Theoretical studies of manganese and iron superoxide dismutases: superoxide binding and superoxide oxidation.
Density-functional calculations indicate that the second sphere of coordination around the metal centers of manganese and iron superoxide dismutases (MnSODs and FeSODs) plays an important role in the binding of O2(-). In these systems, O2(-) prefers to bind to Mn or Fe in end-on configurations. For human and E. coli MnSODs, the bound O2(-) forms hydrogen bonds with the tyrosine and glutamine amino acid residues in the second sphere of coordination. In the cases of E. coli and T. elongates FeSODs, hydrogen bonding occurs between the bound O2(-) and the tyrosine amino acid only because the glutamine is too far away for an effective bonding interaction. The manner in which the O2(-) binds to the metal center in MnSODs and FeSODs can affect the rate of subsequent protonation and determine the mechanism for the formation of H2O2. Both Mn- and Fe-containing superoxide dismutases contain a metal-bound solvent molecule that has been suggested to be involved in the uptake of a H+ upon reduction of the metal center [Bull, C.; Fee, J. A. J. Am. Chem. Soc. 1985, 107, 3295; Miller, A.-F.; Padmakumar, K.; Sorkin, D. L.; Karapetian, A.; Vance, C. K. J. Inorg. Biochem. 2003, 93, 71]. Using density-functional theory, we confirm this suggestion and show the involvement of the second sphere of coordination in the process. We show that the oxidation of superoxide by Mn- or Fe-containing superoxide dismutases is facilitated by a cooperative effect between superoxide binding, protonation of the OH- bound to the metal, and electron transfer from the superoxide molecule to the oxidized metal. In particular, proton transfer through tyrosine-34 on the absence of a bound superoxide is uphill while, once superoxide is bound, the energetic barrier is lowered. It is this barrier that likely keeps the resting state (Mn(III)SOD) of the enzyme with a bound hydroxide, instead of a water. This work provides a model for the mechanism of reaction of superoxide with the oxidized form of the metal within Mn- and FeSODs.