5.B.5.1.1
The multihaem c-type cytochrome quinol:Fe³⁺ /Mn^{3 /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).  The MtrCAB protein complex plays a crucial role in exporting electrons through the outer membrane (OM) to external acceptors. This three protein complex contains 20 hemes. Optimal protein-protein interactions and, consequently, heme-heme interactions facilitate efficient electron transfer through the conduit of hemes (Mandal et al. 2025). The cytochrome MtrA remains mostly inside porin MtrB, and the MtrB barrel contains two calcium ions on its surface. The effect of porin-bound calcium ions on the heme-heme distances in the twenty-heme network has been examined. All-atom molecular dynamics simulations of MtrCAB, in the presence and absence of the MtrB-bound calcium ions was studied. The calcium ions bound to MtrB affect the interfacial heme-heme distance when all of the hemes are oxidized and impact one of the heme-heme distances in MtrC when all of the hemes are reduced. The absence of calcium ions increased the heme-heme distance, highlighting the crucial role of calcium ions in maintaining the heme network, which is essential for long-range charge transport (Mandal et al. 2025).