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Crystal structure of aqua-cobalt(III) tetradehydrocorrin in the heme pocket of horse heart myoglobin
Structural highlights
FunctionMYG_HORSE Serves as a reserve supply of oxygen and facilitates the movement of oxygen within muscles. Publication Abstract from PubMedMyoglobins 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 A 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 pi-back-donation from the cobalt ion to the cyanide ion is lower in rMb(Co(III)(CN)(TDHC)). Furthermore, the pK1/2 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. Crystal Structures and Coordination Behavior of Aqua- and Cyano-Co(III) Tetradehydrocorrins in the Heme Pocket of Myoglobin.,Morita Y, Oohora K, Mizohata E, Sawada A, Kamachi T, Yoshizawa K, Inoue T, Hayashi T Inorg Chem. 2016 Feb 1;55(3):1287-95. doi: 10.1021/acs.inorgchem.5b02598. Epub, 2016 Jan 13. PMID:26760442[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. See AlsoReferences
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