Modelling the three-dimensional structure and electrostatic potential field of the two Cu,Zn superoxide dismutase variants from Xenopus laevis

Cu,Zn superoxide dismutase from Trematomus bernacchii: functional conservation and erratic molecular evolution in Antarctic teleosts

In the present study, we describe the purification and molecular characterization of Cu,Zn superoxide dismutase (SOD) from Trematomus bernacchii, a teleost widely distributed in many areas of Antarctica, that plays a pivotal role in the Antarctic food chain. The amino acid and cDNA sequences have been obtained using both biochemical and molecular biology approaches and are compared with Cu,Zn SODs from other fishes. Assessment of the primary sequences highlights that the catalytically important residues are fully conserved in Cu,Zn SOD from T. bernacchii. Phylogenetic analyses performed on Cu,Zn SOD amino acid sequences permit speculation regarding the evolution of this protein. In particular, the data confirms the erratic differentiation of these proteins and concurs with the theory of the "unclock-like" behaviour of Cu,Zn SOD evolution.

A free cysteine residue at the dimer interface decreases conformational stability of Xenopus laevis copper,zinc superoxide dismutase

The two Cu,Zn superoxide dismutases from the amphibian Xenopus laevis (denoted XSODA and XSODB) display different heat sensitivities, XSODA being more thermolabile than XSODB. In this study, we have investigated the contribution of a free cysteine residue located close to the subunit interface of XSODA to its lower thermal stability. We have found that mutation of residue Cys 150 to Ala in XSODA makes the thermal stability of this enzyme comparable to that of the wild-type XSODB isoenzyme, while the introduction of a cysteine residue in the same position of XSODB renders this enzyme variant much more heat-sensitive. Differential scanning calorimetry experiments showed that XSODA has a melting temperature about 8.5 degrees C lower than that of XSODB. On the contrary, the melting temperature of XSODACys150Ala is very close to that of XSODB, while the melting temperature of XSODBSer150Cys is even lower than that of wild-type XSODA. These data indicate that the free cysteine residue present in XSODA affects not only the reversibility of unfolding of the enzyme but also its conformational stability. We suggest that the large effect of the Cys 150 residue on XSODA stability might be due to incorrect disulfide bond formation or disulfide bond interchange during heat-induced unfolding rather than to alteration of the interaction between the enzyme subunits.

A model for the incorporation of metal from the copper chaperone CCS into Cu,Zn superoxide dismutase

BACKGROUND

Recent studies have identified the human copper chaperone CCS as the presumed factor responsible for copper incorporation into superoxide dismutase (SOD). A lack of knowledge of the chaperone's three-dimensional structure has prevented understanding of how the copper might be transferred.

RESULTS

The three-dimensional structure of CCS was homology modelled using the periplasmic protein from the bacterial mercury-detoxification system and the structure of one subunit of the human SOD dimeric enzyme as templates. On the basis of the three-dimensional model, a mechanism for the transfer of copper from CCS to SOD is proposed that accounts for electrostatic acceptor recognition, copper storage and copper-transfer properties.

CONCLUSIONS

The proposed model identifies a path for copper transfer based on the presence of different metal sites characterized by sulphur ligands. Such a model permits the development of strategies able to interfere with copper incorporation in SOD, providing a possible way to prevent or arrest degeneration in the fatal motor neuron disorder amyotrophic lateral sclerosis.

Evolutionary constraints for dimer formation in prokaryotic Cu,Zn superoxide dismutase

Prokaryotic Cu,Zn superoxide dismutases are characterized by a distinct quaternary structure, as compared to that of the homologous eukaryotic enzymes. Here we report a newly determined crystal structure of the dimeric Cu,Zn superoxide dismutase from Photobacterium leiognathi (crystallized in space group R32, refined at 2.5 A resolution, R-factor 0.19) and analyse it in comparison with that of the monomeric enzyme from Escherichia coli. The dimeric assembly, observed also in a previously studied monoclinic crystal form of P. leiognathi Cu,Zn superoxide dismutase, is based on a ring-shaped subunit contact region, defining a solvated interface cavity. Three clusters of neighbouring residues play a direct role in the stabilization of the quaternary assembly. The present analysis, extended to the amino acid sequences of the other 11 known prokaryotic Cu,Zn superoxide dismutases, shows that at least in five other prokaryotic enzymes the interface residue clusters are under strong evolutionary constraint, suggesting the attainment of a quaternary structure coincident with that of P. leiognathi Cu,Zn superoxide dismutase. Calculation of electrostatic fields for both the enzymes from E. coli and P. leiognathi shows that the monomeric/dimeric association behaviour displayed by prokaryotic Cu, Zn superoxide dismutases is related to the distribution of surface charged residues. Moreover, Brownian dynamics simulations reproduce closely the observed enzyme:substrate association rates, highlighting the role of the active site neighbouring residues in determining the dismutase catalytic properties.

The role of the active site amino acid residues on the catalytic activity of Cu2Zn2SOD

Copper, zinc superoxide dismutase, the enzyme that catalyzes the dismutation of the superoxide ion, is a key enzyme for neurodegenerative processes caused by the formation into tissues of this reactive free radical. Although the structure of enzyme is known, the catalytic mechanism has not yet fully elucidated. The copper ion represents the center, which exchanges electrons with the superoxide ion, but other residues present in the active site seem to play a relevant role. The most recent advancements in clarifying the structure-function relationship in this enzyme are discussed.

Evolutionary conservativeness of electric field in the Cu,Zn superoxide dismutase active site. Evidence for co-ordinated mutation of charged amino acid residues

Equipotential lines were calculated, using the Poisson-Boltzmann equation, for six Cu,Zn superoxide dismutases with different protein electric charge and various degrees of sequence homology, namely those from ox, pig, sheep, yeast, and the isoenzymes A and B from the amphibian Xenopus laevis. The three-dimensional structures of the porcine and ovine superoxide dismutases were obtained by molecular modelling reconstruction using the structure of the highly homologous bovine enzyme as a template. The three-dimensional structure of the evolutionary distant yeast Cu,Zn superoxide dismutase was recently resolved by us, while computer-modelled structures are available for X. laevis isoenzymes. The six proteins display large differences in the net protein charge and distribution of electrically charged surface residues but the trend of the equipotential lines in the proximity of the active sites was found to be constant in all cases. These results are in line with the very similar catlytic rate constants experimentally measured for the corresponding enzyme activities. This analysis shows that electrostatic guidance for the enzyme-substrate interaction in Cu,Zn superoxide dismutases is related to a spatial distribution of charges, arranged so as to maintain, in the area surrounding the active sites, an identical electrostatic potential distribution, which is conserved in the evolution of this protein family.