Crystal structure of cambialistic superoxide dismutase from Porphyromonas gingivalis

The crystal structure of cambialistic superoxide dismutase (SOD) from Porphyromonas gingivalis, which exhibits full activity with either Fe or Mn at the active site, has been determined at 1.8-Angstrom resolution by molecular replacement and refined to a crystallographic R factor of 17.9% (R-free 22.3%). The crystals belong to the space group P2(1)2(1)2(1) (a = 75.5 Angstrom, b = 102.7 Angstrom, c = 99.6 Angstrom) with four identical subunits in the asymmetric unit. Each pair of subunits forms a compact dimer, but not a tetramer, with 222 point symmetry. Each subunit has 191 amino-acid residues most of which are visible in electron density maps, and consists of seven a helices and one three-stranded antiparallel beta sheet. The metal ion, a 3:1 mixture of Fe and Mn, is coordinated with five ligands (His27, His74, His161, Asp157, and water) arranged at the vertices of a trigonal bipyramid. Although the overall structural features, including the metal coordination geometry, are similar to those found in other single-metal containing SODs, P. gingivalis SOD more closely resembles the dimeric Fe-SODs from Escherichia coli rather than another cambialistic SOD from Propionibacterium shermanii, which itself is rather similar to other tetrameric SODs.