Shi Liang, Rosso Kevin M, Clarke Tomas A, Richardson David J, Zachara John M, Fredrickson James K
Pacific Northwest National Laboratory Richland, WA, USA.
Front Microbiol. 2012 Feb 15;3:50. doi: 10.3389/fmicb.2012.00050. eCollection 2012.
In the absence of O(2) and other electron acceptors, the Gram-negative bacterium Shewanella oneidensis MR-1 can use ferric [Fe(III)] (oxy)(hydr)oxide minerals as the terminal electron acceptors for anaerobic respiration. At circumneutral pH and in the absence of strong complexing ligands, Fe(III) oxides are relatively insoluble and thus are external to the bacterial cells. S. oneidensis MR-1 and related strains of metal-reducing Shewanella have evolved machinery (i.e., metal-reducing or Mtr pathway) for transferring electrons from the inner-membrane, through the periplasm and across the outer-membrane to the surface of extracellular Fe(III) oxides. The protein components identified to date for the Mtr pathway include CymA, MtrA, MtrB, MtrC, and OmcA. CymA is an inner-membrane tetraheme c-type cytochrome (c-Cyt) that belongs to the NapC/NrfH family of quinol dehydrogenases. It is proposed that CymA oxidizes the quinol in the inner-membrane and transfers the released electrons to MtrA either directly or indirectly through other periplasmic proteins. A decaheme c-Cyt, MtrA is thought to be embedded in the trans outer-membrane and porin-like protein MtrB. Together, MtrAB deliver the electrons through the outer-membrane to the MtrC and OmcA on the outmost bacterial surface. MtrC and OmcA are the outer-membrane decaheme c-Cyts that are translocated across the outer-membrane by the bacterial type II secretion system. Functioning as terminal reductases, MtrC and OmcA can bind the surface of Fe(III) oxides and transfer electrons directly to these minerals via their solvent-exposed hemes. To increase their reaction rates, MtrC and OmcA can use the flavins secreted by S. oneidensis MR-1 cells as diffusible co-factors for reduction of Fe(III) oxides. Because of their extracellular location and broad redox potentials, MtrC and OmcA can also serve as the terminal reductases for soluble forms of Fe(III). In addition to Fe(III) oxides, Mtr pathway is also involved in reduction of manganese oxides and other metals. Although our understanding of the Mtr pathway is still far from complete, it is the best characterized microbial pathway used for extracellular electron exchange. Characterizations of the Mtr pathway have made significant contributions to the molecular understanding of microbial reduction of Fe(III) oxides.
在缺乏氧气（O₂）和其他电子受体的情况下，革兰氏阴性细菌——奥奈达希瓦氏菌MR-1可以利用铁（III）的（羟基）氧化物矿物作为厌氧呼吸的末端电子受体。在接近中性的pH值且不存在强络合配体的情况下，铁（III）氧化物相对不溶，因此位于细菌细胞外部。奥奈达希瓦氏菌MR-1以及相关的金属还原希瓦氏菌菌株已经进化出了将电子从内膜传递至周质、穿过外膜到达细胞外铁（III）氧化物表面的机制（即金属还原或Mtr途径）。迄今为止，已确定的Mtr途径的蛋白质成分包括CymA、MtrA、MtrB、MtrC和OmcA。CymA是一种内膜四血红素c型细胞色素（c-Cyt），属于萘醌脱氢酶的NapC/NrfH家族。据推测，CymA在内膜中氧化醌，并将释放的电子直接或通过其他周质蛋白间接传递给MtrA。十血红素c-Cyt的MtrA被认为嵌入在外膜转运蛋白和孔蛋白样蛋白MtrB中。MtrAB共同将电子通过外膜传递到最外层细菌表面的MtrC和OmcA。MtrC和OmcA是外膜十血红素c-Cyts，通过细菌II型分泌系统转运穿过外膜。作为末端还原酶，MtrC和OmcA可以结合铁（III）氧化物的表面，并通过其暴露于溶剂中的血红素将电子直接传递给这些矿物。为了提高它们的反应速率，MtrC和OmcA可以利用奥奈达希瓦氏菌MR-1细胞分泌的黄素作为还原铁（III）氧化物的可扩散辅助因子。由于它们位于细胞外且具有广泛的氧化还原电位，MtrC和OmcA也可以作为可溶性铁（III）形式的末端还原酶。除了铁（III）氧化物外，Mtr途径还参与锰氧化物和其他金属的还原。尽管我们对Mtr途径的理解仍远未完善，但它是用于细胞外电子交换的最具特征的微生物途径。对Mtr途径的表征为从分子层面理解微生物对铁（III）氧化物的还原做出了重要贡献。
