Comparison and contrasts between the active site PKs of Mn-superoxide dismutase and those of Fe-superoxide dismutase

The Fe- and Mn-containing superoxide dismutases catalize the same reaction and have almost superimposable active sites. Therefore, the details of their mechanisms have been assumed to be similar. However, we now show that the pH dependence of Eschenchia Escherichia coli MnSOD activity reflects a different active site proton equilibrium in (oxidized) Mn³⁺SOD than the event that affects the active site pK of oxidized FeSOD. We find that the universally conserved Tyr34 that has a pK above 11.5 in Fe³⁺SOD is responsible for the pK near 9.5 of Mn³⁺SOD and, thus, that the oxidized state pK of Mn³⁺SOD corresponds to an outer-sphere event whereas that of Fe^3-SOD corresponds to an inner sphere event [Bull, C.; Fee, J. A. J. Am. Chem. Soc. 1985, 107, 3295-3304]. We also present the first description of a reduced-state pK for MnSOD. Mn²⁺SOD's pK involves deprotonation of Tyr34, as does Fe²⁺SOD's pK [Sorkin, D. L.; Miller A.-F. Biochemistry 1997, 36, 4916-4924]. However, the values of the pKs, 10.5 and 8.5 respectively, are quite different and Mn²⁺SOD's pK affects the coordination geometry of Mn²⁺ most likely via polarization of the conserved Gln146 that hydrogen bonds to axially coordinated H₂O. Our findings are consistent with the different electronic configurations of Mn^2+/3+vs Fe^2+/3+, such as the stronger hydrogen bonding between Gln146 and coordinated solvent in MnSOD than that between the analogous Gln69 and coordinated solvent in FeSOD, and the existence of weakly localized H₂O near the sixth coordination site of Mn^2- in Mn²⁺SOD [Borgstahl et al. J. Mol. Biol. 2000, 296, 951-959].