The three-dimensional structure of the catalytic core of the family 6 cellobiohydrolase II, Cel6A (CBH II), from Humicola insolens has been determined by X-ray crystallography at a resolution of 1.92 Å. The structure was solved by molecular replacement using the homologous Trichoderma reesei CBH II as a search model. The H. insolens enzyme displays a high degree of structural similarity with its T. reeseiequivalent. The structure features both O- (α-linked mannose) and N-linked glycosylation and a hexa-co-ordinate Mg2+ ion. The active-site residues are located within the enclosed tunnel that is typical for cellobiohydrolase enzymes and which may permit a processive hydrolysis of the cellulose substrate. The close structural similarity between the two enzymes implies that kinetics and chain-end specificity experiments performed on the H. insolens enzyme are likely to be applicable to the homologous T. reesei enzyme. These cast doubt on the description of cellobiohydrolases as exo-enzymes since they demonstrated that Cel6A (CBH II) shows no requirement for non-reducing chain-ends, as had been presumed. There is no crystallographic evidence in the present structure to support a mechanism involving loop opening, yet preliminary modelling experiments suggest that the active-site tunnel of Cel6A (CBH II) is too narrow to permit entry of a fluorescenyl-derivatized substrate, known to be a viable substrate for this enzyme.
Co-ordinates for the structure described in this paper have been deposited with the Brookhaven Protein Data Bank with accession code 1BVW.PDB.