Activation of 1-aminocyclopropane-1-carboxylate oxidase by carbon dioxide

1-Aminocyclopropane-1-carboxylate (ACC) oxidase requires CO2/HCO3- as an essential activator for its activity. Taking advantage that the equilibrium concentrations of CO2 and HCO3- vary with pH and that the interconversions of CO2 and HCO3- are slower at low temperature, we identified CO2 rather than HCO3- as the active species involved in the activation process. Preincubation of the enzyme with a saturating concentration of CO2 resulted in increased activation of the enzyme when preincubation pH was raised, indicating that CO2 reacted with an enzyme group having an alkaline pKa. It is suggested that the CO2 activation of ACC oxidase involves the formation of a carbamate. CO2 increases the Vmax of the reaction but decreases the affinity of the enzyme toward its substrate ACC. A plausible reaction scheme accounting for the CO2 activation process is presented.