Role of the tertiary and quaternary structures in the stability of dimericcopper,zinc superoxide dismutases

The equilibrium unfolding process of human Cu,Zn superoxide dismutase has b een quantitatively monitored through circular dichroism and fluorescence sp ectroscopy as a function of increasing guanidinium hydrochloride concentrat ion. The process occurs through the formation of a monomeric intermediate s pecies following a three-state transition equilibrium. Comparison with the stability of the prokaryotic Cu,Zn SOD from P. leiognathi shows that the eu karyotic enzyme is more stable than the prokaryotic enzyme by similar to 3 kcal/mol. This difference is due to the monomer-to-unfolded equilibrium, wh ile the dimer-to-monomer equilibrium is comparable for the two enzymes desp ite their different intersubunit interactions. These results are confirmed by the unfolding of the copper-depleted derivatives. The Cu,Zn superoxide d ismutase represents a good example of how evolution has found two independe nt quaternary assemblies maintaining the same dimer stability. (C) 2000 Aca demic Press.