The pH dependence of the cathodic peak potential of the active sites in bilirubin oxidase.

This is the first study showing pH dependence of three distinct redox sites within bilirubin oxidase (BOD) adsorbed on a nanocomposite modified electrode. The 1st redox centre with the highest redox potential Ec(1st)=404 mV vs. Ag/AgCl (614 mV vs. NHE at pH7.0) exhibited pH dependence with a slope -dEc(1st)/dpH=66(±3) mV under a non-turnover process. The 2nd redox centre with a potential Ec(2nd)=228 mV vs. Ag/AgCl (438 mV vs. NHE at pH7.0) was not dependent on pH in the absence and presence of O2. Finally, the 3rd redox site with a redox potential Ec(3rd)=92 mV vs. Ag/AgCl (302 mV vs. NHE at pH7.0) exhibited pH dependence for a cathodic process with -dEc(3rd)/dpH=70(±6) mV and for anodic process with -dEa(3rd)/dpH=73(±2) mV, respectively. Moreover, two break points for dependence of Ec(1st) or Ec(3rd) on pH were observed for the 1st (T1) site and the 3rd site assigned to involvement of two acidic amino acids (Asp105 and Glu463). A diagram of a potential difference between cathodic peaks of BOD as a dependence on pH is shown. The results obtained can be of interest for construction of biofuel cells based on BOD such as for generation of a low level of electricity from body fluids.