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The multihaem c-type cytochrome quinol:Fe3+ /Mn3 /4+ oxidoreductase, Cym/Mtr (Shi et al., 2007).  MtrABC is composed of two decahaem cytochromes (MtrA & B) brought together inside a transmembrane β-porin (MtrC) to transport electrons across the outer membrane to mineral based electron acceptors (White et al. 2012).  Conduction through MtrCAB directly to Fe(III) oxides occurs both in vitro in liposomes and in vivo, allowing anaerobic, solid-phase iron respiration (White et al. 2013). MtrC interacts with the surface of MtrAB, extending ∼70 Å from the membrane surface and allowing the terminal hemes to interact with both MtrAB and an extracellular acceptor. MtrA fully extends through the length of MtrB, with ∼30 Å being exposed into the periplasm. MtrCAB can reduce Fe(III) citrate with STC as an electron donor, disclosing a direct interaction between MtrCAB and STC (Edwards et al. 2018). MtrC, but not UndA (a paralog of MtrC of 843 aas; F8UWD6), appears to be the primary reductase of flavins to ensure fast indirect extracellular electron transfer (EET), which plays a crucial role in microbial fuel cell (MFC) electricity generation in Shewanella putrefaciens CN32 (Wu et al. 2018). The dimensions of MtrAB are approximately 105 x 60 x 35 Å and approximately 170 x 60 x 45 Å for MtrCAB. Their shapes suggest that MtrC interacts with the surface of MtrAB, extending approximately 70 Å from the membrane surface and allowing the terminal hemes to interact with both MtrAB and an extracellular acceptor. MtrA fully extends through the length of MtrB, with approximately 30 Å being exposed into the periplasm (Edwards et al. 2018). The multihaem proteins can act as transmembrane molecular electron conduits (Stikane et al. 2019). Thus, MtrCAB iss a lipid membrane-spanning building block for compartmentalized photocatalysis that mimics photosynthesis. The atomic structure of an outer membrane spanning protein complex, MtrAB, that is representative of a protein family known to transport electrons between the interior and exterior environments of phylogenetically and metabolically diverse microorganisms, has been solved. The structure is revealed as a naturally insulated biomolecular wire possessing a 10-heme cytochrome, MtrA, insulated from the membrane lipidic environment by embedding within a 26 strand beta-barrel formed by MtrB. MtrAB forms a connection with an extracellular 10-heme cytochrome, MtrC, which presents its hemes across a large surface area for electrical contact with extracellular redox partners, including transition metals and electrodes (Edwards et al. 2020).

Cym/Mtr of Shewanella oneidensis
CymA (quinol dehydrogenase; 187 aas)
(~34% identical to TorY; TC# 5.A.3.4.2)
MtrA (decaheme cytochrome c; 330 aas)
(21% identical to the polyheme membrane associated cytochrome c; TC# 5.B.3.1.1)
MtrB (outer membrane protein precursor with homology to parts of members of the AT family, TC# 1.B.12; 697 aas)
(similar to PioB of the phototrophic iron (Fe2+)
oxidoreductase oxidizing (Pio), CO2 reducing complex of
Rhodopseudomonas palustris; TC# 5.B.5.2.1))
MtrC dodecaheme cytochrome c; 671 aas
OmcA (decaheme cytochrome c; 735 aas;
may be distantly related to cyt c3 of Geobacter sulfurreducens; TC# 5.B.3.1.1)

Four component transenvelope ferrous oxidase, CymA/MtoA/MtoB/MtoD (Beckwith et al. 2015). 

Ferrous oxidase of Sideroxydans lithotrophicus

Extracellular respiratory system with 3 cytochrome protein components, a periplasmic protein, MtrD, of 321 aas, a porin-type protein, MtrE, of 712 aas, and a surface decaheme cytochrome c component, MtrF, of 639 aas.  Each protein has a single N-terminal targeting TMS. They function together in the reduction of extracellular iron and manganese oxides (Barrozo et al. 2018) using a cytoplasmic electron donor. These 3 proteins are parologous to MtrABC (TC# 5.B.5.1.1) and function in parallel, except that in contrast to MtrABC, no increase in mtrDEF gene expression is observed under O2‐limited conditions (Barchinger et al. 2016). This process is being exploited for the generation of renewable energy technologies incorporating microbial catalysts on electrode surfaces for fuel‐to‐electricity (microbial fuel cells) or electricity‐to‐fuel (microbial electrosynthesis) conversion (Rabaey and Rozendal 2010; Santoro et al. 2017).

MtrF of Shewanella oneidensis

Surface localized decaheme cytochrome c lipoprotein of 759 aas.

Lipoprotein of Shewanella oneidensis

The secreted phototrophic iron (Fe2+) oxidase (CO2 reducing), PioABC (Jiao and Newman, 2007). Photoferrotrophy is a form of anoxygenic photosynthesis whereby bacteria utilize soluble or insoluble forms of ferrous iron as an electron donor to fix carbon dioxide using light energy. They can also use poised electrodes as their electron donor via phototrophic extracellular electron uptake (phototrophic EEU). Gupta et al. 2019 showed that the single periplasmic decaheme cytochrome c, PioA, and the outer membrane porin, PioB, form a complex allowing extracellular electron uptake across the outer membrane from both soluble iron and poised electrodes. They observed that PioA undergoes postsecretory proteolysis of its N terminus to produce a shorter heme-attached PioA (holo-PioAC, where PioAC represents the C terminus of PioA), which can exist both freely in the periplasm and in a complex with PioB. The extended N-terminal peptide controls heme attachment, and its processing is required to produce wild-type levels of the holo-PioAC and holo-PioACB complex. It is also conserved in PioA homologs from other phototrophs (Gupta et al. 2019).

PioABC of Rhodopseudomonas palustris
PioA (c-type cytochrome; like MtrA)
PioB (outer membrane β-barrel protein; like MtrB)
PioC (high potential iron sulfer protein, HiPIP similar to the Fe2+ oxidoreducatse (Iro) of Acidithiobacillus ferrooxidans)