Among manganese superoxide dismutases, residues His30 and Tyr174 are highly conserved, forming part of the substrate access funnel in the active site. These two residues are structurally linked by a strong hydrogen bond between His30 NE2 from one subunit and Tyr174 OH from the other subunit of the dimer, forming an important element that bridges the dimer interface. Mutation of either His30 or Tyr174 in Escherichia coli MnSOD reduces the superoxide dismutase activity to 30-40% of that of the wt enzyme, which is surprising, since Y174 is quite remote from the active site metal center. The 2.2 Å resolution X-ray structure of H30A-MnSOD shows that removing the Tyr174→His30 hydrogen bond from the acceptor side results in a significant displacement of the main-chain segment containing the Y174 residue, with local rearrangement of the protein. The 1.35 Å resolution structure of Y174F-MnSOD shows that disruption of the same hydrogen bond from the donor side has much greater consequences, with reorientation of F174 having a domino effect on the neighboring residues, resulting in a major rearrangement of the dimer interface and flipping of the His30 ring. Spectroscopic studies on H30A, H30N, and Y174F mutants show that (like the previously characterized Y34F mutant of E. coli MnSOD) all lack the high pH transition of the wt enzyme. This observation supports assignment of the pH sensitivity of MnSOD to coordination of hydroxide ion at high pH rather than to ionization of the phenolic group of Y34. Thus, mutations near the active site, as in the Y34F mutant, as well as at remote positions, as in Y174F, similarly affect the metal reactivity and alter the effective pKa for hydroxide ion binding. These results imply that hydrogen bonding of the H30 imidazole N-H group plays a key role in substrate binding and catalysis.
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