Ferredoxins comprise a large family of iron–sulfur (Fe–S) proteins that shuttle electrons in diverse biological processes. Human mitochondria contain two isoforms of [2Fe-2S] ferredoxins, FDX1 (aka adrenodoxin) and FDX2, with known functions in cytochrome P450-dependent steroid transformations and Fe–S protein biogenesis. Here, we show that only FDX2, but not FDX1, is involved in Fe–S protein maturation. Vice versa, FDX1 is specific not only for steroidogenesis, but also for heme a and lipoyl cofactor biosyntheses. In the latter pathway, FDX1 provides electrons to kickstart the radical chain reaction catalyzed by lipoyl synthase. We also identified lipoylation as a target of the toxic antitumor copper ionophore elesclomol. Finally, the striking target specificity of each ferredoxin was assigned to small conserved sequence motifs. Swapping these motifs changed the target specificity of these electron donors. Together, our findings identify new biochemical tasks of mitochondrial ferredoxins and provide structural insights into their functional specificity.

铁氧还蛋白包含一大类铁硫 (Fe-S) 蛋白，可在不同的生物过程中传递电子。人类线粒体含有两种同工型 [2Fe-2S] 铁氧还蛋白：FDX1（又名肾上腺氧还蛋白）和 FDX2，在细胞色素 P450 依赖性类固醇转化和 Fe-S 蛋白生物合成中具有已知功能。在这里，我们表明只有 FDX2 而不是 FDX1 参与 Fe-S 蛋白的成熟。反之亦然，FDX1 不仅对类固醇生成具有特异性，而且对血红素a和硫辛酰辅因子生物合成也具有特异性。在后一种途径中，FDX1 提供电子来启动硫辛酰合酶催化的自由基链式反应。我们还确定硫辛酸化是有毒抗肿瘤铜离子载体艾司洛醇的靶标。最后，每种铁氧还蛋白的显着靶标特异性被分配给小的保守序列基序。交换这些基序改变了这些电子供体的靶标特异性。总之，我们的研究结果确定了线粒体铁氧还蛋白的新生化任务，并为其功能特异性提供了结构见解。