Abstract
Excessive manganese exposure is toxic, but a comprehensive biochemical picture of this assault is poorly understood. Whether oxidative stress or reduced energy metabolism under manganese exposure causes toxicity is still a debate. To address this, we chose ΔmntPEscherichia coli, a highly manganese-sensitive strain, in this study. Combining microarray, proteomics, and biochemical analyses, we show that the chronic manganese exposure rewires diverse regulatory and metabolic pathways. Manganese stress affects protein and other macromolecular stability, and envelope biogenesis. Most importantly, manganese exposure disrupts both iron-sulfur cluster and heme-enzyme biogenesis by depleting cellular iron level. Therefore, the compromised function of the iron-dependent enzymes in the tricarboxylic acid cycle, and electron transport chain impede ATP synthesis, leading to severe energy deficiency. Manganese stress also evokes reactive oxygen species, inducing oxidative stress. However, suppressing oxidative stress does not improve oxidative phosphorylation and cell growth. On the contrary, iron supplementation resumed cell growth stimulating oxidative phosphorylation. Therefore, we hypothesize that affected energy metabolism is the primal cause of manganese toxicity.
Highlights
Iron, the most abundant transition metal in biology, serves as a cationic cofactor or remains embedded in the iron-sulfur clusters (ISC) and heme groups of the reaction centers of proteins[1]
The trends of gene expression revealed from quantitative PCR (q-PCR) (Supplementary Fig. S1) matches with the microarray (Supplementary Tables S1–S8)
Using cell penetrable pH sensor fluorescent dye, BCFL-AM, we found that manganese toxicity raised the relative cellular pH (Fig. 2k), indicating that weak proton motive force (PMF) generated in the manganese-fed cells failed to maintain pH homeostasis
Summary
The most abundant transition metal in biology, serves as a cationic cofactor or remains embedded in the iron-sulfur clusters (ISC) and heme groups of the reaction centers of proteins[1]. The major issue with the iron is that it leaches out from the protein complexes upon reacting with endogenous superoxide anions (O2−)[2] Subsequent oxidation of this free metal by hydrogen peroxide (H2O2) generates highly toxic hydroxyl radical (∙OH) in the cell[2]. We unravel that the iron depletion under manganese stress affects heme-protein biogenesis, as observed earlier[9], and disrupts the biogenesis and the function of ISC proteins. These observations, and other biochemical assays promoted us to hypothesize that the loss of both heme and ISC protein functions dramatically affects energy metabolism, which govern manganese toxicity in ΔmntP strain of E. coli
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