NfoR, Chromate Reductase or FMN reductase?

Soil bacteria can detoxify Cr(VI) ions by reduction. Within the last two decades numerous reports of chromate reductase enzymes have been published. These reports purport 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 Staphlococcus aureus that was claimed to be upregulated in chromate rich soils and to have chromate reductase activity (Han H, Ling Z, Zhou T, Xu R, He Y, Liu P, Li X. 2017. Sci Rep 7:15481.). We show that NfoR has structural similarity to known 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 1350 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 slower than this process occurs in solution. We propose that NfoR is an FMN reductase and that the criterium 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. Importance Chromate is a toxic byproduct of multiple industrial processes. Chromate reduction is an important biological activity that ameliorates Cr(VI) toxicity. Numerous researchers have claimed chromate reductase activity by observing chromate reduction. However, all claimed chromate reductase enzymes have flavin as a cofactor or use a flavin as a substrate. We show here that NfoR, an enzyme claimed to be a chromate reductase, is in fact an FMN reductase. In addition, we show that reduction of a flavin is a viable way to transfer electrons to chromate, but that it is unlikely to be the native function of enzymes. We propose that upregulation of a redox active flavoprotein is a viable means to detoxify chromate that relies on adventitious reduction that is not catalyzed.