Electrostatic-Driven Activity, Loading, Dynamics, and Stability of a Redox Enzyme on Functionalized-Gold Electrodes for Bioelectrocatalysis

The oxygen reduction reaction is the limiting step in fuel cells, and many works are in progress to find efficient cathode catalysts. Among them, bilirubin oxidases are copper-based enzymes that reduce oxygen into water with low overpotentials. The factors that ensure electrocatalytic efficiency of the enzyme in the immobilized state are not well understood, however. In this work, we use a multiple methodological approach on a wide range of pH values for protein adsorption and electrocatalysis to demonstrate the effect of electrostatic interactions on the electrical wiring, dynamics, and stability of a bilirubin oxidase adsorbed on self-assembled-monolayers on gold. We show on one hand that the global charge of the enzyme controls the loading on the interface and that the specific activity of the immobilized enzyme decreases with the enzyme coverage. On the other hand, we show that the dipole moment of the protein and the charge in the vicinity of the Cu site acting as the entry point of electrons drive the enzyme orientation. In case of weak electrostatic interactions, we demonstrate that local pH variation affects the electron transfer rate as a result of protein mobility on the surface. On the contrary, stronger electrostatic interactions destabilize the protein structure and affect the stability of the catalytic signal. These data illustrate the interplay between immobilized protein dynamics and local environment that control the efficiency of bioelectrocatalysis.