A trans‐outer membrane porin‐cytochrome protein complex for extracellular electron transfer by Geobacter sulfurreducens PCA

Abstract

The multi-heme, outer membrane c-type cytochrome (c-Cyt) OmcB of Geobacter sulfurreducens was previously proposed to mediate electron transfer across the outer membrane. However, the underlying mechanism has remained uncharacterized. In G. sulfurreducens, the omcB gene is part of two tandem four-gene clusters, each is predicted to encode a transcriptional factor (OrfR/OrfS), a porin-like outer membrane protein (OmbB/OmbC), a periplasmic c-type cytochrome (OmaB/OmaC) and an outer membrane c-Cyt (OmcB/OmcC) respectively. Here, we showed that OmbB/OmbC, OmaB/OmaC and OmcB/OmcC of G. sulfurreducens PCA formed the porin-cytochrome (Pcc) protein complexes, which were involved in transferring electrons across the outer membrane. The isolated Pcc protein complexes reconstituted in proteoliposomes transferred electrons from reduced methyl viologen across the lipid bilayer of liposomes to Fe(III)-citrate and ferrihydrite. The pcc clusters were found in all eight sequenced Geobacter and 11 other bacterial genomes from six different phyla, demonstrating a widespread distribution of Pcc protein complexes in phylogenetically diverse bacteria. Deletion of ombB-omaB-omcB-orfS-ombC-omaC-omcC gene clusters had no impact on the growth of G. sulfurreducens PCA with fumarate but diminished the ability of G. sulfurreducens PCA to reduce Fe(III)-citrate and ferrihydrite. Complementation with the ombB-omaB-omcB gene cluster restored the ability of G. sulfurreducens PCA to reduce Fe(III)-citrate and ferrihydrite.

Enhanced microbial electrosynthesis by using defined co-cultures

“…Microbial metabolism of electrons that are not associated with a chemical element, that is, 'free' electrons, is an intriguing metabolic capacity that has been primarily investigated in insoluble metalreducing microorganisms, such as Shewanella or Geobacter species (Bond and Lovley, 2003;Hartshorne et al, 2009;Coursolle et al, 2010;Clarke et al, 2011;Lovley, 2012;Liu et al, 2014;Malvankar and Lovley, 2014;Pirbadian et al, 2014;TerAvest et al, 2014). In addition to these anodic microbial processes, recent studies have revealed that some microorganisms can take up 'free' cathodic electrons from conductive minerals during interspecies electron transfer (Kato et al, 2012) or from abiotically reduced surfaces such as deep sea hydrothermal vent chimneys (Nakamura et al, 2010).…”

The mechanisms of extracellular electron transfer

“…In addition, the presence of a porincytochrome (Pcc) protein complex has recently been discovered. Composed of three proteins; a porin like outer membrane protein, a periplasmic cytochrome and an outer membrane cytochrome, the Pcc complex is thought to be responsible for facilitating electron transfer to the outside of the cell [33] . Furthermore, the outer membrane cytochrome OmcZ, has been found to be localised at the biofilm-electrode interface of current producing cells of G. sulfurreducens [34] and is proposed to act as an electrochemical gate, permitting the transfer of electrons from the electroactive biofilm to the electrode [34].…”

“…A speculative model of the Geobacter EET based on previous research and suggestions [23,31,33,161,162,171,181] is shown in Figure 5.9. This model suggests that electrons are transferred from the menoquinone pool within the inner membrane to MacA which then transfers electrons PpcA [163], which shuttles the electrons across the periplasm to a Pcc complex [33].…”

“…Several of these cytochromes are known to be involved in G. sulfurreducens respiration with an electrode including periplasmic cytochrome c (PpcA), cytochrome c peroxidase (MacA), the outer membrane c-type cytochromes: B (OmcB), E (OmcE), F (OmcF), G (OmcG), H (OmcH), S (OmcS), T (OmcT), X (OmcX) and Z (OmcZ) [54], and more recently, outer membrane c-type cytochrome, OmcC, periplasmic c-type cytochromes OmaB and OmaC, and porin-like outer membrane proteins OmbB and OmbC [33]. A large number of cytochromes are expressed during respiration in the presence of Fe(III) oxides, consequently, there is still much speculation on the specific EET pathway of G. sulfurreducens, with several different models suggested [23,160,161].…”

“…Electrons are then transferred to PpcA, which facilitates electron transfer across the periplasm [161][162][163][164]. PpcA, and possibly other periplasmic cytochromes then transfer electrons to a Pcc complex (OmbB/OmbC, OmaB/OmaC and OmcB/OmcC porin-cytochrome protein complexes) that transfer the electrons through the outer membrane [33]. The electrons are then either accepted by terminal electron accepters OmcE and OmcS directly [165] or transferred to pilin or nanowires for long range electron transport [31].…”