Conductance\nChannels in a Single-Entity Enzyme

For a long time,\nthe prevailing view in the scientific\ncommunity\nwas that proteins, being complex macromolecules composed of amino\nacid chains linked by peptide bonds, adopt folded structure with insulating\nor semiconducting properties, with high bandgaps. However, recent\ndiscoveries of unexpectedly high conductance levels, reaching values\nin the range of dozens of nanosiemens (nS) in proteins, have challenged\nthis conventional understanding. In this study, we used scanning tunneling\nmicroscopy (STM) to explore the single-entity conductance properties\nof enzymatic channels, focusing on bilirubin oxidase (BOD) as a model\nmetalloprotein. By immobilizing BOD on a conductive carbon surface,\nwe discern its preferred orientation, facilitating the formation of\nelectronic and ionic channels. These channels show efficient electron\ntransport (ETp), with apparent conductance up to the 15 nS range.\nNotably, these conductance pathways are localized, minimizing electron\ntransport barriers due to solvents and ions, underscoring BOD’s\nredox versatility. Furthermore, electron transfer (ET) within the\nBOD occurs via preferential pathways. The alignment of the conductance\nchannels with hydrophilicity maps, molecular vacancies, and regions\naccessible to electrolytes explains the observed conductance values.\nAdditionally, BOD exhibits redox activity, with its active center\nplaying a critical role in the ETp process. These findings significantly\nadvance our understanding of the intricate mechanisms that govern\nETp processes in proteins, offering new insights into the conductance\nof metalloproteins.