveals that the deletion of just a single loop of the active site, coupled with the resultant conformational change in a second 'cellobiohydrolase-spe cific' loop, peels open the active-site tunnel to reveal a substrate-bindin g groove.