FIGURE 10.

Steric constraints on access to the metal binding site in MnSOD in the native folded state. A cross-sectional view of the active site of E. coli MnSOD is shown, with viewing plane oriented on the only surface-accessible channel to the buried metal binding site. The position of the coordinated metal ion (M) is shown relative to ligating residues (H-26 and D-167 in the inner sphere, Q-146 in the outer sphere) and gateway residues (H-30 and Y-34) that restrict access to the active site from within the substrate channel (S). A metal ion entering the buried binding site from solution must penetrate the gateway barrier (white line), which would be expected to require complete desolvation of the metal cation.