Structural basis for metallic-like conductivity in microbial nanowires

Area of Science:

• Microbiology
• Biophysics
• Materials Science

Background:

• Geobacter sulfurreducens pili, known as microbial nanowires, facilitate long-range electron transport.
• These pili exhibit metallic-like conductivity, a rare property for proteins, with implications for biogeochemical cycling and bioelectronics.
• Previous homology models conflicted with experimental conductivity data.

Purpose of the Study:

• To investigate the structural basis for the metallic-like conductivity of Geobacter sulfurreducens pili.
• To resolve discrepancies between structural predictions and measured conductivity.
• To provide insights for designing novel bioelectronic materials.

Main Methods:

• Synchrotron X-ray microdiffraction and rocking-curve X-ray diffraction were employed.
• Pili from wild-type and nonconductive mutant strains (Aro5) were analyzed.
• Structural analysis was correlated with pH-dependent conductivity measurements.

Main Results:

• A periodic 3.2-Å spacing was identified in conductive pili, absent in nonconductive pili lacking aromatic amino acids.
• The intensity of the 3.2-Å peak showed a 100-fold increase with a pH shift that also increased conductivity 100-fold.
• A revised homology model predicted aromatic amino acid packing consistent with experimental findings.

Conclusions:

• The 3.2-Å spacing, attributed to overlapping π-orbitals of aromatic amino acids, is crucial for metallic-like conductivity.
• Geobacter sulfurreducens pili represent a novel class of electronically functional proteins.
• Experimental structural data are essential for accurate modeling and understanding protein conductivity.