The first crystal structure of class III superoxide reductase from Treponema pallidum

Abstract

Superoxide reductase (SOR) is a metalloprotein containing a non-heme iron centre, responsible for the scavenging of superoxide radicals in the cell. The crystal structure of Treponema pallidum (Tp) SOR was determined using soft X-rays and synchrotron radiation. Crystals of the oxidized form were obtained using poly(ethylene glycol) and MgCl 2 and diffracted beyond 1.55 Å resolution. The overall architecture is very similar to that of other known SORs but TpSOR contains an N-terminal domain in which the desulforedoxin-type Fe centre, found in other SORs, is absent. This domain conserves the b-barrel topology with an overall arrangement very similar to that of other SOR proteins where the centre is present. The absence of the iron ion and its ligands, however, causes a decrease in the cohesion of the domain and some disorder is observed, particularly in the region where the metal would be harboured. The C-terminal domain exhibits the characteristic immunoglobulin-like fold and harbours the Fe(His) 4 (Cys) active site. The five ligands of the iron centre are well conserved despite some disorder observed for one of the four molecules in the asymmetric unit. The participation of a glutamate as the sixth ligand of some of the iron centres in Pyrococcus furiosus SOR was not observed in TpSOR. A possible explanation is that either X-ray photoreduction occurred or there was a mixture of redox states at the start of data collection. In agreement with earlier proposals, details in the TpSOR structure also suggest that Lys49 might be involved in attraction of superoxide to the active site.

Crystal Structure of the Tp34 (TP0971) Lipoprotein of Treponema pallidum

“…This is consistent with the prevailing view that T. pallidum is microaerophilic (1). Genome analysis has further revealed that there likely are at least nine T. pallidum proteins (Tp0053, Tp0080, Tp0735, Tp0823, Tp0842, Tp0939, Tp0991, Tp1008, and Tp1038) that require iron as a cofactor (50,65,71,85). Thus, it is plausible that T. pallidum requires iron.…”

“…There is, thus, the intriguing possibility that Tp34 acts either as a treponemal FTR1 accessory protein or as a coppersequestering protein that supplies the cation to another pro-tein. The similarity of the metal-binding site in rTp34 to a treponemal iron-binding site in the superoxide reductase (Tp0823) of T. pallidum (71), however, suggests that Tp34 could be involved in binding and transporting iron.…”

Intermolecular electron transfer in two-iron superoxide reductase: a putative role for the desulforedoxin center as an electron donor to the iron active site

“…This result shows that Fpr is not able to efficiently transfer its electrons to center II in the absence of center I. Such a low electron transfer rate was also observed for the SOR from Treponema pallidum, which naturally lacks center I [13]. Comparable to the C13S SOR mutant, the oxidized form of SOR from T. pallidum is poorly reduced by Fpr ( Figure 3).…”

“…The SOR active site is located at the surface of the protein and consists of a mononuclear iron center, named center II, pentacoordinated in its ferrous state by four nitrogen atoms from histidine residues in an equatorial plane and one sulfur atom from a cysteine residue in an axial position [11][12][13][14]. It displays a high redox potential of about + 300 mV (vs. NHE) at neutral pH and remains mainly in a reduced form in the presence of air [15][16][17][18].…”

“…SORs having this additional iron center, e.g. the enzymes from Desulfovibrio desulfuricans [11], Desulfoarculus baarsii [5,14] and Desulfovibrio vulgaris [21], are named 2Fe-SOR whereas those having only the iron active site, like the enzymes from Pyrococcus furiosus [4,12], Archaeoglobus fulgidus [18] and Treponema pallidum [13] are named 1Fe-SORs.…”