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

Yoshitsugu Morita^†, Koji Oohora^†‡, Eiichi Mizohata^†, Akiyoshi Sawada^§, Takashi Kamachi^§, Kazunari Yoshizawa^§∥, Tsuyoshi Inoue^†, and Takashi Hayashi^*†

^† 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

^§ 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

Inorg. Chem., Article ASAP

DOI: 10.1021/acs.inorgchem.5b02598

Publication Date (Web): January 13, 2016

Copyright © 2016 American Chemical Society
http://pubs.acs.org/doi/10.1021/acs.inorgchem.5b02598

 Synopsis

Myoglobins reconstituted with aqua- and cyano-Co(III) tetradehydrocorrins were prepared as cobalamin models. X-ray crystal structure analyses clearly reveal that the cobalt complexes each form a hexacoordinated structure with axial ligation by His93 and an exogenous molecule in the heme pocket. Acid titration, ¹³C NMR, and IR data indicate that both axial bonds, Co−CN and Co−N(His), are weakened upon replacement of water with cyanide. This finding is supported by DFT calculations.

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 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.