Novel redox chemistry of [3Fe-4S] clusters: Electrochemical characterization of the all-Fe(II) form of the [3Fe-4S] cluster generated reversibly in various proteins and its spectroscopic investigation in Sulfolobus acidocaldarius ferredoxin
Novel redox chemistry of [3Fe-4S] clusters: Electrochemical characterization of the all-Fe(II) form of the [3Fe-4S] cluster generated reversibly in various proteins and its spectroscopic investigation in Sulfolobus acidocaldarius ferredoxin
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DOI：
10.1021/ja961465l
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发表时间：
1996-09-11
影响因子：
15
通讯作者：
Thomson, AJ
中科院分区：
文献类型：
--
作者：
Duff, JLC;Breton, JLJ;Thomson, AJ
通过电化学、EPR、MCD和UV/Vis光谱表征了这种新型的蛋白质结合的[3Fe-4S]团簇，它是比正常还原形式[3Fe-4S](0)低两个电子等价体，因此形式上完全由Fe(II)亚位组成。研究了一系列蛋白质的[3Fe-4S](0)的两电子还原反应，特别是来自酸性硫杆菌、非洲脱硫弧菌和棕色固氮菌的7Fe铁氧还蛋白。在每种情况下，反应在化学上是可逆的，产物令人惊讶地惰性，并且在pH值为7时，pH依赖的还原电位在-700 mV与SHE的范围内，无论蛋白质的特性如何。三种不同铁氧还蛋白在较宽的pH范围内的蛋白质膜伏安研究表明，新物种[3Fe-4S](2-)是由[3Fe-4S](0)的两电子协同还原形成的，相对于全铁(III)态[3Fe-4S](1+)有三个质子的净吸收。质子可能结合在簇上或靠近簇上(这是由于强烈的pH依赖性和对蛋白质宿主的不敏感性)，但尽管[3Fe-4S]2-形成了负势，H-2并没有析出。酸化铁氧化还蛋白在溶液中通过四电子电化学还原可逆地产生超还原物种，在可见光区几乎没有吸收，并在400 nm以下呈现与FeO Rubredox相似的MCD光谱。四电子还原蛋白质的EPR谱与正常两电子还原蛋白质的EPR谱明显不同([3Fe-4S](0)，[4Fe-4S](1+))；归属于[3Fe-4S](0)的g=12处的信号消失，g=1.94区的光谱发生变化，这可归因于与[4Fe-4S](+)团簇自旋耦合的变化。作为执行两电子氧化还原反应的全铁(II)和(可能)质子化物种，[3Fe-4S](2-)代表了迄今仍难以捉摸的铁-硫团簇化学的基本实体。考虑了这种反应性的结构和功能影响。
The novel ''hyper-reduced'' form of protein-bound [3Fe-4S] clusters, which is two electron equivalents below the normal reduced form [3Fe-4S](0) and thus formally composed entirely of Fe(II) subsites, has been characterized by electrochemistry and by EPR, MCD, and UV/visible spectroscopy. The two-electron reduction of [3Fe-4S](0) has been studied for a range of proteins, in particular the 7Fe ferredoxins from Sulfolobus acidocaldarius, Desulfovibrio africanus, and Azotobacter vinelandii. In each case, the reaction is chemically reversible, the product is surprisingly inert, and the pH-dependent reduction potential is in the region of -700 mV vs SHE at pH 7, regardless of the identity of the protein. Protein film voltammetry of three different ferredoxins investigated in detail over a wide pH range shows that the novel species denoted as [3Fe-4S](2-) is formed by a cooperative two-electron reduction of [3Fe-4S](0) and there is a net uptake of three protons relative to the all-Fe(III) state [3Fe-4S](1+). The protons are probably bound at or close to the cluster (accounting for the strong pH dependence and insensitivity to protein host), but H-2 is not evolved despite the negative potential at which [3Fe-4S]2- is formed. The hyper-reduced species which is produced reversibly in solution by four-electron electrochemical reduction of the 7Fe ferredoxin from Sulfolobus acidocaldarius contributes little absorbance in the visible spectral region, and shows an MCD spectrum with transitions below 400 nm that resemble features observed for FeO rubredoxin. The EPR spectrum of the four-electron reduced protein differs significantly from that of the normal two-electron reduced form ([3Fe-4S](0), [4Fe-4S](1+)); the signal at g = 12 assigned to [3Fe-4S](0) disappears and changes occur to the spectrum in the g = 1.94 region which can be attributed to alterations in spin coupling with the [4Fe-4S](+) cluster. As an all-Fe(II) and (probably) protonated species performing two-electron redox reactions, [3Fe-4S](2-) represents a fundamental entity of iron-sulfur cluster chemistry that has so far remained elusive. Structural and functional implications of this reactivity are considered.