THE MUTABLE DIMER OF Cu, Zn SUPEROXIDE DISMUTASE

Pesce, A.^1,2, Bordo,D.^1,2, Rosano,C.^1,2, Stroppolo,ME.³, Desideri,A.³, Battistoni,A.³ , Matak, D.⁴, Djinovic-Carugo, K.⁵, Bolognesi,M.^1,2

1 Advanced Biotechnology Center - IST, Genova, L.go R. Benzi 10, 16132 Genova (Italy)
2 Department of Physics and INFM, University of Genova, Via Dodecaneso 33, 16146 Genova (Italy)
3 Department of Biology, University of Roma "Tor Vergata", Via della Ricerca Scientifica, 00133 Roma (Italy)
4 Laboratory of General and Inorganic Chemistry, Faculty of Science, University of Zagreb, Ulica Kralja Zvonimira , 8., HR-1000 Zagreb (Croatia)
5 EMBL, Meyerhoffstrasse, 1, D-69117 Heidelberg (Germany)

Cu,Zn superoxide dismutases (SODs) are ubiquitous enzymes which are involved in detoxification from free radicals resulting from in vivo oxygen-based biological processes. In fact, the naturally produced superoxide radical (O^2-) is responsible for a series of reactions which eventually lead to chemical denaturation of proteins, cell wall and DNA damage. The SOD enzyme is conserved as a homo-dimer in its three-dimensional structure in all eukaryotic organisms,. Each subunit is based on an eight-stranded (greek-key topology) b-barrel, hosting the binuclear metal center. Within the Cu,Zn pair, the catalytically active species is Cu⁺⁺, which undergoes a redox cycle coupled to the production of O₂ and H₂O₂, starting from the O^2- substrate.

The 3-D structure of Photobacterium leiognathi SOD (space group R32, unit cell a =b = 86.9 A, c = 99.0 A, g = 120°) has been solved by molecular replacement techniques and refined. Moreover, recently grown crystals of Salmonella typhimurium SOD (space group C2, a = 142.1 A, b = 40.7 A, c = 114.3 A, b = 107.7°) have also provided insight into a new structure for a dimeric prokaryotic SOD. Molecular evolution analysis was conducted with programs from the packages CLUSTAL-W and DRAWTREE, leading to the construction of bacterial SOD evolutionary trees.

We have examined, so far, three structures of prokaryotic SODs, finding that, despite overall conservation of amino acid sequences and three-dimensional folds, bacterial SODs show mutable quaternary structure assembly. The prokaryotic SOD main properties can be summarised as: 1) extended amino acid insertion/deletions in loop structural regions; 2) shift of the evolutionary conserved electrostatic residues of the enzyme to the S-S subloop region; 3) conservation of the fast turnover properties of the enzyme; 4) substantial modification of the structural and polarity surface properties of the protein. The latter observation has two direct implications for the quaternary structure of the enzyme: 4.1) the dimerization interface in bacterial SODs is drastically altered with respect to the eukaryotic SODs, leading to a new dimeric assembly; 4.2) in some cases, the amino acid mutations at the molecular surface are such that a monomeric SOD species becomes stable under physiological conditions.