ENDOR and ESEEM investigation of the Ni-containing superoxide dismutase.

Superoxide dismutases (SODs) protect cells against oxidative stress by disproportionating O2(-) to H(2)O(2) and O(2). The recent finding of a nickel-containing SOD (Ni-SOD) has widened the diversity of SODs in terms of metal contents and SOD catalytic mechanisms. The coordination and geometrical structure of the metal site and the related electronic structure are the keys to understanding the dismutase mechanism of the enzyme. We performed Q-band (14)N,(1/2)H continuous wave (CW) and pulsed electron-nuclear double resonance (ENDOR) and X-band (14)N electron spin echo envelope modulation (ESEEM) on the resting-state Ni-SOD extracted from Streptomyces seoulensis. In-depth analysis of the data obtained from the multifrequency advanced electron paramagnetic resonance techniques detailed the electronic structure of the active site of Ni-SOD. The analysis of the field-dependent Q-band (14)N CW ENDOR yielded the nuclear hyperfine and quadrupole coupling tensors of the axial N(delta) of the His-1 imidazole ligand. The tensors are coaxial with the g-tensor frame, implying the g-tensor direction is modulated by the imidazole plane. X-band (14)N ESEEM characterized the hyperfine coupling of N(epsilon) of His-1 imidazole. The nuclear quadrupole coupling constant of the nitrogen suggests that the hydrogen-bonding between N(epsilon)-H and O(Glu-17) present for the reduced-state Ni-SOD is weakened or broken upon oxidizing the enzyme. Q-band (1)H CW ENDOR and pulsed (2)H Mims ENDOR showed a strong hyperfine coupling to the protons(s) of the equatorially coordinated His-1 amine and a weak hyperfine coupling to either the proton(s) of a water in the pocket at the side opposite the axial N(delta) or the proton of a water hydrogen-bonded to the equatorial thiolate ligand.