title: Electron transfer chain reaction of the extracellular flavocytochrome cellobiose dehydrogenase from the basidiomycete Phanerochaete chrysosporium.
abstract: Cellobiose dehydrogenase (CDH) is an extracellular flavocytochrome containing flavin and b-type heme, and plays a key role in cellulose degradation by filamentous fungi. To investigate intermolecular electron transfer from CDH to cytochrome c, Phe166, which is located in the cytochrome domain and approaches one of propionates of heme, was mutated to Tyr, and the thermodynamic and kinetic properties of the mutant (F166Y) were compared with those of the wild-type (WT) enzyme. The mid-point potential of heme in F166Y was measured by cyclic voltammetry, and was estimated to be 25 mV lower than that of WT at pH 4.0. Although presteady-state reduction of flavin was not affected by the mutation, the rate of subsequent electron transfer from flavin to heme was halved in F166Y. When WT or F166Y was reduced with cellobiose and then mixed with cytochrome c, heme re-oxidation and cytochrome c reduction occurred synchronously, suggesting that the initial electron is transferred from reduced heme to cytochrome c. Moreover, in both enzymes the observed rate of the initial phase of cytochrome c reduction was concentration dependent, whereas the second phase of cytochrome c reduction was dependent on the rate of electron transfer from flavin to heme, but not on the cytochrome c concentration. In addition, the electron transfer rate from flavin to heme was identical to the steady-state reduction rate of cytochrome c in both WT and F166Y. These results clearly indicate that the first and second electrons of two-electron-reduced CDH are both transferred via heme, and that the redox reaction of CDH involves an electron-transfer chain mechanism in cytochrome c reduction.
authors.affiliation: Department of Biomaterials Sciences, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Japan. aquarius@mail.ess.u-tokyo.ac.jp
authors.forename: Kiyohiko
authors.initials: K
authors.lastname: Igarashi
authors.forename: Makoto
authors.initials: M
authors.lastname: Yoshida
authors.forename: Hirotoshi
authors.initials: H
authors.lastname: Matsumura
authors.forename: Nobuhumi
authors.initials: N
authors.lastname: Nakamura
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Ohno
authors.forename: Masahiro
authors.initials: M
authors.lastname: Samejima
authors.forename: Takeshi
authors.initials: T
authors.lastname: Nishino
mesh_terms: D002237:Carbohydrate Dehydrogenases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D002475:Cellobiose / Q000378:metabolism; D045304:Cytochromes c / Q000737:chemistry / Q000378:metabolism; D004579:Electron Transport; D005182:Flavin-Adenine Dinucleotide / Q000737:chemistry*; D005415:Flavins / Q000737:chemistry* / Q000378:metabolism; D005656:Fungal Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D006418:Heme / Q000737:chemistry* / Q000378:metabolism; D007700:Kinetics; D009154:Mutation; D010084:Oxidation-Reduction; D020075:Phanerochaete / Q000737:chemistry / Q000201:enzymology*; D013816:Thermodynamics
chemical_list: D005415:Flavins; D005656:Fungal Proteins; D005182:Flavin-Adenine Dinucleotide; D002475:Cellobiose; D006418:Heme; D045304:Cytochromes c; D002237:Carbohydrate Dehydrogenases; C019859:cellobiose-quinone oxidoreductase