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

Journal Article · Wed Feb 08 04:00:00 EST 2012 · Frontiers in Microbiological Chemistry

DOI:https://doi.org/10.3389/fmicb.2012.00037· OSTI ID:1036063

Liu, Juan; Wang, Zheming; Belchik, Sara M; Edwards, Marcus; Liu, Chongxuan; Kennedy, David W; Merkley, Eric D; Lipton, Mary S; Butt, Julea N; Richardson, David; Zachara, John M; Fredrickson, Jim K; Rosso, Kevin M; Shi, Liang

The Gram-negative bacterium Sideroxydans lithotrophicus ES-1 (ES-1) grows on FeCO{sub 3} 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 the genes known for microbial extracellular Fe(II) oxidation revealed that it contained a three-gene cluster encoding an MtrA homologue, an MtrB homologue and a CymA homologue. The homologues of MtrA, MtrB and/or CymA were previously shown to be involved in extracellular Fe(II) oxidation by Rhodopseudomonas palustris TIE-1 and in extracellular Fe(III) reduction by Shewanella oneidensis MR-1 (MR-1). To distinguish them from those found in MR-1, the identified homologues were named MtoAB and CymA{sub ES-1}, respectively. The gene for MtoA was cloned, and cloned mtoA partially complemented an MR-1 mutant without MtrA in ferrihydrite reduction. Following overexpression in MR-1 cells, recombinant MtoA was purified. 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) at pH ranging from 7-9, and optimal oxidation occurred at pH 9, possibly because of the attendant net increase of [Fe(OH){sup +}] at higher pH. MtoA oxidized Fe(II) complexed with different ligands at different rates. The reaction rates followed the order Fe(II)Cl2 > Fe(II)-citrate > Fe(III)-NTA > Fe(II)-EDTA with the second-order rate constants ranging from 5.5 x 10{sup -3} {micro}M{sup -1}s{sup -1} for oxidation of Fe(II)Cl{sub 2} to 1.0 x 10{sup -3} {micro}M{sup -1}s{sup -1} for oxidation of Fe(II)-EDTA. Thermodynamic modeling shows that redox reaction rate differences for the different Fe(II)-complexes correlated with estimated reaction-free energies. Collectively, these results suggest that MtoA is a functional Fe(II)-oxidizing protein that, by working in concert with MtoB and CymAES 1, may oxidize the Fe(II) on 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.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (US)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1036063
• Report Number(s): PNNL-SA-83499; KP1702030
• Journal Information: Frontiers in Microbiological Chemistry, Journal Name: Frontiers in Microbiological Chemistry Vol. 3
• Country of Publication: United States
• Language: English

Related Subjects

59 BASIC BIOLOGICAL SCIENCES
60 APPLIED LIFE SCIENCES
BENZOQUINONES
CYTOCHROMES
ELECTRONS
FUNCTIONALS
GENES
MEMBRANES
MUTANTS
OXIDATION
PROTEINS
REACTION KINETICS
REDOX REACTIONS
RHODOPSEUDOMONAS
SIMULATION
Sideroxydans lithotrophicus ES-1
THERMODYNAMICS
decaheme c-type cytochrome MtoA
extracellular Fe(II) oxidation
ligand complexation
pH-dependent