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Process optimization for enhancing production of cis-4-hydroxy-l-proline by engineered Escherichia coli
© The Author(s) 2017
Received: 30 August 2017
Accepted: 12 November 2017
Published: 22 November 2017
Understanding the bioprocess limitations is critical for the efficient design of biocatalysts to facilitate process feasibility and improve process economics. In this study, a proline hydroxylation process with recombinant Escherichia coli expressing l-proline cis-4-hydroxylase (SmP4H) was investigated. The factors that influencing the metabolism of microbial hosts and process economics were focused on for the optimization of cis-4-hydroxy-l-proline (CHOP) production.
In recombinant E. coli, SmP4H synthesis limitation was observed. After the optimization of expression system, CHOP production was improved in accordance with the enhanced SmP4H synthesis. Furthermore, the effects of the regulation of proline uptake and metabolism on whole-cell catalytic activity were investigated. The improved CHOP production by repressing putA gene responsible for l-proline degradation or overexpressing l-proline transporter putP on CHOP production suggested the important role of substrate uptake and metabolism on the whole-cell biocatalyst efficiency. Through genetically modifying these factors, the biocatalyst activity was significantly improved, and CHOP production was increased by twofold. Meanwhile, to further improve process economics, a two-strain coupling whole-cell system was established to supply co-substrate (α-ketoglutarate, α-KG) with a cheaper chemical l-glutamate as a starting material, and 13.5 g/L of CHOP was successfully produced.
In this study, SmP4H expression, and l-proline uptake and degradation, were uncovered as the hurdles for microbial production of CHOP. Accordingly, the whole-cell biocatalysts were metabolically engineered for enhancing CHOP production. Meanwhile, a two-strain biotransformation system for CHOP biosynthesis was developed aiming at supplying α-KG more economically. Our work provided valuable insights into the design of recombinant microorganism to improve the biotransformation efficiency that catalyzed by Fe(II)/α-KG-dependent dioxygenase.
The microbial production of chemicals and materials has gained increasing attentions as the concerns on environmental problems and limited availability of fossil resource [1, 2]. More and more bioprocesses have been successfully developed to produce chemicals, fuels, or polymers with metabolically engineering microorganisms [3, 4]. Hydroxyprolines, such as trans-4-hydroxy-l-proline (THOP), and cis-4-hydroxy-l-proline (CHOP), are useful materials for pharmaceutical and cosmetic applications [5, 6]. Among them, CHOP could inhibit the collagen synthesis and its extracellular deposition as an analogue of l-proline, thereby reducing the growth of tumors . In some bacteria, CHOP is directly produced via fermentation process and the related enzymes in CHOP synthetic pathway have been identified [8, 9]. Although the product titer is extremely low, these findings show the possibility of utilizing biological processes to produce CHOP.
The whole-cell biotransformation might be an alternative approach for CHOP bioproduction. Shibasaki et al. has already established the whole-cell process for the production of several hydroxyprolines, such as THOP and cis-3-hydroxy-l-proline . However, the whole-cell process for CHOP production has never been investigated. When using whole-cell biocatalysts, numerous factors could potentially interrupt the catalytic performance, such as expression system, substrate uptake, co-substrate supply, substrate or product degradation by host endogenous metabolism, and product toxicity [10, 12]. The limiting factors for the effective CHOP bioproduction process remained largely uncharacterized.
In this study, we focused on investigating the factors that might influence the whole-cell biocatalyst efficiency for CHOP production (e.g., expression system, reaction condition, l-proline uptake, l-proline consumption by endogenous metabolism, and α-KG supply). Furthermore, the CHOP production process was optimized. Meanwhile, a two-strain coupling whole-cell biotransformation system was established for CHOP biosynthesis.
Bacterial strains and media
All bacterial strains used in this study are listed in Additional file 1: Table S1. Bacteria were grown in Luria–Bertani (LB) broth (10 g/L peptone, 5 g/L yeast extract and 5 g/L sodium chloride) or on LB agar (10 g/L peptone, 5 g/L yeast extract, 5 g/L sodium chloride and 10 g/L agar). Ampicillin (100 mg/L), chloramphenicol (15 mg/L), streptomycin (40 mg/L) or kanamycin (50 mg/L) was added when required.
Plasmids used in this study
Expression vector, CmR, PT7, P15A ori
Expression vector, SmR, PT7, CloDF13 ori
Expression vector, KmR, PT7, RSF ori
Expression vector, KmR, PT7, pBR322 ori
Expression vector, AmpR, PT7, pBR322 ori
Expression vector, AmpR, PT7, pBR322 ori
Expression vector, SmR, PTrc, pCDF ori
Gene SmP4H inserted into NdeI/XhoI sites of pACYCDuet-1
Gene SmP4H inserted into NdeI/XhoI sites of pCDFDuet-1
Gene SmP4H inserted into NdeI/XhoI sites of pRSFDuet-1
Gene SmP4H inserted into NdeI/XhoI sites of pET28a
Gene SmP4H inserted into NdeI/XhoI sites of pET22b
Gene SmP4H inserted into NdeI/XhoI sites of pETDuet-1
Gene dCas9 inserted into NcoI/AvrII sites of pACYCDuet-1
anti-putA (high) sgRNA sequences inserted into EcoRI/BamHI sites of pCDF303
Gene putP inserted into NcoI/XhoI sites of pACYCDuet-1
Gene putP inserted into XhoI sites of pET28a-SmP4H
Gene LOGX inserted into NdeI/XhoI sites of pET28a
Construction of CRISPRi system to repress gene putA
A catalytically dead Cas9 mutant (dCas9) derived from Streptococcus pyogenes  was synthesized by Genewiz (Suzhou, China) based on the pdCas9-bacterial plasmid (Addgene; Plasmid #44249), which contains a gene encoding dCas9 protein. NcoI/HindIII-digested dCas9 fragment was ligated into plasmid pACYCDuet-1. The sgRNA chimera, which contains four functional domains including a Trc-inducible promoter, a 20 bp complementary region for binding putA DNA, a 42 bp dCas9-binding hairpin and a 40 bp transcription terminator was synthesized by Genewiz (Nanjing, China) and inserted into EcoRI/BamHI sites of pCDFDuet-1 to give plasmid pCDF-anti-putA (Additional file 1: Table S2). The plasmid pACYC-dCas9 and pCDF-anti-putA were co-transformed into E. coli BL21(DE3) to repress putA expression. PutA activity in crude extracts was determined by measuring P5C production (nmol P5C/min/mg protein) using o-aminobenzaldehyde as previously described .
Cultivation of microorganisms
The recombinant strain was inoculated from a freshly transformed single colony on LB agar plate to 5 mL LB medium as seed culture. When cell growth reached stationary phase, 200 μL of seed culture was re-inoculated to 100 mL LB medium in a 250 mL flask. The cultures were then induced with 1.0 mM IPTG when OD600 reached 0.4–0.6 and allowed to grow for an additional 10 h at 30 °C and 200 rpm. The effects of induction OD600, temperature and IPTG concentration on the whole-cell activity were investigated.
The biosynthesis of CHOP by whole-cell biotransformation
For the production of CHOP, the whole-cell biotransformation was carried out in a 50 mL flask with 20 mL reaction broth containing resting cells (OD600 = 10), 200 mM PBS buffer (pH = 6.5), 10 g/L l-proline, 13 g/L α-KG, 5.0 mM Fe2+, and 1.7 mM l-ascorbate. The reaction was performed at a temperature of 30 °C and stirred at 200 rpm. At the specific intervals of the reaction, samples were taken to measure the concentration of l-proline, α-KG and CHOP. To identify the factors limiting catalytic efficiency, the effects of surfactant solutions (0.5% Tween 80 and 0.5% Triton X-100), pH, Fe2+ concentration, and substrate concentration were investigated.
To supply α-KG economically, an engineered strain BL21/pET28a-LOGX that could convert l-glutamate to α-KG was coupled with CHOP producing strain. The two strains were collected, washed twice, and resuspended in a reaction mixture containing 200 mM PBS (pH 6.5), 10 g/L l-proline, 3.0 mM Fe2+, and 1.7 mM l-ascorbate. 5, 10 or 20 g/L of l-glutamate was supplemented to identify the suitable substrate ratio. In this coupling system, the final cell concentration was about 10 of OD600 for each strain. The reaction was carried out in a 50 mL flask with a work volume of 15 mL at 30 °C and 200 rpm. Samples were taken at the specific intervals to measure the concentration of CHOP, l-proline, l-glutamate, and α-KG.
Cell growth was monitored by measuring absorbance at 600 nm with BioMate 3S UV–visible spectrophotometer (Thermo). Aqueous concentrations of l-proline and CHOP were analyze by high-performance liquid chromatography (HPLC) system (Agilent 1100 series, Santa Clara, CA) equipped with a evaporative light scattering detector (ELSD) and a Prevail C18 column (250 × 4.6 mm, 5 µm, Bio-Rad, Hercules, CA, USA). The column temperature was maintained at 28.5 °C. The mobile phase was consisted of 0.7% (v/v) aqueous trifluoroacetic acid and 0.0653% (v/v) aqueous heptafluorobutyric acid and supplied at a flow rate of 1.0 mL/min. Analysis of α-KG and l-glutamate concentration was performed by a HPLC system (Agilent1290, Santa Clara, CA) equipped with the Agilent G1362A refractive index detector. The samples were separated on an Aminex HPX-87H ion-exchange column (Bio-Rad, Hercules, CA, USA) operating at 55 °C with 5 mM H2SO4 as the mobile phase (0.6 mL/min). The intracellular CHOP amount was analyzed by Waters time-of-flight mass spectrometry (GC–TOF–MS), which was equipped with a DB-5 fused-silica capillary column (30 m × 0.25 mm i.d., flm thickness 0.25 µm, J&W Scientifc, Folsom, CA). Quenching, metabolite extraction and derivatization were performed according to methods described previously . The area of CHOP peak was then normalized to that of internal standard in the same chromatogram.
To develop an efficient whole-cell biotransformation system for microbial production of CHOP, it is critical to identify the key factors limiting whole-cell activity. Here, the factors including l-proline cis-4-hydroxylase expression, l-proline degradation by endogenous metabolism, l-proline uptake, and α-ketoglutarate (KG) supply were investigated.
The expression of l-proline cis-4-hydroxylase in E. coli
The effects of cultivation conditions including induction temperature, IPTG concentration and induction OD600 on whole-cell activity of BL21/pET28a-SmP4H were also evaluated. Results showed that the decreasing induction temperature significantly increased whole-cell activity (Additional file 1: Figure S2). CHOP titer under condition of 20 °C was twofold higher than that under condition of 30 °C. Various IPTG concentrations ranging from 0.1 to 1 mM showed little effect on whole-cell activity. The optimal induction time was observed at the early exponential phase (Additional file 1: Figure S2).
The effect of endogenous l-proline degradation on CHOP production
The effect of l-proline uptake on CHOP production
The production of CHOP by the optimized whole-cell biocatalysts
Finally, the recombinant strain BL21/pET28a-SmP4H-putP+anti-putA was cultivated and collected to perform whole-cell biotransformation process for CHOP production. After bioconversion of 36 h, 4.2 g/L CHOP was produced from 10 g/L l-proline (Fig. 5b). The reaction conditions were also investigated. As shown in Additional file 1: Figure S3, the reaction pH significantly affected the whole-cell catalytic activity, which reached the highest level under the condition of pH 6.5. The concentration of Fe2+ had a minor effect. In addition, a substrate inhibition might exist in our whole-cell system when the l-proline concentration was above 10 g/L. With a fed-batch strategy, total CHOP accumulation reached a titer of approximately 15.6 g/L (Fig. 5c), which was the highest concentration reported thus far.
The development of two-strain coupling whole-cell system to produce CHOP
Hydroxy amino acids are important intermediates in the industrial synthesis of valuable chiral compounds [17–19]. For their microbial production, several free amino acid-hydroxylating enzymes have been characterized . In our current work, to develop an efficient whole-cell process for CHOP production, the recombinant E. coli expressing l-proline cis-3-hydroxylase was constructed, and factors associated with microbial physiology were focused on to identify the process limitations that impact the efficiency of biocatalytic transformation.
Initially, to explore the expression system suitable for SmP4H expression, two different E. coli host strains and six expression plasmids were employed. The engineered strain BL21/pET28a-SmP4H, which showed highest SmP4H expression level, exhibited the highest biocatalytic activity, while SmP4H synthesis in other system was poor (Fig. 2). These results suggested a SmP4H synthesis limitation in E. coli. Several other microbial physiology related factors such as substrate uptake and degradation were then investigated. To reduce the l-proline degradation by host intrinsic enzymes, the expression of gene putA in BL21/pET28a-SmP4H was repressed using a CRISPRi technology. Its beneficial effect on CHOP production and yield suggested that l-proline hydroxylation process was limited by intracellular l-proline availability (Fig. 3). In addition, substrate transporters have been reported as excellent targets for strain improvement in the whole-cell system . By using permeabilized cells, the improved biocatalytic activity inferred the limitations of the l-proline uptake. When a Na+/l-proline permease putP was overexpressed in the SmP4H containing strain BL21/pET28a-SmP4H, the intracellular and extracellular CHOP accumulation were all positively affected correlated with the increased putP expression level. These results further indicate that factors affecting intracellular l-proline availability were critical for improving biocatalysts activity in the whole-cell bioconversion system for CHOP production, which was consistent with the conclusion from the previous studies that l-proline uptake and competition of THOP formation from l-proline by l-proline-4-hydroxylase with l-proline catabolism were the key factors limiting biocatalytic efficiency for THOP production . However, the additive effect of overexpressing putP and repressing putA was slight (Fig. 5). Increasing the extracellular l-proline concentration could not substantially enhance CHOP production. These results suggested that engineering strategies aiming at further improving SmP4H synthesis and catalytic properties might be promising for CHOP biosynthesis.
For the biocatalytic process that utilizes α-KG dependent dioxygenases, α-KG was required as a co-substrate. The efficiency of α-KG dependent biotransformation in resting cells has been reported to be limited by the α-KG supply in the previous study [10, 22, 23]. In our work, the exogenous supply of α-KG was carried out as an alternative method to solve this problem. However, it was a cost burden to the bioconversion process. Thus, the solution of α-KG supply problem has important significance for developing an economically viable process. The major strategy that has been already employed was to reconstitute the tricarboxylic acid (TCA) cycle of E. coli to force and increase α-KG flux through α-KG dependent hydroxylase [23, 24]. However, the α-KG production via these approaches remained far below that required for industrial applications. l-Glutamate was thought as a promising starting material for α-KG production since it was an industry at overcapacity . Along this line of consideration, we developed a novel strategy to relieve α-KG supply limitation during CHOP production process with a two-strain coupling system, where one of the strains expressing LGOX was constructed to supply α-KG from l-glutamate. Excitingly, CHOP was successfully synthesized in our two-strain coupling system (Fig. 6). At a lower level of l-glutamate concentration, the CHOP titer was also limited by the lack of α-KG, probably due to the degradation of α-KG by endogenous metabolism (Additional file 1: Figure S4). With excessive l-glutamate supplementation, the CHOP titer could reach 3.9 g/L, a similar level with the extra addition of α-KG. To improve the efficiency of this system, engineering efforts to reduce α-KG consumption would be focused on in the future studies.
Our work clearly demonstrated the factors that could influence biocatalytic performance in a whole-cell bioconversion system for CHOP production. Through artificially engineering host phenotypes, the biocatalysts efficiency has been largely improved. Meanwhile, with a two-strain coupling system, the whole-cell process was more economical. Finally, 13.5 g/L of CHOP, the highest titer reported so far, was obtained in our work. To our knowledge, this is the first report that E. coli strain was artificially engineered to product CHOP with a whole-cell biotransformation process, especially with a two-strain coupling whole-cell process. However, this titer was remained far below that required for industrialization application. To further investigate this issue, the capacity of the cell needs to be utilized efficiently by optimized l-proline cis-4-hydroxylase expression with some engineering approaches, i.e., using “trial and error” approaches and modifying the structure of the 5′end of the mRNA [26, 27]. In addition, the host strain will be further metabolically engineered to modify characteristics regarding l-proline uptake and α-KG consumption.
Identifying limitations for the whole-cell SmP4H-catalyzed process was demonstrated as the first step toward process development for CHOP production. A strong interference of microbial catalytic activity with the SmP4H expression, and l-proline uptake and metabolism was demonstrated in our study. Taken together, we have established some engineering strategies to address these limitations by optimizing the expression system, repressing putA gene and overexpressing a l-proline transporter putP. Meanwhile, to reduce the material cost and make the process more economically feasible, a two-strain coupling whole-cell system was developed to address the α-KG supply problem. This work highlights the limitations for Fe/αKG-DO dependent process and provided opportunities for further engineering of recombinant biocatalysts to facilitate process feasibility and improve process economics.
WX, OPK and CKQ conceived and designed the experiments. CKQ and PY performed the experiments and analyzed the data. ZBW, FJ and XS contributed to experimental design and also critically revised the manuscript. CKQ and PY wrote the paper. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Availability of data and materials
The datasets supporting the conclusions of this article are included within the article.
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Not applicable. The manuscript does not report data from humans or animals.
This research was financially supported by the National Nature Science Foundation of China (Grant Nos. 21576134, 21606127), “863” program of China (Grant No. 2015AA021005) and the National Key Research and Development Program of China (Grant No. 2016YFA0204300).
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- Chubukov V, Mukhopadhyay A, Petzold C, Keasling J. Synthetic and systems biology for microbial production of commodity chemicals : from target selection to scale-up. npj Syst Biol Appl. 2016;16009:1–11. http://www.nature.com/articles/npjsba20169.
- Becker J, Wittmann C. Advanced biotechnology: metabolically engineered cells for the bio-based production of chemicals and fuels, materials, and health-care products. Angew Chemie Int Ed. 2015;54:3328–50.View ArticleGoogle Scholar
- Borodina I, Nielsen J. Advances in metabolic engineering of yeast Saccharomyces cerevisiae for production of chemicals. Biotechnol J. 2014;9:609–20.View ArticleGoogle Scholar
- Becker J, Wittmann C. Bio-based production of chemicals, materials and fuels—Corynebacterium glutamicum as versatile cell factory. Curr Opin Biotechnol. 2012;23:631–40.View ArticleGoogle Scholar
- Bach TMH, Hara R, Kino K, Ohtsu I, Yoshida N, Takagi H. Microbial production of N-acetyl cis-4-hydroxy-l-proline by coexpression of the rhizobium l-proline cis-4-hydroxylase and the yeast N-acetyltransferase Mpr1. Appl Microbiol Biotechnol. 2013;97:247–57. https://doi.org/10.1007/s00253-012-4204-z.View ArticleGoogle Scholar
- Mai Hoa BT, Hibi T, Nasuno R, Matsuo G, Sasano Y, Takagi H. Production of N-acetyl cis-4-hydroxy-l-proline by the yeast N-acetyltransferase Mpr1. J Biosci Bioeng. 2012;114:160–5. https://doi.org/10.1016/j.jbiosc.2012.03.014.View ArticleGoogle Scholar
- Mueller C, Emmrich J, Jaster R, Braun D, Liebe S, Sparmann G. cis-Hydroxyproline-induced inhibition of pancreatic cancer cell growth is mediated by endoplasmic reticulum stress. World J Gastroenterol. 2006;12:1569–76.View ArticleGoogle Scholar
- Hara R, Kino K. Characterization of novel 2-oxoglutarate dependent dioxygenases converting l-proline to cis-4-hydroxy-l-proline. Biochem Biophys Res Commun. 2009;379:882–6. https://doi.org/10.1016/j.bbrc.2008.12.158.View ArticleGoogle Scholar
- Matsuoka T, Furuya K, Serizawa N. Fermentative production of cis-4-hydroxy-(l)-proline by Helicocerus oryzae and Acrocylindrium oryzae. Biosci Biotechnol Biochem. 1994;58:1747–8.View ArticleGoogle Scholar
- Theodosiou E, Frick O, Bühler B, Schmid A. Metabolic network capacity of Escherichia coli for Krebs cycle-dependent proline hydroxylation. Microb Cell Fact. 2015;14:108. http://www.microbialcellfactories.com/content/14/1/108.
- Shibasaki T, Mori H, Ozaki A. Enzymatic production of trans-4-hydroxy-l-proline by regio- and stereospecific hydroxylation of l-proline. Biosci Biotechnol Biochem. 2000;64:746–50. https://doi.org/10.1271/bbb.64.746.View ArticleGoogle Scholar
- Olaofe OA, Fenner CJ, Gudiminchi R, Smit MS, Harrison STL. The influence of microbial physiology on biocatalyst activity and efficiency in the terminal hydroxylation of n-octane using Escherichia coli expressing the alkane hydroxylase, CYP153A6. Microb Cell Fact. 2013;12:8.View ArticleGoogle Scholar
- Qi LS, Larson MH, Gilbert LA, Doudna JA, Weissman JS, Arkin AP, et al. Repurposing CRISPR as an RNA-guided platform for sequence-specific control of gene expression. Cell. 2017;152:1173–83. https://doi.org/10.1016/j.cell.2013.02.022.View ArticleGoogle Scholar
- Wadhawan S, Gautam S, Sharma A. Involvement of proline oxidase (PutA) in programmed cell death of Xanthomonas. PLoS ONE. 2014;9:1–12.Google Scholar
- Wang X, Jin M, Balan V, Jones AD, Li X, Li BZ, et al. Comparative metabolic profiling revealed limitations in xylose-fermenting yeast during co-fermentation of glucose and xylose in the presence of inhibitors. Biotechnol Bioeng. 2014;111:152–64.View ArticleGoogle Scholar
- Olkhova E, Raba M, Bracher S, Hilger D, Jung H. Homology model of the Na+/proline transporter PutP of Escherichia coli and its functional implications. J Mol Biol. 2011;406:59–74.View ArticleGoogle Scholar
- Hibi M, Ogawa J. Characteristics and biotechnology applications of aliphatic amino acid hydroxylases belonging to the Fe(II)/α-ketoglutarate-dependent dioxygenase superfamily. Appl Microbiol Biotechnol. 2014;98:3869–76. https://doi.org/10.1007/s00253-014-5620-z.View ArticleGoogle Scholar
- Handa T, Yamaguchi K, Sono Y, Yazawa K. Effects of fenugreek seed extract in obese mice fed a high-fat diet. Biosci Biotechnol Biochem. 2005;69:1186–8.View ArticleGoogle Scholar
- Smirnov SV, Kodera T, Samsonova NN, Kotlyarova VA, Rushkevich NY, Kivero AD, et al. Metabolic engineering of Escherichia coli to produce (2S, 3R, 4S)-4-hydroxyisoleucine. Appl Microbiol Biotechnol. 2010;88:719–26. https://doi.org/10.1007/s00253-010-2772-3.View ArticleGoogle Scholar
- Hausinger RP. Fe(II)/α-ketoglutarate-dependent hydroxylases and related enzymes. Crit Rev Biochem Mol Biol. 2004;39:21–68. https://doi.org/10.1080/10409230490440541.View ArticleGoogle Scholar
- Kell DB, Swainston N, Pir P, Oliver SG. Membrane transporter engineering in industrial biotechnology and whole cell biocatalysis. Trends Biotechnol. 2015;33:237–46.View ArticleGoogle Scholar
- Falcioni F, Blank LM, Frick O, Karau A, Bühler B, Schmida A. Proline availability regulates proline-4-hydroxylase synthesis and substrate uptake in proline-hydroxylating recombinant Escherichia coli. Appl Environ Microbiol. 2013;79:3091–100. http://aem.asm.org/content/79/9/3091.
- Lin B, Fan K, Zhao J, Ji J, Wu L, Yang K, et al. Reconstitution of TCA cycle with DAOCS to engineer Escherichia coli into an efficient whole cell catalyst of penicillin G. Proc Natl Acad Sci. 2015;112:9855–9. http://www.pnas.org/content/112/32/9855.abstract.
- Theodosiou E, Breisch M, Julsing MK, Falcioni F, Bühler B, Schmid A. An artificial TCA cycle selects for efficient α-ketoglutarate dependent hydroxylase catalysis in engineered Escherichia coli. Biotechnol Bioeng. 2017;9999:1–10. https://doi.org/10.1002/bit.26281.Google Scholar
- Niu P, Dong X, Wang Y, Liu L. Enzymatic production of α-ketoglutaric acid from l-glutamic acid via l-glutamate oxidase. J Biotechnol. 2014;179:56–62. https://doi.org/10.1016/j.jbiotec.2014.03.021.View ArticleGoogle Scholar
- Nilsson LO, Mannervik B. Improved heterologous expression of human glutathione transferase A4-4 by random silent mutagenesis of codons in the 5′region. Biochim Biophys Acta. 2001;1528:101–6.View ArticleGoogle Scholar
- Papaneophytou CP, Kontopidis G. Statistical approaches to maximize recombinant protein expression in Escherichia coli: a general review. Protein Expr Purif. 2014;94:22–32.View ArticleGoogle Scholar