Proposed models depicting electron transfer pathways for S. oneidensis MR-1 (A) and G. sulfurreducens (B) during dissimilatory reduction of solid metal (hydr)oxides. For simplicity, the quinone-reducing portion of respiratory chain, the peptidoglycan layer and the individual components of the type II secretion system (T2S) and type IV pilus (T4P) biogenesis machine (other than GspD/PilQ, GspF/PilC and pseudopilus/pilus apparatus) are omitted from these models. Identified multihaem c-type cytochromes (c-Cyts) are in red. Yellow arrows indicate the proposed electron transfer (ET) path.
A. As a member of NapC/NirT family of quinol dehydrogenases, inner membrane (IM) c-Cyt CymA of S. oneidensis MR-1 is capable of oxidizing quinol at IM and reducing the redox proteins, such as c-Cyt MtrA, at periplasm (PS). MtrA might also interact with the outer membrane (OM) protein MtrB. Although it is not a c-Cyt, MtrB is speculated to facilitate ET across OM to MtrC, an OM c-Cyt. Pseudopilus apparatus of T2S, whose formation is regulated by a protein complex in the IM, where only GspF is shown, pushes MtrC and OmcA (another OM c-Cyt) from PS through GspD to the surface of bacterial cells where MtrC and OmcA form a functional complex. The cell surface MtrC and OmcA are capable of directly reducing solid Fe(III)/Mn(III, IV) (hydr)oxides.
B. In G. sulfurreducens, OM c-Cyts OmcE and OmcS are suggested to transfer electrons to the T4P apparatus, which then transfers electrons directly to solid Fe(III)/Mn(III, IV) (hydr)oxides. The structural components that mediate ET from the IM to OmcE/OmcS in the OM during reduction of solid metal (hydr)oxides have yet to be identified experimentally.