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Construction of an in Vitro Bypassed Pyruvate Decarboxylation Pathway

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Construction of an in Vitro Bypassed Pyruvate Decarboxylation Pathway Krutsakorn et al. Microbial Cell Factories 2013, 12:91 http://www.microbialcellfactories.com/content/12/1/91 RESEARCH Open Access Construction of an in vitro bypassed pyruvate decarboxylation pathway using thermostable enzyme modules and its application to N-acetylglutamate production Borimas Krutsakorn1, Takashi Imagawa1, Kohsuke Honda1,2*, Kenji Okano1 and Hisao Ohtake1 Abstract Background: Metabolic engineering has emerged as a practical alternative to conventional chemical conversion particularly in biocommodity production processes. However, this approach is often hampered by as yet unidentified inherent mechanisms of natural metabolism. One of the possible solutions for the elimination of the negative effects of natural regulatory mechanisms on artificially engineered metabolic pathway is to construct an in vitro pathway using a limited number of enzymes. Employment of thermostable enzymes as biocatalytic modules for pathway construction enables the one-step preparation of catalytic units with excellent selectivity and operational stability. Acetyl-CoA is a central precursor involved in the biosynthesis of various metabolites. In this study, an in vitro pathway to convert pyruvate to acetyl-CoA was constructed and applied to N-acetylglutamate production. Results: A bypassed pyruvate decarboxylation pathway, through which pyruvate can be converted to acetyl-CoA, was constructed by using a coupled enzyme system consisting of pyruvate decarboxylase from Acetobacter pasteurianus and the CoA-acylating aldehyde dehydrogenase from Thermus thermophilus. To demonstrate the applicability of the bypassed pathway for chemical production, a cofactor-balanced and CoA-recycling synthetic pathway for N-acetylglutamate production was designed by coupling the bypassed pathway with the glutamate dehydrogenase from T. thermophilus and N-acetylglutamate synthase from Thermotoga maritima. N-Acetylglutamate could be produced from an equimolar mixture of pyruvate and α-ketoglutarate with a molar yield of 55% through the synthetic pathway consisting of a mixture of four recombinant E. coli strains having either one of the thermostable enzymes. The overall recycling number of CoA was calculated to be 27. Conclusions: Assembly of thermostable enzymes enables the flexible design and construction of an in vitro metabolic pathway specialized for chemical manufacture. We herein report the in vitro construction of a bypassed pathway capable of an almost stoichiometric conversion of pyruvate to acetyl-CoA. This pathway is potentially applicable not only to N-acetylglutamate production but also to the production of a wide range of acetyl-CoA- derived metabolites. * Correspondence: [email protected] 1Department of Biotechnology, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan 2PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan © 2013 Krutsakorn et al.; licensee BioMed Central Ltd. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Krutsakorn et al. Microbial Cell Factories 2013, 12:91 Page 2 of 9 http://www.microbialcellfactories.com/content/12/1/91 Background important role in the oxidative decarboxylation of pyru- The integration of diverse biocatalytic modules to ex- vate in some anaerobic bacteria and hyperthermophilic pand the versatility of fermentation-based industries has achaea [14-16]. Pyruvate formate-lyase (PFL) is another been widely employed for the production of biofuel, key enzyme involved in the carbohydrate metabolism of pharmaceuticals, and other useful chemicals [1]. How- anaerobic microorganisms [17,18]. However, none are ever, these “metabolic engineering” approaches often suf- readily available in in vitro production systems. PDH is fer from flux imbalances because the naturally occurring one of the largest protein complexes and consists of translational and transcriptional regulation mechanisms multiple copies of three or four subunits [19]. For ex- fail to appropriately function in artificially engineered ample, the PDH of E. coli consists of a central cubic cells [2,3]. One possible approach to overcome this limi- core composed of 24 molecules of dihydrolipoamide tation is to construct an in vitro metabolic pathway in acetyltransferase (E2), onto which up to 24 copies of pyru- which only a limited number of enzymes involved in the vate dehydrogenase (E1) and 12 copies of dihydrolipoamide pathway-of-interest are used as catalytic modules. In vitro dehydrogenase (E3) are assembled [19]. To our knowledge, systems offer a number of potential advantages over the there have been no reports on the functional expression of conventional fermentation-based production processes the complete form of PDH in heterologous hosts probably such as predictable production yield, operational flexibil- owing to their highly complex nature. PFORs require ity, and scalability of the reaction [4,5]. However, the en- specific redox partner proteins, namely ferredoxin and zyme isolation generally requires laborious, costly, and ferredoxin reductase, to exert their catalytic abilities. time-consuming procedures, and thus little attention has In in vitro systems, the enzymes and the partner pro- been paid to the practical application of in vitro metabolic teins freely diffuse in the reaction mixture, resulting in less engineering approaches. Recently, we reported a simple frequent interactions between them. In addition, the poor approach to construct an in vitro artificial metabolic path- oxygen tolerance of PFOR results in operational limita- way using thermostable biocatalytic modules [6,7]. The tions for its in vitro use. Similarly, PFL is highly sensitive basic procedure consists of the following four steps: 1) se- to oxygen stress [20]. lection of suitable thermostable enzymes; 2) expression of To overcome these limitations, in this study, we the enzymes in mesophilic hosts (e.g. Escherichia coli); constructed an in vitro bypassed pathway capable of 3) preheating the cell suspension at high temperature; and converting pyruvate to acetyl-CoA using an enzyme 4) assembly of the catalytic modules at an experimentally couple of thermostable pyruvate decarboxylase and CoA- optimized ratio to achieve stoichiometric production. The acylating aldehyde dehydrogenase. Owing to the ab- denaturation of indigenous enzymes at high temperatures sence of a co-existing pathway, an almost stoichiometric results in a one-step preparation of highly selective and production of acetyl-CoA could be achieved through the thermostable biocatalytic modules. The membrane struc- in vitro pathway. Furthermore, the performance of this ture of E. coli cells is partially or entirely disrupted at high pathway was assessed by integration to an artificial path- temperatures, and consequently, better accessibility be- way for the production of N-acetylglutamate (NAG), which tween the enzymes and substrates can be achieved [8]. can be used as an ingredient in the cosmetic industry as an Acetyl-CoA is a key precursor for the biosynthesis of a anti-odor compound and skin moisturizer [21,22]. wide variety of industrially useful metabolites. In the mevalonate pathway, acetyl-CoA is carboxylated and conju- Results gated to form various isoprenoid and terpenoid compounds Construction of the bypassed pyruvate decarboxylation that can be used as flavors, fragrances, and anticancer drugs pathway [9]. Condensation of two acetyl-CoA units leads to the A bypassed pyruvate decarboxylation pathway was designed formation of acetoacetyl-CoA, which serves as a C4 back- by using an enzyme couple of pyruvate decarboxylase and bone for polyhydroxyalkanoates and butanol biosynthesis CoA-acylating aldehyde dehydrogenase (Figure 1). Among [10-12]. Acetyl-CoA production tends to be a rate- microbial pyruvate decarboxylases, those from mesophilic limiting step of artificially engineered pathways to pro- bacteria, including Zymomonas mobilis [23] and Acetobac- duce target compounds because acetyl-CoA serves a cen- ter pasteurianus [24], are biochemically well characterized. tral intermediate of a wide range of naturally occurring Although BLAST searches of the genomic sequences of metabolic pathways and thus is subjected to depletion (hyper)thermophiles gave no hits when the amino acid se- when it is routed into co-existing pathways [2]. quences of these enzymes were used as queries, the pyru- In natural metabolism, acetyl-CoA is mainly generated vate decarboxylase from A. pasteurianus (ApPDC) has from pyruvate via three routes. Most aerobic microorgan- been reported to have a relatively high thermal stability isms use the large protein complex of pyruvate dehydro- with a half-life of 2 h at 60°C [24,25]. The CoA-acylating genase (PDH) to produce acetyl-CoA [13]. Alternatively, aldehyde dehydrogenase was obtained from those involved pyruvate-ferredoxin oxidoreductase (PFOR) plays an in the gene-expression library of Thermus thermophilus Krutsakorn et al. Microbial Cell Factories 2013, 12:91 Page 3 of 9 http://www.microbialcellfactories.com/content/12/1/91 Tt GDH NH + TmNAGS O OH 4 O OH HO O HO O O NH2 α-Ketoglutarate Glutamate O OH HO O Bypassed pyruvate NAD NADH decarboxylation pathway NH CO2 O OH O O O S CoA N–Acetylglutamate H O Pyruvate Acetaldehyde Acetyl-CoA ApPDC CoA TtADDH Figure
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