Shokhireva Tatiana Kh, Weichsel Andrzej, Smith Kevin M, Berry Robert E, Shokhirev Nikolai V, Balfour Celia A, Zhang Hongjun, Montfort William R, Walker F Ann
Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona 85721, USA.
Inorg Chem. 2007 Mar 19;46(6):2041-56. doi: 10.1021/ic061408l. Epub 2007 Feb 10.
In this work we report the assignment of the majority of the ferriheme resonances of low-spin nitrophorins (NP) 1 and 4 and compare them to those of NP2, published previously. It is found that the structure of the ferriheme complexes of NP1 and NP4, in terms of the orientation of the ligand(s), can be determined with good accuracy by NMR techniques in the low-spin forms and that angle plots proposed previously (Shokhirev, N. V.; Walker, F. A. J. Biol. Inorg. Chem. 1998, 3, 581-594) describe the angle of the effective nodal plane of the axial ligands in solution. The effective nodal plane of low-spin NP1, NP4, and NP2 complexes is in all cases of imidazole and histamine complexes quite similar to the average of the His-59 or -57 and the exogenous ligand angles seen in the X-ray crystal structures. For the cyanide complexes of the nitrophorins, however, the effective nodal plane of the axial ligand does not coincide with the actual histidine-imidazole plane orientation. This appears to be a result of the contribution of an additional source of asymmetry, the orientation of one of the zero-ruffling lines of the heme. Probably this effect exists for the imidazole and histamine complexes as well, but because the effect of asymmetry that occurs from planar exogenous axial ligands is much larger than the effect of heme ruffling the effect of the zero-ruffling line can only be detected for the cyanide complexes, where the only ligand plane is that of the proximal histidine. The three-dimensional structures of the three NP-CN complexes, including that of NP2-CN reported herein, confirm the high degree of ruffling of these complexes. There is an equilibrium between the two heme orientations (A and B) that depends on the heme cavity shape and changes somewhat with exogenous axial ligand. The A:B ratio can be much more accurately measured by NMR spectroscopy than by X-ray crystallography.
在本研究中，我们报道了低自旋硝普蛋白（NP）1和4的大多数高铁血红素共振峰的归属，并将它们与先前发表的NP2的共振峰进行了比较。结果发现，就配体的取向而言，NP1和NP4的高铁血红素配合物的结构可以通过低自旋形式的核磁共振技术以较高的准确度确定，并且先前提出的角度图（Shokhirev，N. V.；Walker，F. A. J. Biol. Inorg. Chem. 1998，3，581 - 594）描述了溶液中轴向配体有效节面的角度。在咪唑和组胺配合物的所有情况下，低自旋NP1、NP4和NP2配合物的有效节面与X射线晶体结构中His - 59或 - 57以及外源性配体角度的平均值非常相似。然而，对于硝普蛋白的氰化物配合物，轴向配体的有效节面与实际的组氨酸 - 咪唑平面取向不一致。这似乎是由于另一个不对称来源的贡献，即血红素的一条零褶皱线的取向。可能这种效应在咪唑和组胺配合物中也存在，但由于平面外源性轴向配体产生的不对称效应远大于血红素褶皱效应，所以只有在氰化物配合物中才能检测到零褶皱线的效应，因为唯一的配体平面是近端组氨酸的平面。本文报道的三种NP - CN配合物（包括NP2 - CN）的三维结构证实了这些配合物的高度褶皱。两种血红素取向（A和B）之间存在平衡，这取决于血红素腔的形状，并随外源性轴向配体略有变化。通过核磁共振光谱法比通过X射线晶体学能够更准确地测量A:B比值。
