Vibat C R, Cecchini G, Nakamura K, Kita K, Gennis R B
Department of Biochemistry, University of Illinois at Urbana-Champaign 61801, USA.
Biochemistry. 1998 Mar 24;37(12):4148-59. doi: 10.1021/bi9716635.
Complex II (succinate:ubiquinone oxidoreductase) from Escherichia coli contains four different subunits. Two of the subunits (SDHC and SDHD) are hydrophobic and anchor the two more hydrophilic (flavin and iron-sulfur) subunits (SDHA and SDHB) to the cytoplasmic membrane. Previous studies have shown that the complex of SDHC/SDHD is required to maintain the heme B component of the enzyme and that the heme B is ligated to the protein by two histidine ligands. In the current work, the histidines within SDHC and SDHD have been systematically mutated. SDHC-His91 and SDHD-His14 were eliminated as potential ligands by these studies. SDHC-His84 and SDHD-His71 have been identified as the most likely heme axial ligands in the E. coli enzyme, suggesting that the heme bridges these two subunits in the membrane. Furthermore, the results show that the four-subunit Complex II assembles and retains function despite the absence of the heme B prosthetic group in the membrane. The results do not rule out completely SDHC-His30 as a candidate for heme ligation, but do show that mutation at this position prevents assembly of Complex II in the membrane.
来自大肠杆菌的复合物II（琥珀酸：泛醌氧化还原酶）包含四个不同的亚基。其中两个亚基（SDHC和SDHD）是疏水的，将另外两个亲水性更强的亚基（黄素和铁硫）（SDHA和SDHB）锚定在细胞质膜上。先前的研究表明，SDHC/SDHD复合物对于维持该酶的血红素B组分是必需的，并且血红素B通过两个组氨酸配体与蛋白质连接。在当前的工作中，对SDHC和SDHD中的组氨酸进行了系统的突变。这些研究排除了SDHC-His91和SDHD-His14作为潜在配体的可能性。已确定SDHC-His84和SDHD-His71是大肠杆菌酶中最可能的血红素轴向配体，这表明血红素在膜中连接这两个亚基。此外，结果表明，尽管膜中不存在血红素B辅基，四亚基复合物II仍能组装并保持功能。结果并未完全排除SDHC-His30作为血红素连接候选者的可能性，但确实表明该位置的突变会阻止复合物II在膜中的组装。
