User:Nikhil Malvankar/Cytochrome nanowires

Background

The electrically conductive nanowires that extend from cells of Geobacter sulfurreducens have long been thought to be pili assembled from PilA protein. However, the evidence was indirect. Here, the structure of filaments of wild type Geobacter sulfurreducens, confirmed to be electrically conductive, was determined by cryo-electron microscopy (6ef8)^[1]. Surprisingly, these nanowires are assembled from outer membrane cytochrome OmcS. These findings were confirmed a short time later (6nef)^[2].

Nanowire Structure

A nanowire model composed of 7 OmcS protein chains, each shown a different color, was constructed from the 3.2-3.7 Å cryo-EM density (). The filament is ~4 nm in diamater, and has a characteristic undulating or sinusoidal form with a wavelength (pitch) of ~20 nm. The OmcS monomers have 407 amino acids each. The .

The amino terminus forms a bulge that fits into the slightly concave carboxy-terminal face of the contacting subunit.

OmcS Structure

The OmcS monomer has . The structure assigned by the authors is 77% loops; Jmol objectively assigns 82% loops. The authors assigned 10% alpha helices, 7% 3₁₀ helices, and 6% beta strands. Alpha Helices, 3₁₀ Helices,  Beta Strands ,  Loops . The OmcS structure determined by Filman et al. was very similar, with 80% loops assigned by the authors (86% by Jmol), having only 3% beta strand but otherwise very similar. We compared OmcS with three other c-type multi-heme cytochrome crystal structures: 1ofw, 3ucp, and 3ov0 had 45%, 49%, and 60% loops respectively.

Hemes

Each OmcS monomer : C O N Fe. The hemes are arranged in parallel-displaced pairs. Each pair is orthogonal to the next pair. The , which likely contributes to the stability of the filament. More importantly, this produces a continuous chain of hemes through the length of the filament. These hemes are believed to be the basis of the electrical conductivity.