Abstract
Treponema pallidum, the causative agent of venereal syphilis, is a microaerophilic obligate pathogen of humans. As it disseminates hematogenously and invades a wide range of tissues, T. pallidum presumably must tolerate substantial oxidative stress. Analysis of the T. pallidum genome indicates that the syphilis spirochete lacks most of the iron-binding proteins present in many other bacterial pathogens, including the oxidative defense enzymes superoxide dismutase, catalase, and peroxidase, but does possess an orthologue (TP0823) for neelaredoxin, an enzyme of hyperthermophilic and sulfate-reducing anaerobes shown to possess superoxide reductase activity. To analyze the potential role of neelaredoxin in treponemal oxidative defense, we examined the biochemical, spectroscopic, and antioxidant properties of recombinant T. pallidum neelaredoxin. Neelaredoxin was shown to be expressed in T. pallidum by reverse transcriptase-polymerase chain reaction and Western blot analysis. Recombinant neelaredoxin is a 26-kDa alpha(2) homodimer containing, on average, 0.7 iron atoms/subunit. Mössbauer and EPR analysis of the purified protein indicates that the iron atom exists as a mononuclear center in a mixture of high spin ferrous and ferric oxidation states. The fully oxidized form, obtained by the addition of K(3)(Fe(CN)(6)), exhibits an optical spectrum with absorbances at 280, 320, and 656 nm; the last feature is responsible for the protein's blue color, which disappears upon ascorbate reduction. The fully oxidized protein has a A(280)/A(656) ratio of 10.3. Enzymatic studies revealed that T. pallidum neelaredoxin is able to catalyze a redox equilibrium between superoxide and hydrogen peroxide, a result consistent with it being a superoxide reductase. This finding, the first description of a T. pallidum iron-binding protein, indicates that the syphilis spirochete copes with oxidative stress via a primitive mechanism, which, thus far, has not been described in pathogenic bacteria.
Highlights
Syphilis is a sexually transmitted disease caused by the noncultivable human pathogen Treponema pallidum
Analysis of the T. pallidum genome indicates that the syphilis spirochete lacks most of the iron-binding proteins present in many other bacterial pathogens, including the oxidative defense enzymes superoxide dismutase, catalase, and peroxidase, but does possess an orthologue (TP0823) for neelaredoxin, an enzyme of hyperthermophilic and sulfate-reducing anaerobes shown to possess superoxide reductase activity
The neelaredoxin transcript was detected in freshly harvested spirochetes at lower levels than the abundantly expressed flagelin subunit. To confirm that these Reverse Transcriptase-Polymerase Chain Reaction (RT-PCR) data were indicative of expression at the protein level, T. pallidum lysates were probed with anti-neelaredoxin serum in Western analysis
Summary
Syphilis is a sexually transmitted disease caused by the noncultivable human pathogen Treponema pallidum. It has recently been hypothesized that T. pallidum has a limited requirement for iron (9 –11), and an analysis of the T. pallidum genome indicates that the syphilis spirochete lacks most of the iron-binding proteins present in other bacterial pathogens, including the oxidative defense enzymes superoxide dismutase, catalase, and peroxidase. It does possess an orthologue (TP0823) for neelaredoxin, a superoxide reductase of hyperthermophilic and sulfate-reducing anaerobes. The four histidine and cysteine residues that coordinate the iron atom of center II is uniformly conserved in all known enzymes of this family, including the T. pallidum homologue (Fig. 1)
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