Thermally activated charge transport in microbial protein nanowires

Abstract

The bacterium Geobacter sulfurreducens requires the expression of conductive protein filaments or pili to respire extracellular electron acceptors such as iron oxides and uranium and to wire electroactive biofilms, but the contribution of the protein fiber to charge transport has remained elusive. Here we demonstrate efficient long-range charge transport along individual pili purified free of metal and redox organic cofactors at rates high enough to satisfy the respiratory rates of the cell. Carrier characteristics were within the orders reported for organic semiconductors (mobility) and inorganic nanowires (concentration), and resistivity was within the lower ranges reported for moderately doped silicon nanowires. However, the pilus conductance and the carrier mobility decreased when one of the tyrosines of the predicted axial multistep hopping path was replaced with an alanine. Furthermore, low temperature scanning tunneling microscopy demonstrated the thermal dependence of the differential conductance at the low voltages that operate in biological systems. The results thus provide evidence for thermally activated multistep hopping as the mechanism that allows Geobacter pili to function as protein nanowires between the cell and extracellular electron acceptors.

Harnessing the power of microbial nanowires

“…Discharging electrons onto the minerals is predicted to be fast. The charge transport capacity of individual pilus fibres purified free of metal and organic cofactors (~ 1 billion electrons per second at 100 mV) is, for example, two orders of magnitude greater than the rates of iron oxide respiration per cell (Lampa‐Pastirk et al ., 2016). These measurements were obtained for purified pilus fibres deposited onto a substrate without extensive evaporation and/or chemical fixation (Lampa‐Pastirk et al ., 2016).…”

“…The charge transport capacity of individual pilus fibres purified free of metal and organic cofactors (~ 1 billion electrons per second at 100 mV) is, for example, two orders of magnitude greater than the rates of iron oxide respiration per cell (Lampa‐Pastirk et al ., 2016). These measurements were obtained for purified pilus fibres deposited onto a substrate without extensive evaporation and/or chemical fixation (Lampa‐Pastirk et al ., 2016). Although these conditions preserved the structural and electronic signatures described for cell‐associated pili (Veazey et al ., 2011), the assay was performed with T4P immobilized on a surface.…”

“…This contrasts with the in‐vivo conditions, where the fibres undergo antagonistic cycles of protrusion and retraction and experience motions that are predicted to promote electronic coupling and charge transport (Feliciano et al ., 2015). Thus, the charge transport rates estimated in vitro (Lampa‐Pastirk et al ., 2016) may in fact underestimate the true transport capacity of the T4P in vivo . Furthermore, numerous fibres protrude from one side of the cell at any given time (Fig.…”

“…Furthermore, the GS pilin lacks the more insulating β‐strand conformations (Shin et al ., 2003), a structural property that contributes to creating a peptide environment optimal for charge transport (Feliciano et al ., 2012). Consistent with these predictions, the GS pilin assembly is conductive whereas PAK pilins form insulating T4P (Lampa‐Pastirk et al ., 2016). …”

“…The HOMO and LUMO regions are also located in regions of the pilin predicted to align once the peptides assemble, offering opportunities for intermolecular electron transfer via HOMO–LUMO and LUMO–LUMO interactions (Feliciano et al ., 2012). Consistent with this, scanning tunnelling microscopy (STM) of the conductive GS T4P reveals a rich electronic molecular structure (Veazey et al ., 2011) that is absent in the insulating PAK T4P (Lampa‐Pastirk et al ., 2016). The electronic features of individual GS fibres resolved in STM images have periodicities matching the structural periodicities (major and minor grooves) of other bacterial T4P (Veazey et al ., 2011), which result from the alignment of T4a pilins with a conserved 10.5‐Å rise (Craig et al ., 2006).…”