Regulation and function of versatile aerobic and anaerobic respiratory metabolism in Pseudomonas aeruginosa

Review Article published: 05 May 2011 doi: 10.3389/fmicb.2011.00103 Regulation and function of versatile aerobic and anaerobic respiratory metabolism in Pseudomonas aeruginosa Hiroyuki Arai* Department of Biotechnology, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo, Japan Edited by: Dara Frank, Medical College of Wisconsin, USA Reviewed by: Michael L. Vasil, University of Colorado Medical School, USA Virginia Clark, University of Rochester, USA Dieter Jahn, University Braunschweig, Germany *Correspondence: Hiroyuki Arai, Department of Biotechnology, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-8657, Japan. e-mail: Pseudomonas aeruginosa is a ubiquitously distributed opportunistic pathogen that inhabits soil and water as well as animal-, human-, and plant-host-associated environments. The ubiquity would be attributed to its very versatile energy metabolism. P. aeruginosa has a highly branched respiratory chain terminated by multiple terminal oxidases and denitrification enzymes. Five terminal oxidases for aerobic respiration have been identified in the P. aeruginosa cells. Three of them, the cbb3-1 oxidase, the cbb3-2 oxidase, and the aa3 oxidase, are cytochrome c oxidases and the other two, the bo3 oxidase and the cyanide-insensitive oxidase, are quinol oxidases. Each oxidase has a specific affinity for oxygen, efficiency of energy coupling, and tolerance to various stresses such as cyanide and reactive nitrogen species. These terminal oxidases are used differentially according to the environmental conditions. P. aeruginosa also has a complete set of the denitrification enzymes that reduce nitrate to molecular nitrogen via nitrite, nitric oxide (NO), and nitrous oxide. These nitrogen oxides function as alternative electron acceptors and enable P. aeruginosa to grow under anaerobic conditions. One of the denitrification enzymes, NO reductase, is also expected to function for detoxification of NO produced by the host immune defense system. The control of the expression of these aerobic and anaerobic respiratory enzymes would contribute to the adaptation of P. aeruginosa to a wide range of environmental conditions including in the infected hosts. Characteristics of these respiratory enzymes and the regulatory system that controls the expression of the respiratory genes in the P. aeruginosa cells are overviewed in this article. Keywords: respiration, terminal oxidase, denitrification, nitric oxide, Pseudomonas aeruginosa Introduction The opportunistic pathogen Pseudomonas aeruginosa has a remarkable ability to grow under a variety of environmental conditions, including soil and water as well as animal-, human-, and plant-hostassociated environments. It is responsible for severe nosocomial infections in immunocompromised patients. In particular, it causes life-threatening chronic lung infection in patients with the inherited disease cystic fibrosis (CF; Lyczak et al., 2002). The genome of P. aeruginosa is relatively large (6.3 Mb) and carries a large number of genes for utilization of various carbon sources, energy metabolisms, and regulatory systems, which might contribute to the environmental adaptability of this bacterium (Stover et al., 2000). The main energy producing system of P. aeruginosa is respiration, which utilizes a proton motive force for ATP synthesis. In the case of eukaryotic respiration in mitochondria, the electron transfer pathway consists of four complexes, NADH dehydrogenase (complex I), succinate dehydrogenase (complex II), a cytochrome bc1 complex (complex III), and a cytochrome c oxidase (complex IV). Protons are pumped across the membrane during electron transfer through complexes I, III, and IV, producing the proton gradient. On the other hand, P. aeruginosa as well as many other bacterial species use a variety of electron donors and acceptors for respiration and therefore have far more complex and flexible electron transfer pathways. At least 17 respiratory dehydrogenases that are predicted to be responsible for feeding electrons from respiratory substrates into the quinone pool, including three types of NADH dehydrogenases and a succinate dehydrogenase, have www.frontiersin.org been annotated in the genome of P. aeruginosa (Williams et al., 2007). P. aeruginosa has five terminal oxidases that catalyze the four-electron reduction of molecular oxygen to water (Matsushita et al., 1982, 1983; Fujiwara et al., 1992; Cunningham and Williams, 1995; Cunningham et al., 1997; Stover et al., 2000; Comolli and Donohue, 2002, 2004). Three of them are cytochrome c oxidases that receive electrons via the cytochrome bc1 complex and c-type cytochromes. The other two are quinol oxidases that receive electrons directly from ubiquinol (Figure 1). The respiratory chain is also branched to the denitrification enzymes that reduce nitrogen oxides. These alternative respiratory branches enable P. aeruginosa to grow under anaerobic conditions in the presence of nitrate or nitrite (Zumft, 1997). P. aeruginosa also has the ability to ferment arginine and pyruvate anaerobically. A fundamental understanding of the respiratory systems and the physiology of aerobic and anaerobic energy metabolism would be necessary for better comprehension of the ubiquity and pathogenicity of P. aeruginosa. Some excellent reviews on the aerobic and anaerobic respiration of P. aeruginosa are now available (Williams et al., 2007; Schobert and Jahn, 2010; Schobert and Tielen, 2010). This article will additionally focus on some recent information on the transcriptional regulation of the aerobic and anaerobic respiratory genes. Multiple terminal oxidases for aerobic respiration Pseudomonas aeruginosa has five terminal oxidases for aerobic respiration (Figure 1; Matsushita et al., 1982, 1983; Fujiwara et al., 1992; Cunningham and Williams, 1995; Cunningham et al., 1997; May 2011 | Volume 2 | Article 103 | 1 Arai Respiratory metabolism of Pseudomonas aeruginosa Stover et al., 2000; Comolli and Donohue, 2002, 2004). Three of them, the cbb3-1 oxidase (Cbb3-1), the cbb3-2 oxidase (Cbb3-2), and the aa3 oxidase (Aa3), are cytochrome c oxidases. The other two, the cytochrome bo3 oxidase (Cyo) and the cyanide-insensitive oxidase (CIO), are quinol oxidases. These terminal oxidases are expected to have their specific affinity for oxygen, efficiency of proton-translocation, and resistance to various stresses such as cyanide and reactive nitrogen species. We have constructed five kinds of quadruple mutant strains, which lack four out of the five terminal oxidase gene clusters, and used them to characterize the features of each terminal oxidase (unpublished data). The Km values of Aa3, CIO, and Cyo for oxygen were high, whereas those of Cbb3-1 and Cbb3-2 were one order lower than those of the other three terminal oxidases, indicating that Aa3, CIO, and Cyo are low affinity enzymes and Cbb3-1 and Cb (...truncated)