Engineering Charge Transport in Multicopper Oxidases for the Development of Bioelectrocatalytic Systems

Abstract

Bioelectrocatalysis has been intensively studied in the context of biofuel cell research. Unlike traditional fuel cells that rely on metal catalysts to produce energy from a fuel, biofuel cells involve biocatalysts, such as enzymes, that make use of bioavailable substrates. Enzymes being more specific to their substrate and more sustainable to produce, show enormous potential for energy production in bioelectrocatalytic systems. In order to increase the commercial viability of biofuel cells, biocatalysts need to be able to function under industrially (or physiologically) relevant conditions, including high temperatures, high salinity and extremes in pH (acidic or alkaline). Multicopper oxidases (MCOs) are a family of oxidoreductase enzymes well suited to bioelectrocatalytic applications. In this work, a bacterial MCO from Bacillus subtilis – BsCotA – was overexpressed in E. coli, and its aptness for electrochemical applications was evaluated. Initial experiments found that this enzyme had poor catalytic efficiency, prompting an investigation to optimise the production of highly active BsCotA. Production optimisation was achieved by varying the conditions of protein expression and purification, and was evaluated by means of enzymatic activity assays, and spectroscopic techniques were used to study the copper co-factors. The largest increase in catalytic efficiency came as a result of removing sodium chloride from all buffers used to manipulate BsCotA. This discovery led a mutagenesis study aimed at increasing the chloride tolerance of BsCotA by mutating residues found at the entrance to the water exit channel. Halide inhibition of the O2 reduction reaction in MCOs is widely reported, but the mechanism is still poorly understood. An electrochemical method based on protein-film electrochemistry, was initially conceived to characterise the chloride inhibition in BsCotA, by evaluating the effect of the inhibitor on the electrocatalytic current. By varying inhibitor and O2 substrate concentrations, the electrochemical assay was used to identify the mode of inhibition of the inhibitor (competitive, non-competitive or uncompetitive), and to determine reaction and inhibition parameters in a potential- and pH-dependent manner. This method was then used to study three other MCOs and extended to other (pseudo)halide inhibitors. Using the newly developed electrochemical assays in combination with other electrochemical techniques, the Cl– inhibition of all four MCOs was investigated in the context of a structure–function study. By relating the results of the electrochemical 29 experiments to surface features found in the crystal structures of the enzymes, a new mechanism for Cl– inhibition in MCOs was proposed.

Date of Award	1 Aug 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Lu Shin Wong (Supervisor) & Christopher Blanford (Supervisor)