Hydrogen-bond-mediated tuning of the redox potential of the non-heme Fe site of superoxide dismutase.

The highly homologous proteins of Fe-containing superoxide dismutase (FeSOD) and MnSOD from Escherichia coli nonetheless exert very different redox tuning on the active site metal ion [Vance; Miller J. Am. Chem. Soc. 1998, 120, 461-467; Biochemistry 2001, 40, 13079-13087]. This was proposed to stem from different hydrogen bonding between the protein and the metal ion's coordinated solvent molecule, and the tight coupling between the protonation state of coordinated solvent and the oxidation state of the metal ion. We now present density functional theory (DFT) calculations on Fe2+ and Fe3+ bound to models of both FeSOD and MnSOD. The calculations support a very important role for the conserved second sphere Gln in MnSOD in specifically destabilizing coordinated H2O relative to coordinated OH-, and thus disfavouring the oxidized state of the metal ion. To test these results we have mutated this Gln to Glu, which is isosteric and isoelectronic to Gln but functions as an H-bond acceptor instead of an H-bond donor and thus should increase the stability of Fe2+-bound H2O. In accordance with the calculations, Q69E-FeSOD displays a significantly higher reduction potential than wild-type FeSOD. Thus we have demonstrated that hydrogen bonds to coordinated solvent can exert strong redox tuning on a metal ion.