Chemicals
L-tryptophan (L-trp), 5-hydroxytryptophan (5-HTP) and 5-hydroxytryptamine (5-HT) standards were purchased from Aladdin Industrial Corporation (Shanghai, China). (6R)-L-erythro-5, 6, 7, 8-tetrahydrobiopterin (BH4), pyridoxal 5-phosphatemonohydrate (PLP) were purchased from Shanghai Yuanye Bio-Technology Co., Ltd. (Shanghai, China). Other chemicals and solvents in this study were purchased from local suppliers and were of analytical grade or higher purity.
Plasmid and Strain Construction
E.coli Trans1T1 was used for plasmid construction and propagation, and E. coli BL21(DE3) was used for protein expression. For the biosynthesis of 5-HTP or 5-HT, the △tnaA gene was knocked out in E. coli BL21(DE3), using the previously established CRISPR/Cas9 protocol [1, 25]. The expression vectors pCDFDuet-1 and pET-28a (+) were supplied by Novagen Co., Ltd.
Plasmid construction and DNA manipulation were performed following the standard molecular cloning protocols. First, the PTPS, SPR, PCD, DHPR human genes and mtrA from Bacillus subtilus were codon-optimized, synthesized and inserted into pCDF-Duet-1 to acquire E. coli BL21(DE3)△tnaA/BH4 [1]. Another cofactor, tetrahydromonapterin (MH4), whose relative genes were also constructed in pCDF-Duet-1, was used to acquire E. coli BL21(DE3)△tnaA/MH4 according to a previous study [18]. SmTPH from Schistosoma mansoni was codon-optimised for E. coli expression, synthesised, and inserted into pET-28a(+), then transferred into E. coli BL21(DE3)△tnaA/BH4 and E. coli BL21(DE3)△tnaA/MH4 to acquire E. coli BL21(DE3)△tnaA/BH4/SmTPH and E. coli BL21(DE3)△tnaA/MH4/SmTPH. HaDDC from Harminia axyridis was also codon- optimised for E. coli expression, synthesized and inserted into pET-28a (+), then transferred into E. coli BL21(DE3)△tnaA to acquire E. coli BL21(DE3)△tnaA/HaDDC.
The gene fragment of HaDDC was amplified from pET-28a (+)-HaDDC using the primer pairs, HaDDC-F-BamHI, HaDDC-R-HindIII, HaDDC-F-EcoRI, and HaDDC-R-SalI. First, BamHI- and HindIII-digested polymerase chain reaction (PCR) products of HaDDC were ligated to BamHI- and HindIII- digested pET28a (+)-SmTPH and EcoRI- and SalI- digested pET28a (+)-SmTPH to construct the plasmids pET28a (+)-SmTPH-HaDDC and pET28a (+)-HaDDC-SmTPH. The plasmids were constructed in E. coli Trans1T1 and transferred into E. coli BL21(DE3)△tnaA/BH4 a to acquire E. coli BL21(DE3)△tnaA/BH4/SmTPH/HaDDC and E. coli BL21(DE3)△tnaA /BH4/HaDDC/SmTPH. The plasmids and strains along with the primers and specific structures of the plasmids, are shown in Table 2.
Table 2
List of strains and plasmids used for the production of 5-HTP and 5-HT.
Name | Description | Reference |
Strains | | |
E. coli BL21(DE3) | Protein expression host | This lab |
E. coli BL21 (DE3)△tnaA | E. coli BL21(DE3)△tnaA | This study |
E. coli Trans 1T1 | Cloning host | This study |
E. coli BL21(DE3) △tnaA /BH4/SmTPH | E. coli BL21(DE3)△tnaA/pCDFDuet-BH4/pET-28a(+)-SmTPH | This study |
E. coli BL21(DE3) △tnaA/MH4/SmTPH | E. coli BL21(DE3)△tnaA/pCDFDuet-MH4/pET-28a(+)-SmTPH | This study |
E. coli BL21(DE3) △tnaA/HaDDC | E. coli BL21(DE3)△tnaA/pET-28a(+)-SmTPH | This study |
E. coli BL21(DE3) △tnaA/BH4/SmTPH/HaDDC | E. coli BL21(DE3)△tnaA/pCDFDuet-BH4/pET-28a(+)-SmTPH-HaDDC | This study |
E. coli BL21(DE3) △tnaA/BH4/HaDDC/SmTPH | E. coli BL21(DE3)△tnaA/pCDFDuet-BH4/pET-28a(+)- HaDDC-SmTPH | This study |
Plasmids | | |
pET-28a (+) | ColE1 ori, KanR, E. coli expression vector | This lab |
pET-28a (+)-SmTPH | pET-28a (+); tryptophan hydroxylase gene from S.mansoni | This study |
pCDFDuet-1 | CloDF13 ori, SmR, E. coli expression vector | This lab |
pCDFDuet-BH4 | pCDFDuet-1; pterin-4-alpha-carbinolamine dehydratase (PCD), dihydropteridine reductase (DHPR), 6-pyruvate-tetrahydropterin synthase (PTPS), sepiapterin reductase (SPR), GTP cyclohydrolase I (GCHI) | [1] |
pCDFDuet-MH4 | pterin-4-alpha-carbinolamine dehydratase (PCD) and dihydropteridine reductase (DHPR) from human | [18] |
pET-28a (+)-HaDDC | pET-28a (+); dopa decarboxylase from H.axyridis | This study |
pET-28a (+)-HaDDC-SmTPH | pET-28a (+); dopa decarboxylase from H. axyridis; tryptophan hydroxylase gene from S. mansoni | This study |
pET-28a (+)-SmTPH- HaDDC | pET-28a (+); tryptophan hydroxylase gene from S. mansoni; dopa decarboxylase from H. axyridis | This study |
Primers | | |
HaDDC-F-BamHI | GGATCCGAATTCATGGAGGCG | This study |
HaDDC -R-HindIII | AAGCTTTTATTCACCCAGGATATC | This study |
HaDDC-F-EcoRI | GAATTCATGGAGGCGAACCAG | This study |
HaDDC-R-SalI | GTCGACTTATTCACCCAGGATATCGTCC | This study |
Luria-Bertani (LB) medium was used for cell cultivation and enzyme expression [26]. Modified M9 medium (M9Y) was used for in vivo hydroxylation of L-trp to 5-HTP [27]. All the single colonies of the recombinant strains were separately inoculated into 5 mL of LB media containing 25 mg/L kanamycin and/or streptomycin and incubated at 37℃ and mixed at 200 rpm for 12 h. Cultures (1 mL) were then inoculated into 100 mL of LB media at 37℃. When the OD600 reached 0.4–0.6, the cells were induced with isopropyl-β-D-thiogalactoside (IPTG) for 12 h at either 25℃ (E. coli BL21(DE3)△tnaA/BH4/SmTPH) or 18℃ (E. coli BL21(DE3)△tnaA/HaDDC).
The effect of cofactor BH 4 and MH4 synthesis and regeneration pathways on the conversion of L-trp into 5-HTP
The fermentation broth of E. coli BL21(DE3)△tnaA/BH4/SmTPH and E. coli BL21(DE3)△tnaA/MH4/SmTPH were centrifuged at 4,000 rpm for 10 min, and the cells were respectively transferred to M9Y media containing 2 g/L of L-trp. Sampling and measuring were conducted at 12-hour intervals.
Purification of E. coli BL21(DE3)△tnaA/HaDDC and its catalysis specificity towards L-trp and 5-HTP
The cells harvested from cultures were washed, resuspended in 50 mM HEPES (pH 7.0 ), and lysed at 4℃ by JY92-IIN ultrasonication (Ningbo Xinzhi Biotechnology, Ltd., Ningbo, China). The lysates were centrifuged at 12, 000 rpm at 4℃ for 20 min, and the supernatant was used as a crude enzyme solution. The recombinant HaDDC was purified using a fast protein liquid chromatography (FPLC) system (GE AKTA Pure 150; General Electric Co., Fairfield, America) with a Ni-nitrilotriacetic acid affinity chromatography (Ni-NTA) column (His Trap™ FF5 mL) according to the manufacturer’s instructions. Protein concentrations were determined at 595 nm using the Bradford method with bovine serum albumin as the standard [28]. All protein samples were analysed by reductive SDS-PAGE with 20 mM β-Mercapto ethanol incubation. A premixed protein marker (Takara Biotechnology Co., Ltd., Nanjing, China) containing 180-, 140-, 100-, 75-, 60- and 45-kDa proteins was used as the molecular mass standard. The method of purification for the recombinant protein SmTPH was same as that for HaDDC.
The recombinant HaDDC activity was assayed at 35℃ in a reaction mixture of 2 mL, containing 50 mM HEPES (pH 7.0), 0.4 mM PLP, 50 µL purified HaDDC (protein concentration of 15 g/L) and 200 µM L-trp or 5-HTP.
One-pot catalysis for 5-HT production in vitro and in vivo
The in vitro cascade of SmTPH and HaDDC was conducted in a reaction system (2 mL) that contained 7 mM DTT, 0.1 M ferrous ammonium sulfate, 0.1 mg/mL catalase, 0.6 mM BH4, 0.1 mM PLP, 0.5 mM L-Trp, 100 µL purified SmTPH (1.0 g/L), 100 µL purified HaDDC (1.0 g/L) and 50 mM HEPES (pH 7.0), and incubated at 30℃ [1, 7]. In addition to these cofactors, reactions were supplemented with catalase and dithiothreitol to protect against oxidative degradation [18].
The in vivo cascade of SmTPH and HaDDC was performed as follows: the E. coli BL21(DE3)△tnaA/BH4/SmTPH/HaDDC and E. coli BL21(DE3)△tnaA /BH4/HaDDC/SmTPH fermentation broths were centrifuged at 4,000 rpm for 10 min and the cells (final cell concentration of 3.8 OD/mL) were transferred to 100 mL of M9Y media containing 2 g/L L-trp [1]. Cells were incubated at pH 7.0 and 30℃. Sampling and measuring were conducted every 12 hours.
Optimisation of the production of 5-HT from L-trp using E.coli BL21(DE3)△tnaA /BH4/HaDDC-SmTPH whole-cell factory
Cells were transferred to 100 mL of M9Y medium in a 500 mL flask and shaken at 200 rpm in various conditions.
In this study, single-factor experiments using four predominant factors (concentration of IPTG, induced time, induced temperature and catalysis temperature) were conducted to investigate the effects of these factors on the production of 5-HT [29]. Investigation of the effect of each factor on the production of 5-HT was carried out based on changes only to that factor, while keeping other variables constant.
The investigation was carried out at varying concentrations of IPTG (0.25, 0.5, 0.75, and 1 mM), induced time (8 h, 12 h, 16 h and 20 h), induced temperature (18℃, 25℃, 30℃and 37℃), and catalysis temperature (25℃, 30℃, 37℃ and 40℃).
Analytical method
The cell growth density at 600 nm (OD600) was measured using an UV1000D ultraviolet-visible spectrophotometer (AOE Instruments, Shanghai Co., Ltd). High-performance liquid chromatographic (HPLC) analysis of the L-trp, 5-HTP and 5-HT contents was performed on an Agilent 1260 series LC system (Agilent Technologies, Santa Clara, CA, USA) with an ultraviolet detector reading at 276 nm [1]. The samples were filtered through Millex-LG filter units (Millipore, Billerica, MA, USA) prior to HPLC analysis. Separation of samples was achieved using a reverse phase Agilent TC-C18 column (5 × 4.6 mm × 250 mm; Agilent) with a constant flow rate of 1 mL/min at 25℃ and an injection volume of 10 µL. The mobile phase consisted of methanol(10 mM) and potassium phosphate buffer (pH 6.5). The substrate and product concentration were measured by HPLC against the L-trp, 5-HTP and 5-HT standards using a calibration curve.
The 5-HT yield from L-trp was calculated according to the following equation:
5-HT yield (mol%) = 5-HTP released (mol)/L-trp reduced (mol)