Mechanism of Chloride Inhibition of Bilirubin Oxidases and Its Dependence on Potential and pH

Bilirubin oxidases (BODs) belong to the multicopper oxidase (MCO) family and efficiently reduce O2 at neutral pH and under physiological conditions where chloride concentrations are >100 mM. BODs were consequently considered to be Cl– resistant, as opposed to laccases. However, there has not been a detailed study of the related effect of chloride and pH on the redox state of immobilized BODs. Here, we investigate by electrochemistry the catalytic mechanism of O2 reduction by the thermostable Bacillus pumilus BOD immobilized on carbon nanofibers in the presence of NaCl. The addition of chloride results in the formation of a redox state of the enzyme, previously observed for different BODs and laccases, which is active only after a reductive step. This behavior has not been previously investigated. We show that the kinetics of formation of this state is strongly dependent on pH, temperature, Cl– concentration, and applied redox potential. Ultraviolet–visible spectroscopy allows us to correlate the inhibition by chloride with the formation of the alternative resting form of the enzyme. We demonstrate that O2 is not required for its formation and show that the application of an oxidative potential is sufficient. In addition, our results suggest that reactivation may proceed through T3β.