the enzyme contains two iron sites: center I, similar to that of desulforedoxin, a small iron protein having a rubredoxin-like FeCys4 center, and center II, another type of iron site, which is named desulfoferrodoxin, Dfx, or rubredoxin. Structure of the FeCys4 center I, oxidized and reduced forms. overview
2Fe-SOR, an iron ion is bound at the catalytic site to four histidines and a cysteine that, in its reduced form, reacts with superoxide anion with a diffusion-limited second order rate constant, metal site structure and mechanism, overview
catalytic Fe2+ binding residues are H49, H69, H74, C115, and H118. With the exception of the class IV (methanoferrodoxins) and the atypical SORs, they all appear to contain one or two iron centers: the catalytic center plus the desulforedoxin-like and rubredoxin-like, Dx/Rb-like, center
Fe-SOR classification, detailed overview. One classification takes into consideration the primary and tertiary structures of SORs some enzymes contain only one Fe ion, but have a longer N-terminus with amino acid sequence and structural similarities with those of the respective domain of desulfoferrodoxins, but lacking the cysteine ligands to the desulforedoxin (Dfxs)-like center. According to the authors, SORs fall into three classes: classes I (Dfxs), II (neelaredoxins), and III (neelaredoxins structurally homologous to desulfoferrodoxins, with only one Fe center). In dendograms constructed from available amino acid sequences, class III enzymes cluster within the class I enzymes, it is plausible that class III SORs evolved from class I proteins by loss of the cysteine residues binding the desulforedoxin-like center, an event that may have occurred more than once because the Dfxs are not monophyletic. This classification misses the family of methanoferrodoxins. Another classification is based on the variability of N-terminal domains classifying SORs into seven classes. Class I or Dx-SOR includes the 2Fe-SORs, where the N-terminal is a desulforedoxin-like (Dx) domain. Class II includes the 1Fe-SORs that have no extra N-terminal domain. Class III SORs are analogous to Dx-SORs but lacking some or all of the Fe cysteine ligands (FeCys4) for the desulforedoxin-like Fe center and therefore lacking the FeCy4 site. Class IV includes SORs with an extra C-terminal domain containing an iron-sulfur center. The fifth class, termed HTH-Dx-SOR, includes Dx-SORs (2Fe-SOR) with an extended N-terminal helix-turn-helix domain present in transcription regulators. The sixth class, termed TAT-SOR, includes SORs from only a few organisms and the sequences are preceded by a putative twin-arginine signal peptide that suggests their periplasmic localization