title: A pulse-radiolysis study of the manganese-containing superoxide dismutase from Bacillus stearothermophilus. A kinetic model for the enzyme action.
abstract: The enzymic reaction mechanism of a manganese-containing superoxide dismutase from Bacillus stearothermophilus was studied by using pulse radiolysis. During catalysis (pH 8.9; 25 degrees C), changes occurring in the kinetics of substrate disappearance and in the visible absorption of the enzyme at 480 nm established that the simple two-step mechanism found for copper- and iron-containing superoxide dismutases is not involved. At a low ratio (less than 15) of substrate concentration to enzyme concentration the decay of O2--is close to exponetial, whereas at much higher ratios (greater than 100) the observed decay is predominantly zero-order. The simplest interpretation of the results invokes a rapid one-electron oxidation-reduction cycle ('the fast cycle') and, concurrently, a slower reaction giving a form of the enzyme that is essentially unreactive towards O2-- but which undergoes a first-order decay to yield fully active native enzyme ('the slow cycle'). The fast cycle involves the native enzyme EA and a form of the enzyme EB which can be obtained also by treating the form EA with H2O2. Computer calculations made with such a simple model predict behaviour in excellent agreement with the observed results.
mesh_terms: D002384:Catalysis; D001411:Geobacillus stearothermophilus / Q000201:enzymology*; D006861:Hydrogen Peroxide / Q000494:pharmacology; D007700:Kinetics; D008345:Manganese; D008954:Models, Biological; D011675:Pulse Radiolysis; D013482:Superoxide Dismutase / Q000378:metabolism*
chemical_list: D008345:Manganese; D006861:Hydrogen Peroxide; D013482:Superoxide Dismutase