SUPEROXIDE DISMUTASE FROM THE HYPERTHERMOPHILE SULFOLOBUS SOLFATARICUS
Thomas Ursby1,
Bianca Stella Adinolfi2,
Salam AlKaradaghi1,
Emmanuele De Vendittis2,
Vincenzo Bocchini2
1Molecular Biophysics, Chemical
Centre, Lund University, P.O.B. 124, S-221 00 Lund, Sweden; Thomas.Ursby@mbfys.lu.se
2Dipartimento di Biochimica e
Biotecnologie Mediche, Universita di Napoli Federico II, Via S.
Pansini 5, I-80131 Napoli, Italy
Keywords: superoxide dismutase, Sulfolobus
solfataricus, hyperthermophile, tyrosine modification.
The crystal structure of the iron containing superoxide dismutase (SOD) from the hyperthermophile Sulfolobus solfataricus was solved by molecular replacement to 2.3 A resolution with an R-factor of 16.3% (Rfree 19.3%) [1].
The monomer, 210 amino acids and 24 kDa, has an overall fold similar to other known Fe- or Mn-SOD structures. Four monomers join to form very compact tetramers, similar to SOD of the hyperthermophile Aquifex pyrophilus [2]. Both these structures show an elevated number of inter-subunit ion-pairs but in contrast to A. pyrophilus SOD the number of intra-subunit ion-pairs and inter-subunit hydrogen bonds is not higher in S. solfataricus SOD than in thermophile and mesophile SOD structures. Average hydrophobicity and amino acid weight of SODs, including that from S. solfataricus, has been shown to be correlated to the optimal growth temperature of the organism [3].
An unexpected covalent modification of a tyrosine was found in the crystal structure. This conserved tyrosine gates the entrance to the substrate binding pocket with its hydroxyl group positioned only 5 A from the metal in the active site. The size of the modification could correspond to a phosphate group covalently bound to the hydroxyl oxygen of the tyrosine and to an additional five or six membered ring bound to the phosphate. As revealed by mass spectrometry the modification was present in both native and recombinant protein, though not in all preparations. Such a modification, which has not been reported for other known SOD structures, reduced the activity of the enzyme.