5.B.8 The Trans-Outer Membrane Electron Transfer Porin/Cytochrome Complex (ET-PCC) Family 

The tandem gene clusters orfR-ombB-omaB-omcB and orfS-ombC-omaC-omcC (encoding two Pcc complexes) of the extracellular metal-reducing bacterium Geobacter sulfurreducens PCA are responsible for trans-outer membrane electron transfer during extracellular reduction of Fe(III)-citrate and ferrihydrite [a poorly crystalline Fe(III) oxide] (Liu et al. 2015). Each gene cluster encodes a putative transcriptional factor (OrfR/OrfS), a porin-like outer-membrane protein (OmbB/OmbC), a periplasmic c-type cytochrome (c-Cyt, OmaB/OmaC) and an outer-membrane c-Cyt (OmcB/OmcC). Elimination of ombB, omaB and omcB in the presence of ombC-omaC-omcC had no impact on reduction of Fe(III)-citrate by G. sulfurreducens PCA. However, loss of ombB, omaB and omcB, in the absence of ombC-omaC-omcC severely impaired the ability to reduce Fe³⁺-citrate as well as ferrihydrite. Thus, there is an overlapping role of ombB-omaB-omcB and ombC-omaC-omcC in extracellular Fe³⁺ reduction. The absence of any protein subunit eliminates the function of the OmaB/OmbB/OmcB protein complex. OrfR regulates expression of this complex as a repressor (Leang and Lovley 2005), and OrfS is probably a similar transcriptional repressor (Liu et al. 2015). 

The isolated Pcc protein complexes reconstituted in proteoliposomes transferred electrons from reduced methyl viologen across the lipid bilayer of liposomes to Fe³⁺-citrate and ferrihydrite (Liu et al. 2014). 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. 

The generalized reaction catalyzed by the Pcc complex is:

  electron (e.g., in reduced cytochrome in the periplasm) + Fe³⁺-citrate or ferrihydrite  (extracellular) → Fe²⁺ (extracellular)