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A five-gene "oxidative stress protection" cluster has recently been described from the strictly anaerobic, acetogenic bacterium, Moorella thermoacetica [Das, A., et al. (2001) J. Bacteriol. 183, 1560-1567]. Within this cluster are two cotranscribed genes, fprA (for A-type flavoprotein) and hrb (for high molecular weight rubredoxin) whose encoded proteins have no known functions. Here we show that FprA and Hrb are expressed in M. thermoacetica under normal anaerobic growth conditions and report characterizations of the recombinant FprA and Hrb. FprA contains flavin mononucleotide (FMN) and a non-heme diiron site. Mössbauer spectroscopy shows that the irons of the diferric site are antiferromagnetically coupled, implying a single-atom, presumably solvent, bridge between the irons. Hrb contains FMN and a rubredoxin-like [Fe(SCys)4] site. NADH does not directly reduce either the FMN or the diiron site in FprA, whereas Hrb functions as an efficient NADH:FprA oxidoreductase. Substitution of zinc for iron in Hrb completely abolished this activity. The observation that homologues of FprA from other organisms show O2 and/or anaerobic NO consumption activity prompted an examination of these activities for M. thermoacetica FprA. The Hrb/FprA combination does indeed have both NADH:O2 and NADH:NO oxidoreductase activities. The NO reductase activity, however, was significantly more efficient due to a lower Km for NO (4 M) and to progressive and irreversible inactivation of FprA during O2 reductase turnover but retention of activity during NO reductase turnover. Substitution of zinc for iron in FprA completely abolished these reductase activities. The stoichiometry of 1 mol of NADH oxidized:2 mol of NO consumed implies reduction to N2O. Fits of an appropriate rate law to the kinetics data are consistent with a mechanism in which 2NO's react at each FprA active site in the committed step. Expression of FprA in an Escherichia coli strain deficient in NO reductase restored the anaerobic growth phenotype of cultures exposed to otherwise toxic levels of exogenous NO. The accumulated results indicate that Hrb/FprA is fully capable of functioning in nitrosative stress protection in M. thermoacetica.

Superoxide reductases (SORs) contain a novel square pyramidal ferrous [Fe(NHis)(4)(SCys)] site that rapidly reduces superoxide to hydrogen peroxide. Here we report extensive pulse radiolysis studies on recombinant two-iron SOR (2Fe-SOR) from Desulfovibrio vulgaris. The results support and elaborate on our originally proposed scheme for reaction of the [Fe(NHis)(4)(SCys)] site with superoxide [Coulter, E. D., Emerson, J. E., Kurtz, D. M., Jr., and Cabelli, D. E. (2000) J. Am. Chem. Soc. 122, 11555-11556]. This scheme consists of second-order diffusion-controlled formation of an intermediate absorbing at approximately 600 nm, formulated as a ferric-(hydro)peroxo species, and its decay to the carboxylate-ligated ferric [Fe(NHis)(4)(SCys)] site with loss of hydrogen peroxide. The second-order rate constant for formation of the 600-nm intermediate is essentially pH-independent (pH 5-9.5), shows no D(2)O solvent isotope effect at pH 7.7, and decreases with increasing ionic strength. These data indicate that formation of the intermediate does not involve a rate-determining protonation, and are consistent with interaction of the incoming superoxide anion with a positive charge at or near the ferrous [Fe(NHis)(4)(SCys)] site. The rate constant for decay of the 600-nm intermediate follows the pH-dependent rate law: k(2)(obs) = k(2)'[H(+)] + k(2)' ' and shows a significant D(2)O solvent isotope effect at pH 7.7. The values of k(2)' and k(2)' ' indicate that the 600-nm intermediate decays via diffusion-controlled protonation at acidic pHs and a first-order process involving either water or a water-exchangeable proton on the protein at basic pHs. The formation and decay rate constants for an E47A variant of 2Fe-SOR are not significantly perturbed from their wild-type values, indicating that the conserved glutamate carboxylate does not directly displace the (hydro)peroxo ligand of the intermediate at basic pHs. The kinetics of a K48A variant are consistent with participation of the lysyl side chain in directing the superoxide toward the active site and in directing the protonation pathway of the ferric-(hydro)peroxo intermediate toward release of hydrogen peroxide.

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The ferrous square-pyramidal [Fe(NHis)4(SCys)] site of superoxide reductases (SORs) has been shown to reduce superoxide at a nearly diffusion-controlled rate. The final products of the reaction are hydrogen peroxide and the ferric hexacoordinated SOR site, with a carboxylate group from a conserved glutamate serving as the sixth ligand trans to the cysteine sulfur. A transient intermediate absorbing at approximately 600 nm in the reaction of the ferrous pentacoordinated site with superoxide has been proposed to be a ferric-(hydro)peroxo complex (Coulter, E.; Emerson, J.; Kurtz, D. M., Jr.; Cabelli, D. J. Am. Chem. Soc. 2000, 122, 11555-11556.). In the present study, DFT and ZINDO/S-CI results are shown to support the description of the 600-nm intermediate as an end-on, low-spin ferricperoxo or--hydroperoxo complex. Side-on peroxo coordination was found to be significantly less stable than end-on because of constraints on the imidazole ligand ring orientations imposed mostly by the protein. The modeled ferric-hydroperoxo complex had a decidedly nonplanar CysC beta-S-Fe-O-O geometry that appears to be imposed by the same constraints. A single prominent visible absorption of the (hydro)peroxo model is shown to be due mainly to a CysS-->Fe(III) pi charge transfer (CT) transition with a minor portion of His-->Fe(III) pi CT character and very little peroxo-->Fe(III) CT character. On the basis of calculations of models with various mono- and diprotonated peroxo ligands, protonation of the iron-bound peroxo oxygen is a key step in the decay of the ferric(hydro)peroxo complex favoring release of hydrogen peroxide over cleavage of the O-O bond, as occurs in the heme structural analogue, cytochrome P450.

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The electronic and vibrational properties of the [Fe(His)(4)(Cys)] site (Center II) responsible for catalysis of superoxide reduction in the two-iron superoxide reductase (2Fe-SOR) from Desulfovibrio vulgaris have been investigated using the combination of EPR, resonance Raman, UV/visible/near-IR absorption, CD, and VTMCD spectroscopies. Deconvolution of the spectral contributions of Center II from those of the [Fe(Cys)(4)] site (Center I) has been achieved by parallel investigations of the C13S variant, which does not contain Center I. The resonance Raman spectrum of ferric Center II has been assigned based on isotope shifts for (34)S and (15)N globally labeled proteins. As for the [Fe(His)(4)(Cys)] active site in 1Fe-SOR from Pyrococcus furiosus, the spectroscopic properties of ferric and ferrous Center II in D. vulgaris 2Fe-SOR are indicative of distorted octahedral and square-pyramidal coordination geometries, respectively. Differences in the properties of the ferric [Fe(His)(4)(Cys)] sites in 1Fe- and 2Fe-SORs are apparent in the rhombicity of the S=5/2 ground state ( E/ D=0.06 and 0.28 in 1Fe- and 2Fe-SORs, respectively), the energy of the CysS(-)(p(pi))-->Fe(3+)(d(pi)) CT transition (15150+/-150 cm(-1) and 15600+/-150 cm(-1) in 1Fe- and 2Fe-SORs, respectively) and in changes in the Fe-S stretching region of the resonance Raman spectrum indicative of a weaker Fe-S(Cys) bond in 2Fe-SORs. These differences are interpreted in terms of small structural perturbations in the Fe coordination sphere with changes in the Fe-S(Cys) bond strength resulting from differences in the peptide N-H.S(Cys) hydrogen bonding within a tetrapeptide bidentate "chelate". Observation of the characteristic intervalence charge transfer transition of a cyano-bridged [Fe(III)-NC-Fe(II)(CN)(5)] unit in the near-IR VTMCD spectra of ferricyanide-oxidized samples of both P. furiosus 1Fe-SOR and D. vulgaris 2Fe-SOR has confirmed the existence of novel ferrocyanide adducts of the ferric [Fe(His)(4)(Cys)] sites in both 1Fe- and 2Fe-SORs.