Evidence That a Respiratory Shield in Escherichia coli Protects a Low-Molecular-Mass Fe-^II Pool from O₂-Dependent Oxidation

Author(s)

Joshua D. Wofford, Texas A&M University
Naimah Bolaji, University of South Carolina - Columbia
Nathaniel Dziuba, Texas A&M University
Franklin Wayne Outten, University of South Carolina - ColumbiaFollow
Paul A. Lindahl, Texas A&M University

ORCID iD

Franklin Wayne Outten: https://orcid.org/0000-0002-9095-0194

Document Type

Article

Abstract

Iron is critical for virtually all organisms, yet major questions remain regarding the systems' level understanding of iron in whole cells. Here, we obtained Mössbauer and EPR spectra of Escherichia coli cells prepared under different nutrient iron concentrations, carbon sources, growth phases, and O₂ concentrations to better understand their global iron content. We investigated wild-type cells and those lacking Fur, FtnA, Bfr, and Dps proteins. The coarse-grain iron content of exponentially growing cells consisted of iron-sulfur clusters, variable amounts of nonheme high-spin Fe^II species, and an unassigned residual quadrupole doublet. The iron in stationary-phase cells was dominated by magnetically-ordered Fe^III ions due to oxyhydroxide nanoparticles. Analysis of cytosolic extracts by size-exclusion chromatography detected by an online inductively-coupled plasma mass spectrometer revealed a low-molecular-mass (⁴LMM) Fe^II pool consisting of two iron complexes with masses of ~ 500 (major) and ~1300 (minor) Da. They appeared to be high-spin Fe^II species with mostly O donor ligands, perhaps a few N donors, and probably no S donor. Surprisingly, the iron content of E. coli and its reactivity with O₂ were remarkably similar to those of mitochondria. In both cases, a "respiratory shield" composed of membrane-bound iron-rich respiratory complexes may protect the LMM Fe^II pool from reacting with O₂. When exponentially growing cells transition to stationary phase, the shield deactivates as metabolic activity declines. Given the universality of oxidative phosphorylation in aerobic biology, the iron content and respiratory shield in other aerobic prokaryotes might be similar to those of E. coli and mitochondria.

Digital Object Identifier (DOI)

Wofford, J., Bolaji, N., Dziuba, N., Outten, F., & Lindahl, P. (2018). Evidence That a Respiratory Shield in Escherichia coli Protects a Low-Molecular-Mass Fe-II Pool from O2-Dependent Oxidation. Journal of Biological Chemistry, 294(1), 50–62. https://doi.org/10.1074/jbc.RA118.005233