A flavodiiron protein and high molecular weight rubredoxin from Moorella thermoacetica with nitric oxide reductase activity

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, 15601567]. 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. Mossbauer spectroscopy shows that the irons of the diferric site are antiferro-magnetically 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 O-2 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:O-2 and NADH:NO oxidoreductase activities. The NO reductase activity, however, was significantly more efficient due to a lower K-m for NO (4 muM) and to progressive and irreversible inactivation of FprA during O-2 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 I 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.