A SPECTROSCOPIC INVESTIGATION OF THE STRUCTURE AND REDOX PROPERTIES OF ESCHERICHIA-COLI CYTOCHROME B-562
A SPECTROSCOPIC INVESTIGATION OF THE STRUCTURE AND REDOX PROPERTIES OF ESCHERICHIA-COLI CYTOCHROME B-562
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DOI：
10.1016/0167-4838(85)90071-8
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发表时间：
1985-01-01
期刊：
影响因子：
--
通讯作者：
MATHEWS, FS
中科院分区：
文献类型：
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作者：
MOORE, GR;WILLIAMS, RJP;MATHEWS, FS
E.大肠杆菌表现出2血红素连接的pH值依赖性转换检测NMR和光学光谱。这些跃迁中只有一个，即Fe（III）配位的His-102的跃迁，可以通过EPR和MCD [磁性圆二色性]检测到;血红素丙酸盐的电离则不能。这两种电离都是氧化还原状态依赖性的，并且蛋白质的中点氧化还原电位显著依赖于pH。在5.0-8.5的pH范围内，电位从260 mV下降到110 mV，至少有5次单质子电离。除了2种光谱鉴定的铁细胞色素离子化外，至少有3种未鉴定的离子化，其中2种发生在亚铁蛋白中。从X射线结构的考虑，连同NMR数据，似乎可能的是，这些电离中至少有一个涉及氨基酸羧酸盐。X射线结构还表明，His-102相对较低的pKa是其接近Arg-98的结果。这些基团之间的明显相互作用要求溶液构象与X射线结构略有不同。电子自交换速率快，超过4.106M-1·cm-1。s ~（-1），这可能反映了如下事实：如X射线结构所示，大量的血红素和轴向组氨酸配体暴露在分子表面，周围的带电基团呈不对称分布。
The 6-coordinate monohaem ferricytochrome b-562 from E. coli exhibits 2 heme-linked pH-dependent transitions detected by NMR and optical spectroscopy. Only 1 of these transitions, that of the Fe(III)-coordinated His-102, is detected by EPR and MCD [magnetic circular dichroism]; the ionization of a heme propionate is not. Both ionizations are redox-state-dependent and the midpoint redox potential of the protein is markedly pH-dependent. Over the pH range 5.0-8.5 the potential drops from 260 mV to 110 mV and at least 5 single proton ionizations are responsible for this. In addition to the 2 spectroscopically identified ferricytochrome ionizations, there are at least 3 unidentified ionizations, 2 of which occur in the ferrous protein. From a consideration of the X-ray structure, together with NMR data, it seems probable that at least 1 of these ionizations involves an amino acid carboxylate. The X-ray structure also suggests that the relatively low pKa of His-102 is a result of its proximity to Arg-98. An appreciable interaction between these groups requires that the solution conformation differs slightly from the X-ray structure. The fast rate of electron self-exchange, over 4.106 M-1 .cntdot. s-1 at 315 K and pH* 7, may be a reflection of the fact that, as shown by the X-ray structure, a large amount of the heme and axial histidine ligand are exposed at the molecular surface with an asymmetric distribution of charged groups surrounding them.