TCDB is operated by the Saier Lab Bioinformatics Group

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).

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)
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)