Engineering of Microbial Cells for L-Valine Production

Gao et al. Microb Cell Fact (2021) 20:172 https://doi.org/10.1186/s12934-021-01665-5 Microbial Cell Factories REVIEW Open Access Engineering of microbial cells for L-valine production: challenges and opportunities Hui Gao†, Philibert Tuyishime†, Xian Zhang, Taowei Yang, Meijuan Xu* and Zhiming Rao* Abstract L-valine is an essential amino acid that has wide and expanding applications with a suspected growing market demand. Its applicability ranges from animal feed additive, ingredient in cosmetic and special nutrients in pharmaceutical and agriculture felds. Currently, fermentation with the aid of model organisms, is a major method for the production of L-valine. However, achieving the optimal production has often been limited because of the metabolic imbalance in recombinant strains. In this review, the constrains in L-valine biosynthesis are discussed frst. Then, we summarize the current advances in engineering of microbial cell factories that have been developed to address and overcome major challenges in the L-valine production process. Future prospects for enhancing the current L-valine production strategies are also discussed. Keywords: L-Valine biosynthesis, Feedstocks, Microbial cell factories, Metabolic engineering, Fermentation Background the pharmaceutical industry, L-valine is widely used as Branched-chain amino acids (BCAAs) comprising valine, a component of third-generation amino acids infusion isoleucine and leucine, are essential amino acids that are and is highly tolerant to the synthesis and decomposi- key components of human and animal nutrition, and tion of muscle protein; it has a critical role in pharma- need to be obtained from the daily diet. [1, 2]. Besides to cological nutrients for patients with chronic liver disease their important role as build blocks of proteins, BCAAs [7]. Moreover, L-valine activate the PI3K/Akt1 signaling earned the greatest reputation to be applied in a large pathway and inhibit the activity of arginase to increase number of industrial applications including the food, the expression of NO. Terefore, L-valine can enhance feed, cosmetic and pharmaceutical industry. Nota- phagocytosis of macrophages to drug-resistant patho- bly, L-valine is the α-amino acid with chemical formula gens and serves as a chemical building block for anti- C5H11NO2 that is commercially important for the need viral drugs and antibiotics (monensin, cervimycin and to supplement food or feed and to use in medical treat- valanimycin) [8]. Because of the increased world amino ment and biochemical synthesis precursors [3]. L-valine acids needs, it is estimated that the production of amino can improve the lactation function of breeding animals acids can yield revenue of US$ 25.6 billion by 2022 and and has been considered as one of the limiting amino animal feed amino acids (BCAAs) is the largest and fast- acids in animal feed for poultry and pigs [4–6]. Te est-growing market with expected revenue of US$10.4 addition of L-valine can make cosmetics have moistur- billion by 2022). In recent years, the global L-valine mar- izing function and promote the synthesis of collagen. In ket has been growing rapidly with a compound annual growth rate (CAGR) of 65% from 2016 to 2019, and is expected to maintain a CAGR of 24% from 2020 to 2023, *Correspondence: [email protected]; [email protected] †Hui Gao and Philibert Tuyishime contributed equally to this study among which feed grade L-valine has the most signif- The Key Laboratory of Industrial Biotechnology, Ministry of Education, cant growth rate, with a CAGR of 48% from 2015 to 2020 School of Biotechnology, Jiangnan University, Wuxi 214122, Jiangsu, [9]. Taking into consideration, this may change the near China future research for efcient production of animal feed © The Author(s) 2021. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. The Creative Commons Public Domain Dedication waiver (http:// creat iveco mmons. org/ publi cdoma in/ zero/1. 0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Gao et al. Microb Cell Fact (2021) 20:172 Page 2 of 16 amino acids. Tus, there is a strong impetus for improv- Encoded by ilvB and ilvN, AHAS catalyzed two mol- ing L-valine production. Traditionally, L-valine produc- ecules of pyruvate to form 2-acetolactate (Fig. 1a). Tis tion is centered on the hydrolysis process with subcritical enzyme consists of two large subunits and two small sub- water technology which use cheap feedstock solid waste units to form the catalytic domain and structural domain. (mainly including skin, feathers, viscera, blood, bones and Environmental oxidation level regulates the activity of residual meat produced in poultry processing) consisting AHAS, and cofactors favin adenine dinucleotide (FAD) of proteins which composed of a variety of amino acids and Mg2+ are also required for AHAS activity. AHAIR [10]. Chemical synthesis or enzymatic processes have encoded by ilvC catalyzes the isomerization reduction of been also applied for L-valine production [11]. However, 2-acetolactate to form dihydroxyisovalerate, whose activ- the complexity of these process and suboptimal produc- ity requires Mg 2+ and NADPH [23]. DHAD ecoded by tion prompt the above strategies less attractive and non- ilvD catalyzed the dehydration of dihydroxyisovalerate sustainable. Te direct fermentation of hexoses (mainly to 2-ketoisovalerate. Finally, TA encoded by ilvE converts glucose) has therefore become one of the great promising 2-ketoisovalerate to L-valine and this reaction also cata- process for the industrial-scale production of L-valine. lyzed by AvtA (encoded by avtA) in C. glutamicum. Te Fermentation pathway with the aid of microorganisms transcription of ilvB, ilvN and ilvC genes is regulated by has received increasing attention because of the new the operon ilvBNC, which has three diferent promot- genetic engineering tools applied to maximize the yield, ers that respectively start the transcription of ilvB, ilvN titer and productivity [12, 13]. Moreover, a great deal of and ilvC genes to form three mRNA of diferent lengths. efort has been devoted to correctly directing metabolic Because ilvC is located at the end of the operon ilvBNC, fux to the L-valine pathway through overexpression of the transcription of ilvC gene was three times that of the the rate-limiting enzymes, transporters, and deactivation other two genes, and the expression efciency of ilvC of the competitive pathways. L-valine-producing strains gene was the highest among the three genes [26]. BrnFE mainly includes Corynebacterium glutamicum subsp. is a two-component osmotic enzyme in C. glutamicum, favum [14, 15], Bacillus subtilis [16], Bacillus licheni- which is responsible for the export of L-valine. Te formis [17, 18], Saccharomyces cerevisiae [19], Escheri- expression of BrnFE is regulated by the global regulatory chia coli [20–22], and Corynebacterium glutamicum [23]. factor Lrp. Lrp is activated to promote the expression of However, it is still difcult to achieve high production of brnFE. Te protein BrnFE transports L-valine to the out- L-valine because of the interdependency between path- side of the cell and the import of L-valine is performed by ways in engineered strains. Te optimization of L-valine the protein BrnQ. biosynthetic pathways in a single host is also hampered Te regulation of L-valine synthesis in E. coli appears by unexpected metabolic burdens that may trigger low to be more complex than in C. glutamicum. In contrast energy and leave the host in an unstable physiological to C. glutamicum, there are three diferent AHAS iso- state [24, 25]. L-valine of the biosynthetic pathway in a enzymes in E. coli with diferent biochemical and regu- single host body is hampered by unexpected metabolic latory properties (Fig. 1b). AHAS I is encoded by ilvBN, burden which could cause low energy, make host in AHAS II is encoded by ilvIH, and AHAS III (encoded by unstable state of physiology. In this manuscript, recent ilvIH) is highly similar to C. glutamicum. 172 amino acids engineering strategies for the production of L-valine by in C. glutamicum (encoded by ilvN) have 39% homol- microbial cell factories are reviewed with accompanying ogy with 163 amino acids in E. coli (encoded by ilvH). examples. We also suggest the potential approaches for It contains an N-terminal ACT domain [26]. Te sites optimizing L-valine biosynthesis pathway with the help responsible for L-valine feedback inhibition are located of system and synthetic biology. in the small subunits of AHAS I (encoded by ilvN) and AHAS III (encoded by ilvH), while AHAS II (encoded Pathway of L‑valine biosynthesis in C. glutamicum, by ilvGM) is resistant to L-valine. At the transcrip- E. coli and B. subtilis tional level, the attenuation of ilvGMEDA (ilvE encodes Te synthesis of L-valine starts from pyruvate and transaminase, ilvD encodes dihydroxyacid dehydratase, involves four enzymes, namely acetohydroxyacid syn- ilvA encodes L-threonine dehydratase) operon is medi- thase (AHAS), acetohydroxyacid isomeroreductase ated by three BCAAs (L-valine, L-leucine, L-isoleucine), (AHAIR), dihydroxyacid dehydratase (DHAD), and and the absence of L-valine and L-leucine will trigger the transaminase B (TA) [26].

Load more