The active site of manganese-containing superoxide dismutase from Bacillus stearothermophilus studied by 1H and 19F magnetic relaxation.

The dependence of the magnetic relaxation rates of 1H and 19F- on temperature, frequency, pH and N-3 concentration, were measured in solutions of Manganese-containing superoxide dismutase of Bacillus stearothermophilus, and were compared to activity measurements, in order to obtain some information on the structure and dynamics at Mn(III) present in the active site of the enzyme. The experimental data lead us to hypothesize the presence of two binding sites in the coordination sphere of the enzyme bound Mn(III), which are accessible to water and anions and have different chemical and spectroscopic properties. NMR measurements carried out in the presence of competitive inhibitors and the pH dependence of both NMR relaxation rates suggest that F-, N-3 and OH- ions bind to one site, while a water molecule binds to the other one. The stability constant values of the complexes between these anions and the enzyme are reported. The influence of the anions on activity and the pH dependence of NMR parameters are discussed.