Abstract
Microorganisms can transfer electrons directly to extracellular acceptors, during which organic compounds are oxidized to carbon dioxide. One of these microbes, Geobacter sulfurreducens, is well known for the “metallic-like” conductivity of its type IV pili. However, there is no consensus on what the mechanism for electron transfer along these conductive pili is. Based on the aromatic distances and orientations of our predicted models, the mechanism of electron transfer in the Geobacter sulfurreducens (GS) pili was explored by quantum chemical calculations with Marcus theory of electron transfer reactions. Three aromatic residues from the N-terminal α-helix of the GS pilin subunit are packed together, resulting in a continuous pi-pi interaction chain. The theoretical conductance (4.69 μS/3.85 μS) of the predicted models is very similar to that in the experiments reported recently (3.40 μS). These findings offer a new concept that the GS pili belongs to a new class of proteins that can transport electrons through pi-pi interaction between aromatic residues and also provide a valuable tool for guiding further researches of these conductive pili, to investigate their roles in biogeochemical cycling, and potential applications in biomaterials, bioelectronics, and bioenergy.
Highlights
Direct Extracellular Electron Transfer (DEET) offers the possibility of novel, sustainable, cost-effective bioenergy strategies, and cleaning up environmental contaminants associated with traditional sources of energy [1]. ese are attractive because the oxidation of the organic compound only releases fixed carbon back into the atmosphere
Structural Basis of Electron Transfer for the Geobacter sulfurreducens (GS) Pili. e GS pilin (PDB ID : 2M7G) was first refined in solution by MD simulation based on the steepest descent algorithm and the conjugate gradient algorithm (Amber 12) [31, 32], because its nuclear magnetic resonance conformation was derived from pilin subunits submerged in lipid micelles [33]
E distances between proximal aromatic ring centers were defined as interaromatic distances, and interaromatic distances for each pair of aromatic residues in the refined GS pilin were measured (Figure 1(a)). ese distances range from 8.7 Ato 41.8 A, which suggest that aromatic residues in independent GS pilin could not form pi-pi interactions and could not construct an efficient electron transfer pathway. erefore, possible aromatic contacts should be examined in the GS pili superstructure. e structures of the GS pili were constructed by assembling the refined GS pilin [34]
Summary
Direct Extracellular Electron Transfer (DEET) offers the possibility of novel, sustainable, cost-effective bioenergy strategies, and cleaning up environmental contaminants associated with traditional sources of energy [1]. ese are attractive because the oxidation of the organic compound only releases fixed carbon back into the atmosphere. Previous experiments of site-directed mutagenesis suggested that the GS pili transfer electrons along pili is at least partly attributed to aromatic amino acids [15, 16, 19,20,21]. En, one kind of proposed mechanism of DEET for the GS pili is attributed to electron hopping between aromatic residues [22, 23] It cannot adequately interpret the high conductivity (18.75 S·m− 1) of the GS pili [16, 17, 22, 23]. It cannot adequately interpret the high conductivity (18.75 S·m− 1) of the GS pili [16, 17, 22, 23]. erefore, the role of aromatic residues in promotion of long-distance electron transport has not been fully established
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