title: Catalytic properties and classification of cellobiose dehydrogenases from ascomycetes.
abstract: Putative cellobiose dehydrogenase (CDH) genes are frequently discovered in various fungi by genome sequencing projects. The expression of CDH, an extracellular flavocytochrome, is well studied in white rot basidiomycetes and is attributed to extracellular lignocellulose degradation. CDH has also been reported for plant-pathogenic or saprotrophic ascomycetes, but the molecular and catalytic properties of these enzymes are currently less investigated. This study links various ascomycetous cdh genes with the molecular and catalytic characteristics of the mature proteins and suggests a differentiation of ascomycete class II CDHs into two subclasses, namely, class IIA and class IIB, in addition to the recently introduced class III of hypothetical ascomycete CDHs. This new classification is based on sequence and biochemical data obtained from sequenced fungal genomes and a screening of 40 ascomycetes. Thirteen strains showed CDH activity when they were grown on cellulose-based media, and Chaetomium atrobrunneum, Corynascus thermophilus, Dichomera saubinetii, Hypoxylon haematostroma, Neurospora crassa, and Stachybotrys bisbyi were selected for detailed studies. In these strains, one or two cdh-encoding genes were found that stem either from class IIA and contain a C-terminal carbohydrate-binding module or from class IIB without such a module. In several strains, both genes were found. Regarding substrate specificity, class IIB CDHs show a less pronounced substrate specificity for cellobiose than class IIA enzymes. A pH-dependent pattern of the intramolecular electron transfer was also observed, and the CDHs were classified into three groups featuring acidic, intermediate, or alkaline pH optima. The pH optimum, however, does not correlate with the CDH subclasses and is most likely a species-dependent adaptation to different habitats.
authors.affiliation: Department für Lebensmittelwissenschaften und -technologie, Universität für Bodenkultur, Muthgasse 18/2, A-1190 Vienna, Austria.
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Harreither
authors.forename: Christoph
authors.initials: C
authors.lastname: Sygmund
authors.forename: Manfred
authors.initials: M
authors.lastname: Augustin
authors.forename: Melanie
authors.initials: M
authors.lastname: Narciso
authors.forename: Mikhail L
authors.initials: ML
authors.lastname: Rabinovich
authors.forename: Lo
authors.initials: L
authors.lastname: Gorton
authors.forename: Dietmar
authors.initials: D
authors.lastname: Haltrich
authors.forename: Roland
authors.initials: R
authors.lastname: Ludwig
mesh_terms: D000595:Amino Acid Sequence; D001203:Ascomycota / Q000201:enzymology*; D001483:Base Sequence; D002237:Carbohydrate Dehydrogenases / Q000145:classification* / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D002482:Cellulose / Q000378:metabolism; D003470:Culture Media / Q000737:chemistry; D004271:DNA, Fungal; D007700:Kinetics; D008969:Molecular Sequence Data; D017422:Sequence Analysis, DNA; D013379:Substrate Specificity
chemical_list: D003470:Culture Media; D004271:DNA, Fungal; D002482:Cellulose; D002237:Carbohydrate Dehydrogenases; C019859:cellobiose-quinone oxidoreductase