Cofactor-specific Anchoring of Horseradish Peroxidase onto a Polythiophene-modified Electrode

Abstract

Horseradish peroxidase apoprotein (apoHRP) was immobilized on a heme-modified polythiophene film, which was electropolymerized on an indium tin oxide surface to yield immobilized and reconstituted HRP. The HRP-immobilized electrode exhibits a 6-fold enhanced electrochemical response toward hydrogen peroxide reduction relative to having the protein randomly immobilized on the polythiophene electrode. This indicates that the insertion of the heme moiety into the heme pocket of HRP leads to an increase in the electrocatalytic current. Well-organized electron-transfer interfaces between redoxactive proteins and conductive materials have been a critical target for the development of biosensors and biodevices.1 In particular, redox-active hemoproteins are expected to be useful for the construction of functional protein-immobilized electrode materials due to their diverse functions, which include electron transfer, catalysis, and sensing. Hemoprotein-immobilized electrodes have thus been studied for their electrochemical behavior 2 and as components of functional systems, such as biocatalysts, 3 biosensors, 4 and biofuel cells. 5Methodologies that provide a means for efficient electrochemical communication have been explored by immobilization of proteins via noncovalent interactions, such as electrostatic interactions with self-assembled monolayers on electrodes, 6 affinity interactions with tag peptides, 7 and specific proteinligand interactions.8 Hemoproteins generally retain their redox-active heme cofactors by noncovalent interactions and at least one metal protein coordination. This unique feature enables oriented anchoring of a hemoprotein via the hemeheme pocket interaction. 9We have recently reported the development of a process for programmed assembly of hemoproteins on metal electrodes and the surfaces of nanoparticles based on the specific hemeheme pocket interaction.10 To increase the electrochemical communication between the redox-active site of the immobilized proteins and the electrode, a fabrication technique for immobilizing sufficient quantities of proteins with favorable orientations is required. In this paper, we demonstrate the immobilization of an apo-form of horseradish peroxidase (HRP), apoHRP, on the surface of a polythiophene (PT) electrode via the hemeheme pocket interaction (Figure 1). Conductive polymers including polythiophene have been studied in the field of solar cells, field effect transistors, and actuators. Abundance of applications is facilitated by the ease of tailoring properties such as processability, functionality, electronic properties, and electrical stability.11 Polythiophene is easily fabricated by electropolymerization on the electrode, and are thus attractive electrode materials for promoting bioelectrocatalysis. 12 We thus apply the immobilization of hemoproteins by hemeanchoring on the polythiophene-modified electrode. The characterization and electrochemical properties of the indium tin oxide (ITO) electrode containing oriented HRP immobilized on the surface of t...