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

Prokaryotic Cu,Zn superoxide dismutases are characterized by a distinct qua ternary 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 Angstrom resolution, R-factor 0.19) and analyse i t in comparison with that of the monomeric enzyme from Escherichia coli. Th e dimeric assembly, observed also in a previously studied monoclinic crysta l form of P. leiognathi Cu,Zn superoxide dismutase, is based on a ring-shap ed subunit contact region, defining a solvated interface cavity. Three clus ters of neighbouring residues play a direct role In the stabilization of th e quaternary assembly. The present analysis, extended to the amino acid seq uences of the other 11 known prokaryotic Cu,Zn superoxide dismutases, shows that at least in five other prokaryotic enzymes the interface residue clus ters are under strong evolutionary constraint, suggesting the attainment of a quaternary structure coincident with that of P. leiognathi Cu,Zn superox ide dismutase. Calculation of electrostatic fields for both the enzymes fro m E. coli and P. leiognathi shows that the monomeric/dimeric association be haviour displayed by prokaryotic Cu,Zn superoxide dismutases is related to the distribution of surface charged residues. Moreover, Brownian dynamics s imulations reproduce closely the observed enzyme:substrate association rate s, highlighting the role of the active site neighbouring residues in determ ining the dismutase catalytic properties. (C) 1999 Academic Press.