Iron superoxide dismutase from the archaeon Sulfolobus solfataricus: Analysis of structure and thermostability

The crystal structure of superoxide dismutase (SOD) from the hyperthermophi le Sulfolobus solfataricus has been determined at 2.3 Angstrom resolution b y molecular replacement and refined to a crystallographic X-factor of 16.8% (R-free 19.8%). The crystals belong to the space group C2 (a = 76.3 Angstr om, b = 124.3 Angstrom, c = 60.3 Angstrom, beta = 128.8 degrees) with two i dentical monomers in the asymmetric unit. The monomer has a molecular weigh t of 24 kDa and consists of 210 amino acid residues of which 205 are visibl e in the electron density map. The overall fold of the monomer of S. solfat aricus SOD is similar to that of the other known Fe or Mn-SODs. S. solfatar icus SOD forms a very compact tetramer of a type similar to that of SOD fro m the hyperthermophile Aquifex pyrophilus. Both structures show an elevated number of inter-subunit ion-pairs compared with the mesophilic SOD from My cobacterium tuberculosis and the thermophilic SOD from Thermus thermophilus . However, in contrast to the A. pyrophilus SOD structure, the number of in tra-subunit ion-pairs as well as intersubunit hydrogen bonds is not higher than in the compared mesophilic and thermophilic SOD structures. The electr on density also revealed an unexpected and unusual covalent modification of a conserved tyrosine in the active site. Its involvement in the specific a ctivity of the enzyme is discussed. (C) 1999 Academic Press.