School of Chemistry and Molecular Biosciences, The University of Queensland, St. Lucia, QLD, 4072, Australia.
Cell Mol Life Sci. 2013 Mar;70(6):977-92. doi: 10.1007/s00018-012-1098-y. Epub 2012 Aug 21.
SoxAX cytochromes are heme-thiolate proteins that play a key role in bacterial thiosulfate oxidation, where they initiate the reaction cycle of a multi-enzyme complex by catalyzing the attachment of sulfur substrates such as thiosulfate to a conserved cysteine present in a carrier protein. SoxAX proteins have a wide phylogenetic distribution and form a family with at least three distinct types of SoxAX protein. The types of SoxAX cytochromes differ in terms of the number of heme groups present in the proteins (there are diheme and triheme versions) as well as in their subunit structure. While two of the SoxAX protein types are heterodimers, the third group contains an additional subunit, SoxK, that stabilizes the complex of the SoxA and SoxX proteins. Crystal structures are available for representatives of the two heterodimeric SoxAX protein types and both of these have shown that the cysteine ligand to the SoxA active site heme carries a modification to a cysteine persulfide that implicates this ligand in catalysis. EPR studies of SoxAX proteins have also revealed a high complexity of heme dependent signals associated with this active site heme; however, the exact mechanism of catalysis is still unclear at present, as is the exact number and types of redox centres involved in the reaction.
SoxAX 细胞色素是一类含有硫醇盐的血红素蛋白，在细菌硫代硫酸盐氧化中起着关键作用，它们通过催化硫代硫酸盐等硫底物与载体蛋白中保守半胱氨酸的结合，启动多酶复合物的反应循环。SoxAX 蛋白具有广泛的系统发育分布，并形成一个至少有三种不同类型 SoxAX 蛋白的家族。SoxAX 细胞色素的类型在蛋白中存在的血红素基团数量（二血红素和三血红素版本）以及亚基结构上有所不同。虽然两种 SoxAX 蛋白类型是异二聚体，但第三组包含一个额外的亚基 SoxK，它稳定了 SoxA 和 SoxX 蛋白的复合物。两种异二聚体 SoxAX 蛋白类型的代表都有晶体结构，这两种结构都表明 SoxA 活性位点血红素的半胱氨酸配体带有一个半胱氨酸过硫化物的修饰，这表明该配体参与了催化。EPR 研究 SoxAX 蛋白也揭示了与该活性位点血红素相关的高度复杂的血红素依赖信号；然而，目前催化的确切机制以及反应中涉及的氧化还原中心的确切数量和类型仍不清楚。
