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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)

References associated with 5.B.8 family:

Leang, C. and D.R. Lovley. (2005). Regulation of two highly similar genes, omcB and omcC, in a 10 kb chromosomal duplication in Geobacter sulfurreducens. Microbiology 151: 1761-1767. 15941985
Liu, Y., J.K. Fredrickson, J.M. Zachara, and L. Shi. (2015). Direct involvement of ombB, omaB, and omcB genes in extracellular reduction of Fe(III) by Geobacter sulfurreducens PCA. Front Microbiol 6: 1075. 26483786
Liu, Y., Z. Wang, J. Liu, C. Levar, M.J. Edwards, J.T. Babauta, D.W. Kennedy, Z. Shi, H. Beyenal, D.R. Bond, T.A. Clarke, J.N. Butt, D.J. Richardson, K.M. Rosso, J.M. Zachara, J.K. Fredrickson, and L. Shi. (2014). A trans-outer membrane porin-cytochrome protein complex for extracellular electron transfer by Geobacter sulfurreducens PCA. Environ Microbiol Rep 6: 776-785. 25139405