Refined crystal structure of a superoxide dismutase from the hyperthermophilic archaeon Sulfolobus acidocaldarius at 2.2 angstrom resolution

The extremely thermostable superoxide dismutase from the hyperthermophilic archaeon Sulfolobus acidocaldarius was crystallized and the three-dimension al structure was determined by X-ray diffraction methods. The enzyme crysta llized in the monoclinic spacegroup C2 with the cell dimensions a = 168.1 A ngstrom, b = 91.3 Angstrom, c = 85.7 Angstrom, beta = 91.4 degrees. The dif fraction limit of these crystals was 2.2 Angstrom. The crystals were very stable in the X-ray beam and measured diffraction da ta of a single crystal had a completeness of 99.5 % up to a resolution of 2 .2 Angstrom. The crystal structure of S. acidocaldarius superoxide dismutas e was solved by Patterson search methods using a dimer of Thermus thermophi lus superoxide dismutase as a search model. The asymmetric unit accommodate s three dimers. Two dimers form a tetramer by using only local symmetries; the third dimer forms a tetramer as well, however, by using the crystallogr aphic 2-fold symmetry. The three-dimensional structure of the S. acidocaldarius dismutase has typi cal features of tetrameric dismutases. Secondary structure elements as well as residues important for the catalytic activity of the enzyme were found to be highly conserved. The model was refined at a resolution of 2.2 Angstr om and yielded a crystallographic R-value of 17.4% (R-free = 22.3 %). A str uctural comparison of the two extremely stable tetrameric dismutases from S , acidocaldarius and Aquifex pyrophilus with the less stable enzyme from T. thermophilus and Mycoplasma tuberculosis revealed the structural determina nts which are probably responsible for the high intrinsic stability of S. a cidocaldarius dismutase. The most obvious factor which may give rise to the extraordinary thermal stability of S. acidocaldarius dismutase (melting te mperature of about 125 degrees C) is the increase in intersubunit ion pairs and hydrogen bonds and, more importantly, the significant reduction of sol vent-accessible hydrophobic surfaces, as well as an increase in the percent age of buried hydrophobic residues. (C) 1999 Academic Press.