Beem K M, Richardson D C, Rajagopalan K V
Biochemistry. 1977 May 3;16(9):1930-6. doi: 10.1021/bi00628a027.
Silver-copper and silver-cobalt proteins have been prepared in which Ag+ resides in the native copper site of superoxide dismutase and either Cu2+ of Co2+ reside in the zinc site. The electron paramagnetic resonance (EPR) spectrum of the copper and the visible absorption spectrum of the cobalt greatly resemble those of either Cu4 of Cu2,Cu2,Co2 proteins, respectively, in which the copper of the native copper sites has been reduced. It was found that, unlike cyanide, azide anion would not perturb the EPR spectrum of Ag2,Cu2 protein. Since azide produces the same perturbation upon the EPR spectrum of native and Cu2 proteins, it must bind to the copper and not the zinc of superoxide dismutase. A model of the metal sites of the enzyme has been fitted to a 3-A electron-density map using an interactive molecular graphics display. The model shows that histidine-61, which appears to bind both copper and zinc, does not lie in the plane of the copper and its three other histidine ligands, but occupies a position intermediate between planar and axial. This feature probably accounts for the rhombicity of the EPR spectrum and the activity of the enzyme.
已制备出银 - 铜和银 - 钴蛋白，其中Ag⁺位于超氧化物歧化酶的天然铜位点，而Cu²⁺或Co²⁺位于锌位点。铜的电子顺磁共振（EPR）谱和钴的可见吸收谱分别与Cu⁴、Cu²、Cu²、Co²蛋白的谱极为相似，在这些蛋白中天然铜位点的铜已被还原。结果发现，与氰化物不同，叠氮阴离子不会干扰Ag₂、Cu₂蛋白的EPR谱。由于叠氮化物对天然蛋白和Cu²蛋白的EPR谱产生相同的干扰，它必定是与超氧化物歧化酶的铜而非锌结合。利用交互式分子图形显示，已将该酶的金属位点模型拟合到一个3埃的电子密度图上。该模型表明，似乎同时结合铜和锌的组氨酸 - 61并不位于铜及其其他三个组氨酸配体所在的平面内，而是占据一个介于平面和轴向之间的位置。这一特征可能解释了EPR谱的菱形度以及该酶的活性。
