Pierik A J, Hagen W R
Department of Biochemistry, Agricultural University, Wageningen, The Netherlands.
Eur J Biochem. 1991 Jan 30;195(2):505-16. doi: 10.1111/j.1432-1033.1991.tb15731.x.
Sulfite reductases contain siroheme and iron-sulfur cluster prosthetic groups. The two groups are believed to be structurally linked via a single, common ligand. This chemical model is based on a magnetic model for the oxidized enzyme in which all participating iron ions are exchange coupled. This description leads to two serious discrepancies. Although the iron-sulfur cluster is assumed to be a diamagnetic cubane, [4Fe-4S]2+, all iron appears to be paramagnetic in Mössbauer spectroscopy. On the other hand, EPR spectroscopy has failed to detect anything but a single high-spin heme. We have re-addressed this problem by searching for new EPR spectroscopic clues in concentrated samples of dissimilatory sulfite reductase from Desulfovibrio vulgaris (Hildenborough). We have found several novel signals with effective g values of 17, 15.1, 11.7, 9.4, 9.0, 4. The signals are interpreted in terms of an S = 9/2 system with spin-Hamiltonian parameters g = 2.00, D = -0.56 cm-1, magnitude of E/D = 0.13 for the major component. In a reductive titration with sodium borohydride the spectrum disappears with Em = -205 mV at pH 7.5. Contrarily, the major high-spin siroheme component has S = 5/2, g = 1.99, D = +9 cm-1, magnitude of E/D = 0.042, and Em = -295 mV. The sum of all siroheme signals integrates to 0.2 spin/half molecule, indicating considerable demetallation of this prosthetic group. Rigorous quantification procedures for S = 9/2 are not available, however, estimation by an approximate method indicates 0.6 S = 9/2 spin/half molecule. The S = 9/2 system is ascribed to an iron-sulfur cluster. It follows that this cluster is probably not a cubane, is not necessarily exchange-coupled to the siroheme, and, therefore, is not necessarily structurally close to the siroheme. It is suggested that this iron-sulfur prosthetic group has a novel structure suitable for functioning in multiple electron transfer.
亚硫酸盐还原酶含有西罗血红素和铁硫簇辅基。据信这两组通过单个共同配体在结构上相连。该化学模型基于氧化酶的磁模型，其中所有参与的铁离子都是交换耦合的。这种描述导致了两个严重的差异。尽管铁硫簇被假定为抗磁性的立方烷，[4Fe - 4S]2 +，但在穆斯堡尔光谱中所有铁似乎都是顺磁性的。另一方面，电子顺磁共振光谱除了单个高自旋血红素外未能检测到任何东西。我们通过在来自普通脱硫弧菌（希登伯勒）的异化亚硫酸盐还原酶的浓缩样品中寻找新的电子顺磁共振光谱线索来重新解决这个问题。我们发现了几个有效g值为17、15.1、11.7、9.4、9.0、4的新信号。这些信号根据一个S = 9/2系统来解释，其自旋哈密顿参数为g = 2.00，D = -0.56 cm - 1，主要成分的E/D大小为0.13。在用硼氢化钠进行的还原滴定中，在pH 7.5时，Em = -205 mV时光谱消失。相反，主要的高自旋西罗血红素成分具有S = 5/2，g = 1.99，D = +9 cm - 1，E/D大小为0.042，Em = -295 mV。所有西罗血红素信号的总和积分到0.2自旋/半分子，表明该辅基有相当程度的脱金属。然而，对于S = 9/2没有严格的定量程序，通过一种近似方法估计表明为0.6 S = 9/2自旋/半分子。S = 9/2系统归因于一个铁硫簇。由此可见，这个簇可能不是立方烷，不一定与西罗血红素交换耦合，因此，不一定在结构上靠近西罗血红素。有人认为这个铁硫辅基具有适合在多电子转移中起作用的新结构。
