Reduced ferredoxin: CO2 oxidoreductase from Clostridium pasteurianum. Effect of ligands to transition metals on the activity and the stability of the enzyme

Abstract

Reduced ferredoxin: CO2 oxidoreductase (CO2-reductase) from Clostridium pasteurianum catalyzes the reduction of CO2 to formate at the expense of reduced ferredoxin, an isotopic exchange between CO2 and formate in the absence of ferredoxin, and the oxidation of formate to CO2 with oxidized ferredoxin. The three activities were found to be equally affected by monovalent anions known to be ligands to transition metals: The enzyme was reversibly inhibited by azide (Ki = 0.004mM), cyanate (Ki = 0.3 mM), thiocyanate (Ki = 1mM), nitrite (Ki = 0.4mM), nitrate (Ki = 6mM), chlorate (Ki = 3mM), fluoride (Ki = 5mM), and by chloride, bromide, iodide (Ki greater than 5mM). There was no observable effect of pH on the inhibition constants. The enzyme was not inhibited by carbon monoxide. The enzyme was irreversibly inactivated by low concentrations (10muM) of cyanide. The rate of inactivation increased with increasing pH with an inflection point near pH 9.5. Reduced ferredoxin and formate rather than oxidized ferredoxin or CO2 protected the enzyme from inactivation by cyanide. The enzyme was protected by azide and cyanate from inactivation. In the presence of high concentrations of the monovalent anions the rate of inactivation by heat (55 degrees C), by molecular oxygen, and by cyanide was decreased by a factor of more than 100. Half maximal protection was observed at the Ki concentrations of the two reversible inhibitors. The data are interpreted to indicate that a transition metal of weak "a class" character and a disulfide are catalytically significant groups of CO2-reductase from C. pasteurianum.