The first member of this family (hereafter abbreviated AlvinFdx) to be identified was that of the purple HMPL-504 molecular weight sulfur γ-proteobacterium Allochromatium vinosum, originally named Chromatium vinosum, and it was initially classified among other [4Fe 4S] ‘bacterial’ Fdxs (as opposed to ‘plant’ [2Fe 2S] Fdxs) [11]. It was later found that the characteristic sequence differences of proteins of the AlvinFdx family shifted the reduction potential of the [4Fe 4S] clusters to very negative values,

below -450 mV with reference to the Normal Hydrogen Electrode, with one reaching -650 mV or less [12]. Because of this unusual property, it is not easy to find an efficient physiological reductant for such proteins, especially in non-photosynthetic organisms. Additional unique spectroscopic [13] and structural [10, PLX3397 cell line 14, 15] properties have also been evidenced in these proteins. Figure 1 Characteristic features of Fdx of the AlvinFdx family. (A) Sequence alignment of selected 2[4Fe-4S] Fdxs from γ-proteobacteria [1]Pseudomonas aeruginosa PAO1, [2]Allochromatium vinosum DSM180, [3]Escherichia coli K12-MG1655; δ-proteobacteria [4]Anaeromyxobacter dehalogenans 2CP-C, [5]Plesiocystis

pacifica SIR-1; ε-proteobacteria [6]Helicobacter pylori 26695, [7]Campylobacter jejuni NCTC 11168, Cj0354 sequence; Chloroflexi [8]Dehalococcoides sp. VS; β-proteobacteria see more [9]Azoarcus sp. (or Aromatoleum aromaticum) EbN1 (locus NT01AE0820), [10]Thauera aromatica K172; α-proteobacteria [11]Rhodopseudomonas palustris CGA009; [12]Clostridium acidurici as an example of heterotrophic anaerobic bacteria; [13]Azoarcus sp. EbN1 (locus NT01AE3314) belonging to the bcr cluster; [14]Campylobacter jejuni NCTC 11168 Cj0333 sequence. nX stands for insertions of n aminoacids. Stars on the consensus line for proteins of the AlvinFdx family indicate identical residues and colons are for conserved

residues. The ① and ② symbols lie under non-conserved residues belonging to the fragment between cysteine ligands and the turn and helix addition, respectively, that characterize the AlvinFdx family as indicated in the structure of Figure 1B. The lengths of the compared sequences are given at the end of the alignment, and [4Fe-4S] cysteine ligands are boxed. (B) View of the P. aeruginosa Fdx structure [10]. The general fold is shown (light grey RG7420 mw tube) with the 8 amino acid stretch between two cysteine ligands of one cluster (labelled ①) and the turn and helix at the C-terminus ② colored in dark grey. Iron and inorganic sulfur atoms are represented as spheres. A well defined function for members of this family of Fdxs has only been found in bacteria catabolizing aromatic compounds in the absence of oxygen [16]. The Thauera aromatica Fdx participates in an electron transfer chain, as electron acceptor from 2-oxoglutarate:Fdx oxidoreductase and donor to benzoyl-CoA reductase [17].