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.