Beinert H, Emptage M H, Dreyer J L, Scott R A, Hahn J E, Hodgson K O, Thomson A J
Proc Natl Acad Sci U S A. 1983 Jan;80(2):393-6. doi: 10.1073/pnas.80.2.393.
Beef heart aconitase contains 3Fe clusters in its inactive and 4Fe clusters in its active form. The fully active form can be restored from the inactive one by insertion of Fe(2+), whereas S(2-) is not required. Chemical analyses for iron and labile sulfide yield Fe/S(2-) ratios of 0.66-0.74 for the inactive and 0.90-1.03 for the active form. Sulfane sulfur (S(0)) was not detected. We propose on the basis of these data that the inactive form may arise from the active one by loss of one iron only per cluster with the sulfur remaining as S(2-) in a [3Fe-4S] structure. Measurements by extended x-ray absorption fine structure (EXAFS) spectroscopy on the 3Fe form of aconitase yield a Fe..S distance of 2.24 A and a Fe..Fe distance of 2.71 A. This Fe..Fe distance is in agreement with that obtained by EXAFS on ferredoxin II of Desulfovibrio gigas, another 3Fe protein, but disagrees with Fe..Fe distances observed for the 3Fe cluster of Azotobacter vinelandii ferredoxin I by x-ray diffraction-namely, 4.1 A. We suggest that this difference may be due to the presence of a [3Fe-3S] structure in the Azotobacter ferredoxin I crystals vs. a [3Fe-4S] structure in liquid or frozen solutions of aconitase. The [3Fe-3S] cluster has been shown to have a relatively flat twist-boat structure, whereas a [3Fe-4S] cluster could be expected to essentially maintain the compact structure of the [4Fe-4S] cluster. This would explain the differences in Fe..Fe distances. Two possible structural models for a [3Fe-4S] cluster are discussed.
牛心乌头酸酶在其无活性形式中含有3Fe簇，在其活性形式中含有4Fe簇。通过插入Fe(2+)可使完全活性形式从无活性形式恢复，而不需要S(2-)。对铁和不稳定硫化物的化学分析得出，无活性形式的Fe/S(2-)比率为0.66 - 0.74，活性形式的比率为0.90 - 1.03。未检测到硫烷硫(S(0))。基于这些数据，我们提出无活性形式可能是由活性形式通过每个簇仅损失一个铁而产生的，硫以S(2-)的形式保留在[3Fe - 4S]结构中。通过扩展X射线吸收精细结构(EXAFS)光谱对乌头酸酶的3Fe形式进行测量，得到Fe..S距离为2.24 Å，Fe..Fe距离为2.71 Å。这个Fe..Fe距离与通过EXAFS对巨大脱硫弧菌的铁氧化还原蛋白II（另一种3Fe蛋白）得到的结果一致，但与通过X射线衍射观察到的棕色固氮菌铁氧化还原蛋白I的3Fe簇的Fe..Fe距离（即4.1 Å）不同。我们认为这种差异可能是由于棕色固氮菌铁氧化还原蛋白I晶体中存在[3Fe - 3S]结构，而乌头酸酶的液体或冷冻溶液中存在[3Fe - 4S]结构。[3Fe - 3S]簇已被证明具有相对扁平的扭曲船型结构，而[3Fe - 4S]簇预计基本上保持[4Fe - 4S]簇的紧凑结构。这将解释Fe..Fe距离的差异。讨论了[3Fe - 4S]簇的两种可能结构模型。
