title: Structure of the catalytic core module of the Chaetomium thermophilum family GH6 cellobiohydrolase Cel6A.
abstract: Cellulases, including cellobiohydrolases and endoglucanases, are important enzymes involved in the breakdown of the polysaccharide cellulose. These catalysts have found widescale industrial applications, particularly in the paper and textile industries, and are now finding use in `second-generation' conversion of biomass to biofuels. Despite this considerable biotechnological application, and undoubted future potential, uncertainty remains as to the exact reaction mechanism of the inverting cellulases found in the GH6 family of carbohydrate-active enzymes. In order to gain additional understanding as to how these societally beneficial biocatalysts function, the crystal structure of a GH6 cellobiohydrolase from Chaetomium thermophilum, CtCel6A, has been solved. This structure reveals a distorted α/β-barrel fold comprising a buried tunnel-like active site quite typical of Cel6A enzymes. Analysis of an enzyme-product complex (cellobiose in the -3 and -2 subsites and cellotetraose in subsites +1 to +4) supports the hypothesis that this group of enzymes act via an atypical single-displacement mechanism. Of particular note in this analysis is an active-centre metal ion, Li(+), the position of which matches the position of the positively charged anomeric carbon of the oxocarbenium-ion-like transition state.
authors.affiliation: Structural Biology Laboratory, Department of Chemistry, University of York, York YO10 5DD, England.
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Thompson
authors.forename: Tia
authors.initials: T
authors.lastname: Heu
authors.forename: Tarana
authors.initials: T
authors.lastname: Shaghasi
authors.forename: Romil
authors.initials: R
authors.lastname: Benyamino
authors.forename: Aubrey
authors.initials: A
authors.lastname: Jones
authors.forename: Esben P
authors.initials: EP
authors.lastname: Friis
authors.forename: Keith S
authors.initials: KS
authors.lastname: Wilson
authors.forename: Gideon J
authors.initials: GJ
authors.lastname: Davies
mesh_terms: D000465:Algorithms; D000595:Amino Acid Sequence; D056804:Biofuels; D018533:Biomass; D002240:Carbohydrate Sequence; D020134:Catalytic Domain; D002475:Cellobiose / Q000737:chemistry; D002482:Cellulose / Q000737:chemistry; D043366:Cellulose 1,4-beta-Cellobiosidase / Q000737:chemistry*; D002597:Chaetomium / Q000201:enzymology*; D019295:Computational Biology / Q000379:methods; D018360:Crystallography, X-Ray / Q000379:methods; D000431:Ethanol / Q000737:chemistry; D005656:Fungal Proteins / Q000737:chemistry*; D006868:Hydrolysis; D007477:Ions; D008956:Models, Chemical; D015233:Models, Statistical; D008969:Molecular Sequence Data; D011485:Protein Binding; D011487:Protein Conformation; D017386:Sequence Homology, Amino Acid
chemical_list: D056804:Biofuels; D005656:Fungal Proteins; D007477:Ions; D002475:Cellobiose; D000431:Ethanol; D002482:Cellulose; D043366:Cellulose 1,4-beta-Cellobiosidase