title: Structural, biochemical, and computational characterization of the glycoside hydrolase family 7 cellobiohydrolase of the tree-killing fungus Heterobasidion irregulare.
abstract: Root rot fungi of the Heterobasidion annosum complex are the most damaging pathogens in temperate forests, and the recently sequenced Heterobasidion irregulare genome revealed over 280 carbohydrate-active enzymes. Here, H. irregulare was grown on biomass, and the most abundant protein in the culture filtrate was identified as the only family 7 glycoside hydrolase in the genome, which consists of a single catalytic domain, lacking a linker and carbohydrate-binding module. The enzyme, HirCel7A, was characterized biochemically to determine the optimal conditions for activity. HirCel7A was crystallized and the structure, refined at 1.7 Å resolution, confirms that HirCel7A is a cellobiohydrolase rather than an endoglucanase, with a cellulose-binding tunnel that is more closed than Phanerochaete chrysosporium Cel7D and more open than Hypocrea jecorina Cel7A, suggesting intermediate enzyme properties. Molecular simulations were conducted to ascertain differences in enzyme-ligand interactions, ligand solvation, and loop flexibility between the family 7 glycoside hydrolase cellobiohydrolases from H. irregulare, H. jecorina, and P. chrysosporium. The structural comparisons and simulations suggest significant differences in enzyme-ligand interactions at the tunnel entrance in the -7 to -4 binding sites and suggest that a tyrosine residue at the tunnel entrance of HirCel7A may serve as an additional ligand-binding site. Additionally, the loops over the active site in H. jecorina Cel7A are more closed than loops in the other two enzymes, which has implications for the degree of processivity, endo-initiation, and substrate dissociation. Overall, this study highlights molecular level features important to understanding this biologically and industrially important family of glycoside hydrolases.
authors.affiliation: Department of Molecular Biology, Swedish University of Agricultural Sciences, SE-751 24 Uppsala, Sweden.
authors.forename: Majid Haddad
authors.initials: MH
authors.lastname: Momeni
authors.forename: Christina M
authors.initials: CM
authors.lastname: Payne
authors.forename: Henrik
authors.initials: H
authors.lastname: Hansson
authors.forename: Nils Egil
authors.initials: NE
authors.lastname: Mikkelsen
authors.forename: Jesper
authors.initials: J
authors.lastname: Svedberg
authors.forename: Åke
authors.initials: Å
authors.lastname: Engström
authors.forename: Mats
authors.initials: M
authors.lastname: Sandgren
authors.forename: Gregg T
authors.initials: GT
authors.lastname: Beckham
authors.forename: Jerry
authors.initials: J
authors.lastname: Ståhlberg
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D056804:Biofuels; D002480:Cellulase / Q000737:chemistry / Q000378:metabolism; D002482:Cellulose / Q000378:metabolism; D043366:Cellulose 1,4-beta-Cellobiosidase / Q000378:metabolism*; D003198:Computer Simulation; D018360:Crystallography, X-Ray / Q000379:methods; D006026:Glycoside Hydrolases / Q000737:chemistry / Q000378:metabolism / Q000502:physiology*; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D035901:Hypocrea / Q000378:metabolism; D008024:Ligands; D008968:Molecular Conformation; D008969:Molecular Sequence Data; D020075:Phanerochaete / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid; D014197:Trees / Q000382:microbiology*; D014242:Trichoderma / Q000378:metabolism*
chemical_list: D056804:Biofuels; D008024:Ligands; D002482:Cellulose; D006026:Glycoside Hydrolases; D002480:Cellulase; D043366:Cellulose 1,4-beta-Cellobiosidase