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.
Highlights
At the most fundamental level energy transduction in all living systems depends on electron transfer (ET) reactions catalyzed by redox-active proteins, often using metal[1], flavin[2] and quinone[3] cofactors
Individual pili were deposited across the edge of gold electrodes nanofabricated onto an electrically insulating SiO2 substrate and the tip of a conductive-probe atomic force microscope (CP-AFM) was positioned on regions of the pilus at various distances from the electrode edge to form the contacts for a two-point electronic transport measurement (Fig. 1A)
The results demonstrate that the pilus protein fiber transports charges, consistent with the predicted role of pili as protein nanowires between the cell and external electron acceptors such as Fe(III) oxides,14uranium[44] and matrix-associated c-cytochromes in electrochemically active biofilms[23]
Summary
At the most fundamental level energy transduction in all living systems depends on electron transfer (ET) reactions catalyzed by redox-active proteins, often using metal[1], flavin[2] and quinone[3] cofactors. Distances of sometimes more than 20 Å have been reported to permit successive short and fast hopping reactions in proteins[6], where the charges (electron or holes) reside for small periods of time in some amino acids or in the peptide backbone and “hop” as they travel distances between donor and acceptor[7]. In enzymes such as oxygenases, dioxygenases, and peroxidases, hole hopping between tyrosines and tryptophans transports potentially oxidizing equivalents away from the active center and toward surface regions to prevent oxidative damage[8]. The aromatic contacts never formed at the same time, as would have been expected in a metal wire[23]
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