title: Crystal Structures and Coordination Behavior of Aqua- and Cyano-Co(III) Tetradehydrocorrins in the Heme Pocket of Myoglobin.
abstract: Myoglobins reconstituted with aqua- and cyano-Co(III) tetradehydrocorrins, rMb(Co(III)(OH2)(TDHC)) and rMb(Co(III)(CN)(TDHC)), respectively, were prepared and investigated as models of a cobalamin-dependent enzyme. The former protein was obtained by oxidation of rMb(Co(II)(TDHC)) with K3[Fe(CN)6]. The cyanide-coordinated Co(III) species in the latter protein was prepared by ligand exchange of rMb(Co(III)(OH2)(TDHC)) with exogenous cyanide upon addition of KCN. The X-ray crystallographic study reveals the hexacoordinated structures of rMb(Co(III)(OH)(TDHC)) and rMb(Co(III)(CN)(TDHC)) at 1.20 and 1.40 Å resolution, respectively. The (13)C NMR chemical shifts of the cyanide in rMb(Co(III)(CN)(TDHC)) were determined to be 108.6 and 110.6 ppm. IR measurements show that the cyanide of rMb(Co(III)(CN)(TDHC)) has a stretching frequency peak at 2151 cm(-1) which is higher than that of cyanocobalamin. The (13)C NMR and IR measurements indicate weaker coordination of the cyanide to Co(III)(TDHC) relative to cobalamin, a vitamin B12 derivative. Thus, the extent of π-back-donation from the cobalt ion to the cyanide ion is lower in rMb(Co(III)(CN)(TDHC)). Furthermore, the pK(1/2) values of rMb(Co(III)(OH2)(TDHC)) and rMb(Co(III)(CN)(TDHC)) were determined by a pH titration experiment to be 3.2 and 5.5, respectively, indicating that the cyanide ligation weakens the Co-N(His93) bond. Theoretical calculations also demonstrate that the axial ligand exchange from water to cyanide elongates the Co-N(axial) bond with a decrease in the bond dissociation energy. Taken together, the cyano-Co(III) tetradehydrocorrin in myoglobin is appropriate for investigation as a structural analogue of methylcobalamin, a key intermediate in methionine synthase reaction.
authors.affiliation: Department of Applied Chemistry, Graduate School of Engineering, Osaka University , Suita 565-0871, Japan.
authors.forename: Yoshitsugu
authors.initials: Y
authors.lastname: Morita
authors.affiliation: Department of Applied Chemistry, Graduate School of Engineering, Osaka University , Suita 565-0871, Japan.|Frontier Research Base for Global Young Researchers, Graduate School of Engineering, Osaka University , Suita 565-0871, Japan.
authors.forename: Koji
authors.initials: K
authors.lastname: Oohora
authors.affiliation: Department of Applied Chemistry, Graduate School of Engineering, Osaka University , Suita 565-0871, Japan.
authors.forename: Eiichi
authors.initials: E
authors.lastname: Mizohata
authors.affiliation: Institute for Materials Chemistry and Engineering and International Research Center for Molecular Systems, Kyushu University , Nishi-ku, Fukuoka 819-0395, Japan.
authors.forename: Akiyoshi
authors.initials: A
authors.lastname: Sawada
authors.affiliation: Institute for Materials Chemistry and Engineering and International Research Center for Molecular Systems, Kyushu University , Nishi-ku, Fukuoka 819-0395, Japan.
authors.forename: Takashi
authors.initials: T
authors.lastname: Kamachi
authors.affiliation: Institute for Materials Chemistry and Engineering and International Research Center for Molecular Systems, Kyushu University , Nishi-ku, Fukuoka 819-0395, Japan.|Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University , Nishikyo-ku, Kyoto 615-8520, Japan.
authors.forename: Kazunari
authors.initials: K
authors.lastname: Yoshizawa
authors.affiliation: Department of Applied Chemistry, Graduate School of Engineering, Osaka University , Suita 565-0871, Japan.
authors.forename: Tsuyoshi
authors.initials: T
authors.lastname: Inoue
authors.affiliation: Department of Applied Chemistry, Graduate School of Engineering, Osaka University , Suita 565-0871, Japan.
authors.forename: Takashi
authors.initials: T
authors.lastname: Hayashi
mesh_terms: D003035:Cobalt / Q000737:chemistry*; D018360:Crystallography, X-Ray; D003486:Cyanides / Q000737:chemistry*; D006418:Heme / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D008665:Metalloporphyrins / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D009211:Myoglobin / Q000737:chemistry*; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D014867:Water / Q000737:chemistry*
chemical_list: D003486:Cyanides; D008665:Metalloporphyrins; D009211:Myoglobin; D009942:Organometallic Compounds; D014867:Water; D003035:Cobalt; D006418:Heme