Crystal structure of cambialistic superoxide dismutase from Porphyromonas gingivalis

The crystal structure of cambialistic superoxide dismutase (SOD) from Porph yromonas gingivalis, which exhibits full activity with either Fe or Mn at t he 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 Angstr om, 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 residue s 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 coordinatio n geometry, are similar to those found in other single-metal containing SOD s, P. gingivalis SOD more closely resembles the dimeric Fe-SODs from Escher ichia coli rather than another cambialistic SOD from Propionibacterium sher manii, which itself is rather similar to other tetrameric SODs.