| Heavy metals play an important role in organisms, but in elevated concentrations all metals become toxic due to their interference with normal biological processes. The aim of this study was to determine how Pseudomonas fluorescens reprograms its metabolic networks to overcome metal toxicity stemming from zinc (Zn), an environmental pollutant. Under Zn stress, this microbe survived despite the fact that two important pathways, the tricarboxylic acid cycle (TCA cycle) and oxidative phosphorylation were ineffective. P. fluorescens upregulated the enzymes citrate lyase (CL), phosphoenolpyruvate carboxylase (PEPC) and pyruvate phosphate dikinase (PPDK), a strategy which enabled the metabolism of citrate, the sole source of carbon, into pyruvate and ATP. Levels of pyruvate were found to be high in the spent fluid and soluble CFE of Zn-stressed cells. Phosphoenolpyruvate (PEP) provided an important route to ATP via substrate-level phosphorylation. Antioxidant systems such as catalase and superoxide dismutase (SOD) were stimulated by Zn toxicity. The increased activity of NADPH-producing enzymes such as isocitrate dehydrogenase (ICDH), malic enzyme (ME), and glucose 6-phopshate dehydrogenase (G6PDH) was necessary to create a reductive environment. This metabolic shift is critical to counter the oxidative stress triggered by zinc toxicity and to generate ATP in an 02-independent manner. This metabolic engineering evoked by Zn may be tailored in bioremediation and metal chelation technologies. |
