Role of the tertiary and quaternary structures in the stability of dimeric copper, zinc superoxide dismutases.

The equilibrium unfolding process of human Cu,Zn superoxide dismutase has been quantitatively monitored through circular dichroism and fluorescence spectroscopy as a function of increasing guanidinium hydrochloride concentration. The process occurs through the formation of a monomeric intermediate species following a three-state transition equilibrium. Comparison with the stability of the prokaryotic Cu,Zn SOD from P. leiognathi shows that the eukaryotic enzyme is more stable than the prokaryotic enzyme by approximately 3 kcal/mol. This difference is due to the monomer-to-unfolded equilibrium, while the dimer-to-monomer equilibrium is comparable for the two enzymes despite their different intersubunit interactions. These results are confirmed by the unfolding of the copper-depleted derivatives. The Cu,Zn superoxide dismutase represents a good example of how evolution has found two independent quaternary assemblies maintaining the same dimer stability.