Identification and Characterization of MtoA: A Decaheme c-Type Cytochrome of the Neutrophilic Fe(II)-Oxidizing Bacterium Sideroxydans lithotrophicus ES-1

Liu, Juan, Wang, Zheming, Belchik, Sara M., Edwards, Marcus J., Liu, Chongxuan, Kennedy, David W., Merkley, Eric D., Lipton, Mary S., Butt, Julea N. ORCID: https://orcid.org/0000-0002-9624-5226, Richardson, David J. ORCID: https://orcid.org/0000-0002-6847-1832, Zachara, John M., Fredrickson, James K., Rosso, Kevin M. and Shi, Liang (2012) Identification and Characterization of MtoA: A Decaheme c-Type Cytochrome of the Neutrophilic Fe(II)-Oxidizing Bacterium Sideroxydans lithotrophicus ES-1. Frontiers in Microbiology, 3. ISSN 1664-302X

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Abstract

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.

Item Type: Article
Additional Information: © 2012 Liu, Wang, Belchik, Edwards, Liu, Kennedy, Merkley, Lipton, Butt, Richardson, Zachara, Fredrickson, Rosso and Shi. This is an open-access article distributed under the terms of the Creative Commons Attribution Non Commercial License, which permits non-commercial use, distribution, and reproduction in other forums, provided the original authors and source are credited.
Faculty \ School: Faculty of Science > School of Biological Sciences
Depositing User: Rachel Smith
Date Deposited: 16 Feb 2012 14:50
Last Modified: 02 Oct 2022 00:28
URI: https://ueaeprints.uea.ac.uk/id/eprint/37174
DOI: 10.3389/fmicb.2012.00037

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