Amino acid residues associated with cluster N3 in the NuoF subunit of the proton‐translocating NADH‐quinone oxidoreductase from Escherichia coli

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Abstract
The NuoF subunit, which harbors NADH‐binding site, of Escherichia coli NADH‐quinone oxidoreductase (NDH‐1) contains five conserved cysteine residues, four of which are predicted to ligate cluster N3. To determine this coordination, we overexpressed and purified the NuoF subunit and NuoF + E subcomplex in E. coli. We detected two distinct EPR spectra, arising from a [4Fe–4S] cluster (g x,y,z = 1.90, 1.95, and 2.05) in NuoF, and a [2Fe–2S] cluster (g x,y,z = 1.92, 1.95, and 2.01) in NuoE subunit. These clusters were assigned to clusters N3 and N1a, respectively. Based on the site‐directed mutagenesis experiments, we identified that cluster N3 is ligated to the 351Cx2Cx2Cx40C398 motif.