Crystal Structures and Coordination Behavior of Aqua- and Cyano-Co(III) Tetradehydrocorrins in the Heme Pocket of Myoglobin

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• 作者：Yoshitsugu Morita ; Koji Oohora ; Eiichi Mizohata ; Akiyoshi Sawada ; Takashi Kamachi ; Kazunari Yoshizawa ; Tsuyoshi Inoue ; Takashi Hayashi
• 刊名：Inorganic Chemistry
• 出版年：2016
• 出版时间：February 1, 2016
• 年：2016
• 卷：55
• 期：3
• 页码：1287-1295
• 全文大小：526K
• ISSN：1520-510X

文摘

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 ? resolution, respectively. The ¹³C 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 ¹³C 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.