title: Crystal structure of the catalytic core domain of the family 6 cellobiohydrolase II, Cel6A, from Humicola insolens, at 1.92 A resolution.
abstract: 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 A. 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. reesei equivalent. The structure features both O- (alpha-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.
authors.affiliation: Structural Biology Laboratory, Department of Chemistry, University of York, Heslington, York Y01 5DD, U.K.
authors.forename: A
authors.initials: A
authors.lastname: Varrot
authors.forename: S
authors.initials: S
authors.lastname: Hastrup
authors.forename: M
authors.initials: M
authors.lastname: Schülein
authors.forename: G J
authors.initials: GJ
authors.lastname: Davies
mesh_terms: D020134:Catalytic Domain; D002480:Cellulase / Q000737:chemistry*; D043366:Cellulose 1,4-beta-Cellobiosidase; D003198:Computer Simulation; D018360:Crystallography, X-Ray; D003904:Mitosporic Fungi / Q000201:enzymology*; D008956:Models, Chemical; D008958:Models, Molecular; D011487:Protein Conformation; D015203:Reproducibility of Results; D013045:Species Specificity; D014242:Trichoderma / Q000201:enzymology
chemical_list: D002480:Cellulase; D043366:Cellulose 1,4-beta-Cellobiosidase