ENDOR and ESEEM Investigation of Ag^I₂Cu^II₂ Bovine Superoxide Dismutase

Ag^I₂Cu^II₂SOD, a derivative of bovine superoxide dismutase containing Ag and Cu rather than Cu and Zn, has been studied by a wide range of paramagnetic resonance techniques. Evidence based on the linear electric field effect (LEFE) in pulsed EPR supports the original conclusion that Cu(II) occupies the Zn(II) site of the native protein, substantially retaining the pseudo-tetrahedral coordination geometry. Cw ENDOR investigation at X (9 GHz) and Q (35 GHz) bands and pulsed ENDOR at X band demonstrate at least two populations of imidazole-¹⁴N directly coordinated to Cu(II) in the Zn(II) site, one with an isotropic hyperfine coupling of about 42 MHz, normal for Cu imidazole complexes, and the other, which is inferred to have two members, with a coupling of about 28 MHz. Electron spin echo envelope modulation (ESEEM) investigation also detects two populations of remote ¹⁴N of imidazole coupled to Cu(II). One population consists of a single ¹⁴N with an isotropic hyperfine coupling constant of about 2.0 MHz and is assigned to the imidazole with the 42-MHz coupling. The second population has a coupling of about 1 MHz and is assigned to imidazole with the 28-MHz coupling. This investigation demonstrates that Cu(II) forms inequivalent bonds to the three imidazoles in the distorted tetrahedral environment of the Zn(II) site and is consistent with the view that a bimetallic imidazolate bridge found in the native Cu₂Zn₂ protein is broken in Ag^I₂Cu^II₂SOD.