engleski

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. In this communication 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 analyze 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 eleven 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 attainement 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 neighboring residues in determining the dismutase catalytic properties.

monomeric Cu; Zn superoxide dismutase; protein-subunit recognition; quaternary structure; Poisson-Boltzmann equation; protein electrostatics

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