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 f rom the anaerobic bacterium Propionibacterium shermanii as active enzy mes with either iron or manganese, which were naturally incorporated i nto 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 dismuta se is much more dependent on the protein structure than on the metal i ncorporated into the active site. The secondary structures of both for ms were not influenced by variations in pH, whereas the EPR spectra of the Fe-superoxide dismutase changed as a function of pH. The EPR spec tra apparently consist of two overlapping species. Steady-state experi ments proved that all iron-containing species show catalytic activity, but the species predominating in the alkaline pH range displays a low er reaction rate. The Michaelis constant and maximal turnover number f or 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 indic ate that, in anaerobic bacteria under physiological conditions, the su peroxide dismutase is not saturable with O-2(-) and the catalytic acti vity is similar to that of metal-specific Fe- or Mn-superoxide dismuta ses from aerobic organisms.