Kinetic and spectroscopic studies on a superoxide dismutase from Propionibacterium shermanii that is active with iron or manganese: pH-dependence

Kinetic studies were performed on the superoxide dismutases isolated from the anaerobic bacterium Propionibacterium shermanii as active enzymes with either iron or manganese, which were naturally incorporated into the same molecule depending on the metal supply. Both the Fe- and Mn- forms showed decreasing activity with increasing pH. This suggests the protonation of some groups near the metal, possibly a metal-bound water molecule. Thus the kinetic behaviour of this superoxide dismutase is much more dependent on the protein structure than on the metal incorporated into the active site. The secondary structures of both forms were not influenced by variations in pH, whereas the EPR spectra of the Fe-superoxide dismutase changed as a function of pH. The EPR spectra apparently consist of two overlapping species. Steady-state experiments proved that all iron-containing species show catalytic activity, but the species predominating in the alkaline pH range displays a lower reaction rate. The Michaelis constant and maximal turnover number for the Fe-superoxide dismutase were determined polarographically as K(m) = 0.54 mmol/l and V(max.)= 2000 mol s^-1 at pH 9.5. These data indicate that, in anaerobic bacteria under physiological conditions, the superoxide dismutase is not saturable with O₂^- and the catalytic activity is similar to that of metal-specific Fe- or Mn-superoxide dismutases from aerobic organisms.