The active species of 'CO2' utilized by reduced ferredoxin:CO2 oxidoreductase from Clostridium pasteurianum

Reduced ferredoxin:CO2 oxidoreductase (CO2 reductase) from Clostridium pasteurianum catalyzes the reduction of 'CO2' to formate with 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 active species of 'CO2', i.e. CO2 or HCO3 (H2CO3), utilized by the enzyme was determined. The method employed for the species identification was that of Copper et al. (1968). Both 'CO2' reduction to formate and the exchange reaction were studied. Data were obtained which are compatible with those expected if CO2 is the active species. The V and the dissociation constant Ks of the enzyme - CO2 complex in dependence of pH were determined from initial velocity studies of the exchange reaction. V was found to be only slightly affected by pH between 5.5 and 7.5. Ks was markedly dependent on pH; the constant increased with decreasing pH from 0.2 mM at pH 7.5 to 3 mM at pH 5.5.