MtrB-PioB is a family of bacterial putative outer membrane porins. This family, is secreted as part of the pio (phototrophic iron oxidation) operon that has been found to couple the oxidation of ferrous iron [Fe(II)] to reductive CO2 fixation using light energy. PioABC is found in Rhodopseudomonas palustris and MtrB-PioB is likely to be a ß-barrel porin. Similar to other outer membrane porins, PioB and MtrB are predicted to have long loops protruding into the extracellular space and short turns on the periplasmic side. The Gram-negative Shewanella is one of the most studied bacterial genera capable of transferring electrons out of the cell to solid-phase Fe(III) and Mn(IV) minerals in order to support anaerobic respiration. The icosaheme protein complex comprising three subunits (MtrA, MtrB, MtrC) from the outer membrane of Shewanella oneidensis and shown that this Mtr complex is able to conduct electrons bidirectionally across proteoliposome bilayers sustaining electron transport rates over 8,500 e s–1. MtrAB is proposed to span the outer membrane and provide electrical connection between the catabolic electron transfer network within the cell, and the electron dispersing MtrC outside the cell. The crystal structure reveals a heterotrimeric complex that consists of two cytochromes, MtrC and MtrA, and a 26 strand ß-barrel protein, MtrB, that sheathes and insulates MtrA.