Cryo-trapping the six-coordinate, distorted-octahedral active site of manganese superoxide dismutase

Superoxide dismutase protects organisms from potentially damaging oxygen ra dicals by catalyzing the disproportionation of superoxide to oxygen and hyd rogen peroxide. We report the use of cryogenic temperatures to kinetically capture the sixth ligand bound to the active site of manganese superoxide d ismutase (MnSOD). Synchrotron X-ray diffraction data was collected from Esc herichia coli MnSOD crystals grown at pH 8.5 and cryocooled to 100 K. Struc tural refinement to 1.55 Angstrom resolution and close inspection of the ac tive site revealed electron density for a sixth ligand that was interpreted to be a hydroxide ligand. The six-coordinate, distorted-octahedral geometr y assumed during inhibition by hydroxide is compared to the room temperatur e, five-coordinate, trigonal bipyramidal active site determined with crysta ls grown from practically identical conditions. The gateway residues Tyr34, His30 and a tightly bound water molecule are implicated in closing-off the active site and blocking the escape route of the sixth ligand. (C) 2000 Ac ademic Press.