Crystal Structures and Coordination Behavior of Aqua- and Cyano-Co(III) Tetradehydrocorrins in the Heme Pocket of Myoglobin
Crystal Structures and Coordination Behavior of Aqua- and Cyano-Co(III) Tetradehydrocorrins in the Heme Pocket of Myoglobin
复制标题
肌红蛋白血红素袋中水-和氰基-Co(III)四氢可林的晶体结构和配位行为
DOI：
10.1021/acs.inorgchem.5b02598
复制
发表时间：
2016
影响因子：
4.6
通讯作者：
T. Hayashi
中科院分区：
文献类型：
--
作者：
Y. Morita;K. Oohora;E. Mizohata;A. Sawada;T. Kamachi;K. Yoshizawa;T. Inoue;T. Hayashi
制备了水和氰基四氢考林重组肌红蛋白，分别为CoIII(OH2)(TDHC)和人民币(CoIII(CN)(TDHC))，并作为钴胺依赖酶的模型进行了研究。前者是用K3[Fe(CN)6]氧化RMB(CoII(TDHC))得到的。后一种蛋白质中的氰基配位Co(III)物种是通过加入KCN后与外源氰化物进行配体交换而形成的。X射线结晶学研究表明，在1.20和1.40？分辨率下，人民币(CoIII(OH)(TDHC))和人民币(CoIII(CN)(TDHC))分别具有六配位结构。测定了人民币(CoIII(CN)(TDHC))中氰化物的~(13)CNMR化学位移为108.6和110.6 ppm。红外光谱测量表明，氰化物(CoIII(CN)(TDHC))在2151 cm-1处有一个伸缩频率峰，比氰基钴胺的伸缩频率高。~(13)C核磁共振和红外光谱测量表明，与维生素B12的衍生物钴胺相比，氰化物与CoIII(TDHC)的配位作用较弱。因此，钴离子对氰离子的π回馈程度较低(CoIII(CN)(TDHC))。此外，通过pH滴定实验测得人民币(CoIII(OH2)(TDHC))和人民币(CoIII(CN)(TDHC))的pK1/2分别为3.2和5.5，表明氰基连接削弱了Co-N(His93)键。理论计算还表明，从水到氰化物的轴向配体交换延长了Co-N(轴向)键，并降低了键的解离能。综上所述，肌红蛋白中的氰基钴(III)四氢考林是甲钴胺的结构类似物，甲钴胺是蛋氨酸合成酶反应的关键中间体。
Myoglobins reconstituted with aqua- and cyano-Co(III) tetradehydrocorrins, rMb(CoIII(OH2)(TDHC)) and rMb(CoIII(CN)(TDHC)), respectively, were prepared and investigated as models of a cobalamin-dependent enzyme. The former protein was obtained by oxidation of rMb(CoII(TDHC)) with K3[Fe(CN)6]. The cyanide-coordinated Co(III) species in the latter protein was prepared by ligand exchange of rMb(CoIII(OH2)(TDHC)) with exogenous cyanide upon addition of KCN. The X-ray crystallographic study reveals the hexacoordinated structures of rMb(CoIII(OH)(TDHC)) and rMb(CoIII(CN)(TDHC)) at 1.20 and 1.40 Å resolution, respectively. The13C NMR chemical shifts of the cyanide in rMb(CoIII(CN)(TDHC)) were determined to be 108.6 and 110.6 ppm. IR measurements show that the cyanide of rMb(CoIII(CN)(TDHC)) has a stretching frequency peak at 2151 cm–1which is higher than that of cyanocobalamin. The13C NMR and IR measurements indicate weaker coordination of the cyanide to CoIII(TDHC) relative to cobalamin, a vitamin B12derivative. Thus, the extent of π-back-donation from the cobalt ion to the cyanide ion is lower in rMb(CoIII(CN)(TDHC)). Furthermore, the pK1/2values of rMb(CoIII(OH2)(TDHC)) and rMb(CoIII(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.