%A Liu,Juan %A Wang,Zheming %A Belchik,Sara %A Edwards,Marcus %A Liu,Chongxuan %A Kennedy,David %A Merkley,Eric %A Lipton,Mary %A Butt,Julea %A Richardson,David %A Zachara,John %A Fredrickson,James %A Rosso,Kevin %A Shi,Liang %D 2012 %J Frontiers in Microbiology %C %F %G English %K decaheme c-type cytochrome MtoA,extracellular Fe(II) oxidation,ligand complexation,pH-dependent,Sideroxydans lithotrophicus ES-1 %Q %R 10.3389/fmicb.2012.00037 %W %L %M %P %7 %8 2012-February-08 %9 Original Research %+ Dr Liang Shi,Pacific Northwest National Laboratory,Richland,99352,WA,United States,Liang.Shi@pnnl.gov %# %! Identification and Characterization of MtoA %* %< %T Identification and Characterization of MtoA: A Decaheme c-Type Cytochrome of the Neutrophilic Fe(II)-Oxidizing Bacterium Sideroxydans lithotrophicus ES-1 %U https://www.frontiersin.org/articles/10.3389/fmicb.2012.00037 %V 3 %0 JOURNAL ARTICLE %@ 1664-302X %X The Gram-negative bacterium Sideroxydans lithotrophicus ES-1 (ES-1) grows on FeCO3 or FeS at oxic–anoxic interfaces at circumneutral pH, and the ES-1-mediated Fe(II) oxidation occurs extracellularly. However, the molecular mechanisms underlying ES-1’s ability to oxidize Fe(II) remain unknown. Survey of the ES-1 genome for candidate genes for microbial extracellular Fe(II) oxidation revealed that it contained a three-gene cluster encoding homologs of Shewanella oneidensis MR-1 (MR-1) MtrA, MtrB, and CymA that are involved in extracellular Fe(III) reduction. Homologs of MtrA and MtrB were also previously shown to be involved in extracellular Fe(II) oxidation by Rhodopseudomonas palustris TIE-1. To distinguish them from those found in MR-1, the identified homologs were named MtoAB and CymAES-1. Cloned mtoA partially complemented an MR-1 mutant without MtrA with regards to ferrihydrite reduction. Characterization of purified MtoA showed that it was a decaheme c-type cytochrome and oxidized soluble Fe(II). Oxidation of Fe(II) by MtoA was pH- and Fe(II)-complexing ligand-dependent. Under conditions tested, MtoA oxidized Fe(II) from pH 7 to pH 9 with the optimal rate at pH 9. MtoA oxidized Fe(II) complexed with different ligands at different rates. The reaction rates followed the order Fe(II)Cl2 >  Fe(II)–citrate > Fe(II)–NTA > Fe(II)–EDTA with the second-order rate constants ranging from 6.3 × 10−3 μM−1 s−1 for oxidation of Fe(II)Cl2 to 1.0 × 10−3 μM−1 s−1 for oxidation of Fe(II)–EDTA. Thermodynamic modeling showed that redox reaction rates for the different Fe(II)-complexes correlated with their respective estimated reaction-free energies. Collectively, these results demonstrate that MtoA is a functional Fe(II)-oxidizing protein that, by working in concert with MtoB and CymAES-1, may oxidize Fe(II) at the bacterial surface and transfer released electrons across the bacterial cell envelope to the quinone pool in the inner membrane during extracellular Fe(II) oxidation by ES-1.