Abstract
The metallic-like electrical conductivity of Geobacter sulfurreducens pili has been documented with multiple lines of experimental evidence, but there is only a rudimentary understanding of the structural features which contribute to this novel mode of biological electron transport. In order to determine if it was feasible for the pilin monomers of G. sulfurreducens to assemble into a conductive filament, theoretical energy-minimized models of Geobacter pili were constructed with a previously described approach, in which pilin monomers are assembled using randomized structural parameters and distance constraints. The lowest energy models from a specific group of predicted structures lacked a central channel, in contrast to previously existing pili models. In half of the no-channel models the three N-terminal aromatic residues of the pilin monomer are arranged in a potentially electrically conductive geometry, sufficiently close to account for the experimentally observed metallic like conductivity of the pili that has been attributed to overlapping pi-pi orbitals of aromatic amino acids. These atomic resolution models capable of explaining the observed conductive properties of Geobacter pili are a valuable tool to guide further investigation of the metallic-like conductivity of the pili, their role in biogeochemical cycling, and applications in bioenergy and bioelectronics.
Highlights
Understanding the mechanisms for electron transport along the electrically conductive Type IV pili of Geobacter species is important because these ‘microbial nanowires’ enable extracellular electron exchange that is of environmental and practical significance[1,2,3,4,5,6]
Alternative modeling approaches to more fully explore the possibilities of G. sulfurreducens pilus structure are warranted to provide more information to facilitate further hypothesis-driven experimental investigation of the conductivity mechanisms
We report that the application of this modeling approach suggests options, different than those derived from homology modeling, for how the G. sulfurreducens pilus assembles to yield the experimentally observed close-packing of aromatic amino acids
Summary
Understanding the mechanisms for electron transport along the electrically conductive Type IV pili of Geobacter species is important because these ‘microbial nanowires’ enable extracellular electron exchange that is of environmental and practical significance[1,2,3,4,5,6]. Alternative modeling approaches to more fully explore the possibilities of G. sulfurreducens pilus structure are warranted to provide more information to facilitate further hypothesis-driven experimental investigation of the conductivity mechanisms. Another strategy for modeling pilus structure[17] successfully reconstructed the known structures of the type IVa pili of Neisseria gonorrhoeae[18]; the type IVb pili of Vibrio cholera[19,20]; and the T2SS pseudopili of Klebsiella oxytoca[21] from the structure of the pilin monomer, by employing the Rosetta Symmetric Docking protocol[22] with a combination of randomized structural parameters and distance constraints. We report that the application of this modeling approach suggests options, different than those derived from homology modeling, for how the G. sulfurreducens pilus assembles to yield the experimentally observed close-packing of aromatic amino acids
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