Nuclear magnetic relaxation dispersion in protein solutions. V. Bovine erythrocyte superoxide dismutase

Abstract

The paramagnetic contribution Rp to the spin-lattice relaxation rate of solvent protons in aqueous solutions of bovine erythrocyte superoxide dismutase has been measured for magnetic fields in the range 5 Oe to 12 kOe and temperatures in the range 0 °C to 25 °C. We find a peak in Rp as a function of magnetic field, as has been observed recently by several investigators for Mn2+-macromolecular complexes, indicating that the spin-lattice relaxation time τs of the Cu2+ electron spin moments is magnetic field dependent. τs is approx. 5·10-10 s at low field and begins to increase above approx. 750 Oe. From the magnitude of Rp and its qualitative behavior with magnetic field and temperature, combined with previous EPR and inhibition studies, we conclude that there is at least one rapidly exchanging water molecule (exchange time τM in the range 4·10-6 to 10-8 s) on each of the two Cu2+ per protein molecule. We speculate that the water binding sites on the Cu2+, with a Cu-O bond distance approx. 2.0 Å, are similarly accessible to the superoxide anion and are the loci for the catalytic activity of the enzyme. © 1972.