A 14.7 kDa Protein from <i>Francisella tularensis subsp. novicida</i> (Named FTN_1133), Involved in the Response to Oxidative Stress Induced by Organic Peroxides, Is Not Endowed with Thiol-Dependent Peroxidase Activity

<div><p><i>Francisella</i> genus comprises Gram-negative facultative intracellular bacteria that are among the most infectious human pathogens. A protein of 14.7 KDa named as FTN_1133 was previously described as a novel hydroperoxide resistance protein in <i>F. tularensis subsp. novicida</i>, implicated in organic peroxide detoxification and virulence. Here, we describe a structural and biochemical characterization of FTN_1133. Contrary to previous assumptions, multiple amino acid sequence alignment analyses revealed that FTN_1133 does not share significant similarity with proteins of the Ohr/OsmC family or any other Cys-based, thiol dependent peroxidase, including conserved motifs around reactive cysteine residues. Circular dichroism analyses were consistent with the <i>in silico</i> prediction of an all-α-helix secondary structure. The pK<sub>a</sub> of its single cysteine residue, determined by a monobromobimane alkylation method, was shown to be 8.0±0.1, value that is elevated when compared with other Cys-based peroxidases, such as peroxiredoxins and Ohr/OsmC proteins. Attempts to determine a thiol peroxidase activity for FTN_1133 failed, using both dithiols (DTT, thioredoxin and lipoamide) and monothiols (glutathione or 2-mercaptoethanol) as reducing agents. Heterologous expression of <i>FTN_1133</i> gene in <i>ahpC</i> and <i>oxyR</i> mutants of <i>E. coli</i> showed no complementation. Furthermore, analysis of <i>FTN_1133</i> protein by non-reducing SDS-PAGE showed that an inter-molecular disulfide bond (not detected in Ohr proteins) can be generated under hydroperoxide treatment, but the observed rates were not comparable to those observed for other thiol-dependent peroxidases. All the biochemical and structural data taken together indicated that FTN_1133 displayed distinct characteristics from other thiol dependent peroxidases and, therefore, suggested that FTN_1133 is not directly involved in hydroperoxide detoxification.</p></div>