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The PEP–Pyruvate–Oxaloacetate Node As the Switch Point for Carbon flux Distribution in Bacteria

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The PEP–Pyruvate–Oxaloacetate Node As the Switch Point for Carbon flux Distribution in Bacteria View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by RERO DOC Digital Library FEMS Microbiology Reviews 29 (2005) 765–794 www.fems-microbiology.org The PEP–pyruvate–oxaloacetate node as the switch point for carbon flux distribution in bacteria Uwe Sauer a, Bernhard J. Eikmanns b,* a Institute of Biotechnology, ETH Zu¨rich, 8093 Zu¨rich, Switzerland b Department of Microbiology and Biotechnology, University of Ulm, 89069 Ulm, Germany Received 18 August 2004; received in revised form 27 October 2004; accepted 1 November 2004 First published online 28 November 2004 We dedicate this paper to Rudolf K. Thauer, Director of the Max-Planck-Institute for Terrestrial Microbiology in Marburg, Germany, on the occasion of his 65th birthday Abstract In many organisms, metabolite interconversion at the phosphoenolpyruvate (PEP)–pyruvate–oxaloacetate node involves a struc- turally entangled set of reactions that interconnects the major pathways of carbon metabolism and thus, is responsible for the dis- tribution of the carbon flux among catabolism, anabolism and energy supply of the cell. While sugar catabolism proceeds mainly via oxidative or non-oxidative decarboxylation of pyruvate to acetyl-CoA, anaplerosis and the initial steps of gluconeogenesis are accomplished by C3- (PEP- and/or pyruvate-) carboxylation and C4- (oxaloacetate- and/or malate-) decarboxylation, respectively. In contrast to the relatively uniform central metabolic pathways in bacteria, the set of enzymes at the PEP–pyruvate–oxaloacetate node represents a surprising diversity of reactions. Variable combinations are used in different bacteria and the question of the sig- nificance of all these reactions for growth and for biotechnological fermentation processes arises. This review summarizes what is known about the enzymes and the metabolic fluxes at the PEP–pyruvate–oxaloacetate node in bacteria, with a particular focus on the C3-carboxylation and C4-decarboxylation reactions in Escherichia coli, Bacillus subtilis and Corynebacterium glutamicum.We discuss the activities of the enzymes, their regulation and their specific contribution to growth under a given condition or to bio- technological metabolite production. The present knowledge unequivocally reveals the PEP–pyruvate–oxaloacetate nodes of bacte- ria to be a fascinating target of metabolic engineering in order to achieve optimized metabolite production. Ó 2004 Federation of European Microbiological Societies. Published by Elsevier B.V. All rights reserved. Keywords: C3-carboxylation; C4-decarboxylation; Anaplerosis; Gluconeogenesis; Metabolic flux; PEP–pyruvate–oxaloacetate node Contents 1. Introduction . 766 2. The enzymes at the PEP–pyruvate–oxaloacetate node in bacteria . 767 2.1. C3-carboxylating enzymes . ............................................................ 767 2.2. C4-decarboxylating enzymes ............................................................ 770 3. The PEP–pyruvate–oxaloacetate node in E. coli ................................................... 772 3.1. Anaplerosis in E. coli................................................................. 772 * Corresponding author. Tel.: +49 0 731 50 22707; fax: +49 0 731 50 22719. E-mail address: [email protected] (B.J. Eikmanns). 0168-6445/$22.00 Ó 2004 Federation of European Microbiological Societies. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.femsre.2004.11.002 766 U. Sauer, B.J. Eikmanns / FEMS Microbiology Reviews 29 (2005) 765–794 3.2. C4-decarboxylating reactions in E. coli .................................................... 773 3.3. Carbon fluxes and regulation at the PEP–pyruvate–oxaloacetate node in E. coli ....................... 774 3.4. Metabolic engineering of the node. ....................................................... 776 4. The PEP–pyruvate–oxaloacetate node in B. subtilis................................................. 777 4.1. Anaplerosis and C4-decarboxylation reactions in B. subtilis ...................................... 777 4.2. Carbon fluxes and regulation at the PEP–pyruvate–oxaloacetate node in B. subtilis ..................... 778 5. The PEP–pyruvate–oxaloacetate node in C. glutamicum ............................................. 779 5.1. Anaplerosis in C. glutamicum ........................................................... 780 5.2. C4-decarboxylation reactions in C. glutamicum............................................... 781 5.3. Carbon fluxes and regulation at the corynebacterial PEP–pyruvate–oxaloacetate node . .................. 782 5.4. Metabolic engineering of the node for amino acid production . .................................. 784 6. Concluding remarks. 785 Acknowldegements . 786 References . 786 1. Introduction the TCA cycle intermediates oxaloacetate or malate are converted to pyruvate and PEP by decarboxylation In most chemotrophic, aerobic and facultatively (C4-decarboxylation) [1,4] and thus, the PEP–pyru- anaerobic bacteria, the Embden–Meyerhof–Parnas vate–oxaloacetate node provides the direct precursors pathway (glycolysis) or the Entner–Doudoroff pathway for gluconeogenesis. and the tricarboxylic acid (TCA) cycle are the main Although essential, the carbon flux through the PEP– pathways of central metabolism. The former two are pyruvate–oxaloacetate node is flexible, and hence it is the primary routes for carbohydrate breakdown to reasonable that the cells tightly adjust these fluxes to phosphoenolpyruvate (PEP), pyruvate and acetyl-CoA, the energetic and anabolic demands under a given con- thereby providing energy and building blocks for the dition. In some bacteria and under some conditions, synthesis of cellular components. The TCA cycle also the regulation of the carbon flux at the PEP–pyruvate– serves a dual role in catabolism and anabolism by cata- oxaloacetate node is rather simple and straightforward. lyzing complete oxidation of acetyl-CoA to CO2 for A prominent example is catabolite repression, which en- respiratory ATP formation and by providing carbon sures absence of C4-decarboxylating enzymes at glucose precursor metabolites and NADPH for biosynthetic processes. Upon growth on TCA cycle intermediates or on substrates that enter central metabolism via ace- Glucose tyl-CoA (e.g. acetate, fatty acids and ethanol), the cycle Glucose (extracellular) intermediates malate or oxaloacetate must be converted PEPCk PEP Glucose-6P to pyruvate and PEP for the synthesis of glycolytic inter- AMP ADP (intracellular) PEPCx mediates. This gluconeogenic formation of sugar phos- PPS PYK PTS CO 2 ATP CO ATP phates from PEP is accomplished by the reversible 2 CO2 PQO Pyruvate Acetate reactions of glycolysis and one further enzyme, fruc- NDP ATP + CoA PCx NAD Quinone Quinol tose-1,6-bisphosphatase. PDHC CO2 CO2 The metabolic link between glycolysis/gluconeogene- NTP ADP NADH sis and the TCA cycle is represented by the PEP– Acetyl-CoA pyruvate–oxaloacetate node, also referred to as the Oxaloacetate anaplerotic node (Fig. 1). This node comprises a set of NADH ODx Citrate Quinol reactions that direct the carbon flux into appropriate MDH CO2 + MQO NAD directions and thus, it acts as a highly relevant switch NAD(P)H Quinone point for carbon flux distribution within the central Malate + MAE NAD(P) metabolism. Under glycolytic conditions, the final prod- ucts of glycolysis PEP and pyruvate enter the TCA cycle via acetyl-CoA (oxidative pyruvate decarboxylation and Fig. 1. The enzymes at the PEP–pyruvate–oxaloacetate node in fueling of the cycle) and via formation of oxaloacetate aerobic bacteria. Abbreviations: MAE, malic enzyme; MDH, malate by carboxylation (C3-carboxylation). This latter route dehydrogenase; MQO, malate: quinone oxidoreductase; ODx, oxalo- acetate decarboxylase; PCx, pyruvate carboxylase; PDHC, pyruvate is referred to as anaplerosis, a process to replenish dehydrogenase complex; PEPCk, PEP carboxykinase; PEPCx, PEP TCA cycle intermediates that were withdrawn for ana- carboxylase; PPS, PEP synthetase; PQO, pyruvate: quinone oxidore- bolic purposes [1–3]. Under gluconeogenic conditions, ductase; PTS, phosphotransferase system; PYK, pyruvate kinase. U. Sauer, B.J. Eikmanns / FEMS Microbiology Reviews 29 (2005) 765–794 767 excess and presence during growth on gluconeogenic enzymes for acetyl-CoA formation from pyruvate are substrates [5–8]. Recent studies revealed, however, that pyruvate-formate lyase (in enterobacteria) and pyru- the carbon flux control at the PEP–pyruvate–oxaloace- vate-ferredoxin oxidoreductase (in saccharolytic clos- tate node is often more complex than simple on/off reg- tridia) [12] (not shown in Fig. 1). Oxidative ulation under a given condition. In some bacteria, two decarboxylation of pyruvate is also accomplished by C3-carboxylating and up to three C4-decarboxylating the FAD-containing pyruvate oxidase (H2O2 and ace- enzymes are simultaneously active, even during growth tyl-phosphate-forming, in lactobacilli) [13–16] and on glucose as sole carbon and energy source (e.g. pyruvate: quinone oxidoreductase (acetate-forming, in [9,10]). One enzyme, although operating in the same E. coli and C. glutamicum)([17–19], M. Schreiner direction, can fulfil a different function under certain and B.J. Eikmanns, submitted for publication). Under conditions (e.g. [11]). In other organisms or under differ- fermentative conditions, pyruvate can be the substrate ent conditions, the same enzyme can operate in the re- for pyruvate decarboxylase (e.g. in yeasts and in Zymo- verse direction and thus contribute to a third function. monas
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