Role of Glyoxylate Shunt in Oxidative Stress Response

Woojun Park,In-Ae Jang,Sungeun Ahn,Jaejoon Jung,Eugene L Madsen

doi:10.1074/jbc.m115.708149

Woojun Park, In-Ae Jang + Show 3 more

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https://doi.org/10.1074/jbc.m115.708149

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Abstract

The glyoxylate shunt (GS) is a two-step metabolic pathway (isocitrate lyase, aceA; and malate synthase, glcB) that serves as an alternative to the tricarboxylic acid cycle. The GS bypasses the carbon dioxide-producing steps of the tricarboxylic acid cycle and is essential for acetate and fatty acid metabolism in bacteria. GS can be up-regulated under conditions of oxidative stress, antibiotic stress, and host infection, which implies that it plays important but poorly explored roles in stress defense and pathogenesis. In many bacterial species, including Pseudomonas aeruginosa, aceA and glcB are not in an operon, unlike in Escherichia coli In P. aeruginosa, we explored relationships between GS genes and growth, transcription profiles, and biofilm formation. Contrary to our expectations, deletion of aceA in P. aeruginosa improved cell growth under conditions of oxidative and antibiotic stress. Transcriptome data suggested that aceA mutants underwent a metabolic shift toward aerobic denitrification; this was supported by additional evidence, including up-regulation of denitrification-related genes, decreased oxygen consumption without lowering ATP yield, increased production of denitrification intermediates (NO and N2O), and increased cyanide resistance. The aceA mutants also produced a thicker exopolysaccharide layer; that is, a phenotype consistent with aerobic denitrification. A bioinformatic survey across known bacterial genomes showed that only microorganisms capable of aerobic metabolism possess the glyoxylate shunt. This trend is consistent with the hypothesis that the GS plays a previously unrecognized role in allowing bacteria to tolerate oxidative stress.

Highlights

The glyoxylate shunt (GS)2 is known to be essential for utilizing acetate and fatty acids as carbon sources under physiological conditions requiring gluconeogenesis [1]
Physiological and Phenotypic Alterations in the Absence of the Glyoxylate Shunt—To ascertain the broad physiological role of the glyoxylate shunt in P. aeruginosa, PA2634 (⌬aceA) and PA0482 (⌬glcB) mutants were tested in a variety of assays
To further investigate this surprising growth-enhanced phenotype in the mutants, we raised the level of superoxide-induced oxidative stress in the cells by adding PQ to the LB medium; growth enhancement was retained in the aceA mutant, whereas the glcB mutant showed a severe growth defect (Fig. 1B)

Summary

Introduction

The glyoxylate shunt (GS) is known to be essential for utilizing acetate and fatty acids as carbon sources under physiological conditions requiring gluconeogenesis [1]. The glyoxylate cycle is a variant of the tricarboxylic acid cycle and shares five of the eight enzymes; the glyoxylate cycle bypasses the carbon dioxide-generating steps of the tricarboxylic acid cycle, which are catalyzed by isocitrate dehydrogenase and ␣-ketoglutarate dehydrogenase [2]. The glyoxylate shunt is known to be up-regulated when acetyl-CoA is a direct product of a metabolic pathway, for example via degradation of acetate, fatty acids, and alkanes [5]. A global bioinformatics analysis across sequenced bacterial genomes suggests that this role may extend broadly across the bacterial domain

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