Azide and Fluoride Binding to E. Coli Iron Superoxide Dismutase as Studied by Solvent Proton Magnetic Relaxation Dispersion

Abstract

Nuclear magnetic relaxation dispersion (NMRD) is used to probe solvent exchange with the FeIIIsite in E. Coli superoxide dismutase (FeSOD). We Find that the water proton magnetic relaxation rates in aqueous solutions of the native enzyme are largely determined by innersphere interactions with FeIIIbut that the outersphere contribution is not negligible. Azide binding dramatically increases the FeSOD relaxivity and alters the functional form of the NMRD profile. The enhanced relaxivity of the FeSOD-N3-complex is proposed to result from a rapidly exchanging water molecule that is hydrogen bonded to coordinated azide, with an FeIII-1H distance of ∼3 Å. In marked contrast, fluoride binding to the FeIIIions of FeSOD does not significantly alter the FeSOD relaxivity. Coordinated azide and fluoride must therefore interact with solvent very differently, suggesting that these two FeSOD-anion complexes have different structures. © 1987, American Chemical Society. All rights reserved.