Soil bacteria can detoxify Cr(VI) ions by reduction. Within the last 2 decades, numerous reports of chromate reductase enzymes have been published. These reports describe catalytic reduction of chromate ions by specific enzymes. These enzymes each have sequence similarity to known redox-active flavoproteins. We investigated the enzyme NfoR from Staphylococcus aureus, which was reported to be upregulated in chromate-rich soils and to have chromate reductase activity (H. Han, Z. Ling, T. Zhou, R. Xu, et al., Sci Rep 7:15481, 2017, https://doi.org/10.1038/s41598-017-15588-y). We show that NfoR has structural similarity to known flavin mononucleotide (FMN) reductases and reduces FMN as a substrate. NfoR binds FMN with a dissociation constant of 0.4 μM. The enzyme then binds NADPH with a dissociation constant of 140 μM and reduces the flavin at a rate of 1,350 s⁻¹. Turnover of the enzyme is apparently limited by the rate of product release that occurs, with a net rate constant of 0.45 s⁻¹. The rate of product release limits the rate of observed chromate reduction, so the net rate of chromate reduction by NfoR is orders of magnitude lower than when this process occurs in solution. We propose that NfoR is an FMN reductase and that the criterion required to define chromate reduction as enzymatic has not been met. That NfoR expression is increased in the presence of chromate suggests that the survival adaption was to increase the net rate of chromate reduction by facile, adventitious redox processes.

中文翻译：

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NfoR、铬酸盐还原酶或 FMN 还原酶？

土壤细菌可以通过还原使 Cr(VI) 离子解毒。在过去的 2 年里，已经发表了大量关于铬酸盐还原酶的报道。这些报告描述了特定酶对铬酸盐离子的催化还原。这些酶各自与已知的氧化还原活性黄素蛋白具有序列相似性。我们研究了金黄色葡萄球菌中的 NfoR 酶，据报道在富含铬酸盐的土壤中被上调并具有铬酸盐还原酶活性（H. Han, Z. Ling, T. Zhou, R. Xu, et al., Sci Rep 7:15481, 2017, https:/ /doi.org/10.1038/s41598-017-15588-y）。我们显示 NfoR 与已知的黄素单核苷酸 (FMN) 还原酶具有结构相似性，并将 FMN 还原为底物。NfoR 以 0.4 μM 的解离常数结合 FMN。然后该酶以 140 μM 的解离常数结合 NADPH，并以 1,350 s ^-1的速率还原黄素。酶的周转率显然受到发生的产物释放速率的限制，净速率常数为 0.45 s ^-1. 产品释放速率限制了观察到的铬酸盐还原速率，因此 NfoR 的铬酸盐还原净速率比该过程在溶液中发生时低几个数量级。我们建议 NfoR 是一种 FMN 还原酶，并且尚未满足将铬酸盐还原定义为酶促所需的标准。NfoR 表达在铬酸盐存在的情况下增加表明生存适应是通过简单的、不定的氧化还原过程增加铬酸盐还原的净速率。