CHAPTER 3 – Results
3.9. Biosynthesis of MGGBQ in recombinant E. coli
3.9.4. Elucidation of the chemical structure of MGGBQ by NMR
149 3.9.3. Characterisation of MGGBQ using LC-MS
HPLC analysis of E. coli CS6 samples resulted in identical retention time and identical UV maximum absorption for peak 1 and 2, as that obtained for in E. coli DH5/pCAS29 and E.
coli BW25113/pCAS29 samples. Peak 1 and peak 2 in E. coli DH5/pCAS29 samples were characterized with LC-MS (shown in figure 3.17). To confirm whether the HPLC peak 1 and peak 2, detected in E. coli CS6 corresponded to MGGBQ reduced and MGGBQ oxidised respectively, the extract sample (bioreactor cultivation MM-Glucose) was characterized by LC-MS using the same method described in section 2.2.4.4. These analyses were performed by Dr. W. Armbruster, Universität Hohenheim, Stuttgart. The results of LC-MS can be seen in appendix Figure A3-3. Strong signals in Fig. A3-3 (B) at 397.2 Da and 414.2 Da in positive mode corresponded to the mass of MGGBQ reduced i.e. [M+H+]and [M+NH4+].
Where M is the mass of sample in Da when calculated resulted in 396 Da. Similarly, in figure A3 (C), the signals at 395.2 Da and 412.2 Da shown in positive mode corresponded to MGGBQ oxidised i.e. [M+H+]and [M+NH4+]. After calculation, M resulted in 394 Da which corresponded to the expected mass of oxidized MGGBQ. Hence, from the LC-MS results it was confirmed that both MGGBQ reduced and MGGBQ oxidized were produced during cultivation of E. coli CS6 in MM-Glucose and MM-Glycerol in bioreactor.
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carried out according to the method described in (section 2.2.5.3). Estimated MGGBQ reduced
obtained after purification was 11.6 mg and was analyzed by NMR for elucidation of the chemical structure. NMR analysis and its interpretation was performed at Universität Hohenheim, Stuttgart at Institute of Bioorganic Chemistry by Dr. Jürgen Conrad. 1H and 13C NMR spectrum and data are included in appendix A3-4 to A3-6.
Heteronuclear adiabatic HSQC and HMBC correlations along with ROESY correlations allowed the unambiguous assignment of the substitution patterns of the aromatic ring and the side chain (figure A3-5 in appendix). Evaluation of the ROESY spectrum revealed an all-(E) configuration in the side chain. Band selective HSQC and HMBC experiments were applied to clearly assign closely resonating 13C NMR signals e.g. δ values of 40.52, 40.43, and 40.39 ppm in crowded regions of the NMR spectra, thus enabling both, a complete NMR assignment and the verification of the proposed structure. All hydrogen atoms and carbon atoms on isoprenoid chain from 1´ to 15´ (shown in figure A3-6) were detected from 1H and
13C NMR analysis. Magnified 1H & 13C NMR spectrum for small range of values can be seen in appendix. Clear signals indicating the 3 methyl groups on side chain were also obtained. Signals representing the 3 trans double bonds on the side chain were also detected (seen from ROESY spectrum in appendix). Signals representing hydrogen and carbon atoms forming the aromatic ring, the methyl group at carbon 6 position on aromatic ring, were obtained. Signals showing the 2 hydroxyl groups on carbon 1 and 4 positions were also obtained. 2D NMR analysis results (shown in appendix) for these sample resulted in data confirming the characteristic properties of the proposed MGGBQ structure.
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3.10. Biosynthesis of -tocotrienol in a chromosomal integrated E. coli strain 3.10.1. Construction of E. coli CS7 strain
Chromosomal Integration of cycAt expression cassette in E. coli CS6 and analysis of protein expression by 2D-Gel electrophoresis
Plasmid pCAS50 was modified by ligation of HindIII digested 1.1 kB, FRT-cat-FRT fragment in 6.4 kB HindIII digested pCAS50 fragment. Plasmids were isolated from different transformants obtained after transforming ligation mixture in E. coli DH5. Digestion of these plasmids from different clones with restriction enzyme HindIII resulted in 1.1 kB & 6.4 kB and digestion with NdeI/BamHI resulted in 1.3 kB and 6.2 kB. This corresponded to the expected sizes calculated using Clone Manager program. This new plasmid was named pCAS50-FRT-cat-FRT.
pCAS50-FRT-cat-FRT was used as template for PCR amplification. Primers xylA-integr and xylB-integr (i.e. primer nr. 21 & 22 from table 2.1.2.5) were used for amplification of a fragment Ptac- cycAt -FRT-cat-FRT. On each side of this fragment 50 nucleotides are homologous to a region in xylA and xylB locus. E. coli CS6 strain carrying a plasmid pKD46 (expressing -recombinase induced by arabinose) was transformed by electroporation with the fragment Ptac- cycAt -FRT-cat-FRT. 21 transformants obtained were tested on MacConkey agar plates with xylose, with there respective control strain. These transformants were screened on MacConkey agar plate each supplemented with 4 different sugars (fucose, maltose, lactose and xylose). Two transformants were found to be positive as they didn’t turn red on any of the McConkey agar plates with xylose, or maltose, or lactose or fucose. The new strain was named as E. coli CS7-cat. PCR control test with primers P27 and P11 and P28 and P11 yielded PCR product of 1.05 kb and 1.30 kb sizes which corresponds with the expected sizes calculated using Clone manager 7.0. Elimination of cat resistance gene was carried out by using temperature sensitive recombinase (pCP20) (method 2.2.2.5) yielded E. coli CS7 which can be seen in figure 3.25.
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Figure 3.55: Scheme showing chromosomal integration of cycAt gene in xylose locus. (a) Showing the intact loci yiaA, xylB, xylA and xylF in E. coli CS6 strain before integration. This E. coli CS6 strain already has hptSyn, crtE and hpd expression cassettes integrated in lactose, maltose and fucose operon resp. (b) The site of xylose locus after integration of Ptac- cycAt-FRT-cat-FRT in xylA & xylB region of E. coli CS6 chromosome. (c) The final strain obtained i.e. E. coli CS7 after removal of chloramphenicol residual cassette (cat) with Ptac-cycAt integrated at the site of xylA and xylB i.e. xylAB::Ptac-cycAt.
The newly obtained strain E. coli CS7, is shown schematically with comparison to the E. coli CS6 strain in figure 3.56A.
Figure 3.56A: Scheme showing chromosomally integrated strains E. coli CS6 & E. coli CS7. a) E.
coli CS6 which consists of hpt-Syn, hpd and crtE expression cassettes in lactose, fucose and maltose operons resp. b) E. coli CS7 which consists of an additional cyc-At expression cassette in xylose operon in E. coli CS6.
After confirming that cycAt expression cassette was correctly integrated in xylose operon in the desired location of the newly constructed strain E. coli CS7, expression of Hpd, CrtE, Hpt –Syn and Cyc-At was tested on 2D gel electrophoresis. E. coli CS7 was cultivated in LB medium in shaking flask (2.2.3.1) and as control E. coli CS6 was cultivated. Cultures were induced with 0.25 mM IPTG at OD 600nm of 0.8. Sample 6 h after induction was tested for protein expression on 2D gel electrophoresis shown in figure 3.56. The Hpd and CrtE protein at 39.6 kDa and 31.4 kDa resp. seen in both strains perfectly overlapped each other (marked as Hpd and CrtE resp. in figure 3.56B). Hpt-Syn being hydrophobic protein could not be detected in any of the 2 samples (calculated values of 34.4 kDa and pI of 9.02). An additional protein spot in red colour was visible in E. coli CS7 sample which was absent in E.
coli CS6. This red spot had a mass of 46.2 kDa and pI of 5.88 (based on Rf method) which closely corresponded to the calculated properties (ExPASy) of Cyc-At protein.
hpt-Syn ptac RBS
(lacZYA)
crtE ptac RBS
(malEFG)
hpd RBS
(fucIP)
cyc-At RBS
(xylAB)
FRT FRT FRT
hpt-Syn ptac RBS
(lacZYA)
crtE ptac RBS
(malEFG)
hpd RBS
(fucIP)
FRT FRT FRT a) E. coli CS6
b) E. coli CS7
yiaA xylB xylA xylF
cyc-At FRT
xylF cat
Ptac FRT
Ptac
(a)E. coliCS6
(b)E. coliCS7-cat (c) E. coliCS7 yiaA
yiaA xylF
FRT RBS
RBS cyc-At
yiaA xylB xylA xylF
cyc-At FRT
xylF cat
Ptac FRT
Ptac
(a)E. coliCS6
(b)E. coliCS7-cat (c) E. coliCS7 yiaA
yiaA xylF
FRT RBS
RBS cyc-At
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Figure 3.56B: 2 D gel electrophoresis for E. coli CS6 and E. coli CS7 samples. Gels were overlayed i.e. E. coli CS6 (in green) and E. coli CS7 (in red). Hpd and CrtE protein seen in both samples are marked with arrow. Additional protein band (red spot) in E. coli CS7 sample at approx. 46.2 kDa was seen. Cyc-At has calculated size of 47 kDa and pI of 5.95. An unidentified protein was observed on the 2 D gel in E. coli CS7 strain shown by block arrow.
-Tocotrienol Biosynthesis in E. coli CS7 in Shaking Flask
After confirming the Cyc-At expression in E. coli CS7, it was important to check the activity of the cyclase level expressed. This was done in-vivo in E. coli CS7 strain by cultivation in shaking flask in LB – Glycerol, at 30 °C. As control E. coli CS6 strain was cultivated under identical conditions. If Cyc-At expressed in E. coli CS7 is active, E. coli CS7 cells would produce MGGBQ along with -tocotrienol. The control strain E. coli CS6 which do not carry cyc-At gene would not produce -tocotrienol. This difference should be visible during the HPLC analysis of samples extracted.
Both strains reached almost identical cell densities till the time of induction. Cultures were induced with 0.25 mM IPTG at approx. OD600nm of 0.8 (4.5 h). Results are shown in figure 3.57. 4 h after induction, cell growth in E. coli CS7 cultures had already started to retard when compared to that in E. coli CS6 cultures. After 72 h, E. coli CS7 cells reached a final OD600nm of 4.01 while E. coli CS6 reached a final OD600nm of 4.45. Cell pellet samples extracted and analyzed by HPLC showed MGGBQ (in reduced and oxidized form) in both strains. E. coli CS7 produced 1.6 times more total MGGBQ in terms of µg/g CDW as compared to E. coli CS6 after 24 h. Perhaps the maximum MGGBQ level attained in E. coli CS6 was within the first 24 h of cultivation. This could not be confirmed as no sample for MGGBQ/-tocotrienol extraction was taken between 4 h and 24 h. Lower cell dry weight achieved in E. coli CS7 as compared to E. coli CS6 after 24 h also automatically contributed to higher MGGBQ yields. The 48 h sample showed that the total MGGBQ productivity and
M
50 kDa 37 kDa
25 kDa 75 kDa
20 kDa
- +
pH 10 pH 3
E. coli CS7 E. coli CS6
Cyc-At
CrtE
Hpd
M
50 kDa 37 kDa
25 kDa 75 kDa
20 kDa
- +
pH 10 pH 3
E. coli CS7 E. coli CS6 E. coli CS7 E. coli CS6
Cyc-At
CrtE
Hpd
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yield in both cultures decreased by approx. 30-35 % to what it was after 24 h. Decrease in total MGGBQ in E. coli CS6 of 40 % continued between 48 h and 72 h while the decrease in total MGGBQ in E. coli CS7 during this period was 14 %. HPLC analysis of the same samples was able to detect -tocotrienol in E. coli CS7 cells while no -tocotrienol was detected in any of the cell samples of E. coli CS6. Small amount (0.13 + 0.02 µg/g CDW or 0.17 + 0.03 µg/l) of -tocotrienol was produced in E. coli CS7 cells after 24 h. -tocotrienol productivity and yield doubled from 24 h to 48 h, and increased marginally from 0.39 + 0.05 µg/g CDW (0.52 + 0.06 µg/l) to 0.45 + 0.05 µg/g CDW (0.58 + 0.07 µg/l) between 48 and 72 h. There was only a small increase in -tocotrienol and small decrease in total MGGBQ despite of approx. 200 µg/g CDW MGGBQ was still available for cyclization reaction between 48 and 72 h in E. coli CS7. This indicates that the tocopherol cyclase activity was rate limiting.
Results in figure 3.57 showed that Cyc-At expressed from the single copy of cyc-At gene integrated in the chromosome of E. coli CS7 strain was able to catalyze the cyclization reaction by converting MGGBQ into small amount of -Tocotrienol in complex medium using glycerol as additional carbon source.
Figure 3.57: Comparison of cell growth curve, total MGGBQ and -Tocotrienol production in chromosomally integrated strain in shaking flask in LB medium with 2 % glycerol. Cultures induced with 0.25 mM IPTG at OD600nm of 0.8.
a) Cell densities achieved by E. coli CS6 and E. coli CS7 cultures in shaking flask
b Total MGGBQ produced in E. coli CS6 and E. coli CS7
c) -tocotrienol produced in E. coli CS7. No product detected in control E. coli CS6.
0 100 200 300 400
0 24 48 72
Time [h]
Total MGGBQ [µg/g CDW]
E. coli CS6 E. coli CS7
0 1 2 3 4
0 24 48 72
Time [h]
Tocotrienol [µg/g CDW]
E. coli CS6 E. coli CS7 0
1 2 3 4 5
0 10 20 30 40 50 60 70 80
Time [h]
OD600 nm [-]
E. coli CS6 E. coli CS7
0 100 200 300 400
0 24 48 72
Time [h]
Total MGGBQ [µg/g CDW]
E. coli CS6 E. coli CS7
0 1 2 3 4
0 24 48 72
Time [h]
Tocotrienol [µg/g CDW]
E. coli CS6 E. coli CS7 0
1 2 3 4 5
0 10 20 30 40 50 60 70 80
Time [h]
OD600 nm [-]
E. coli CS6 E. coli CS7
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3.10.2-Tocotrienol Biosynthesis in Bioreactor in E. coli CS7
To study if E. coli CS7 was able to produce -tocotrienol in minimal medium using glucose or glycerol as sole carbon and energy source, it was cultivated in bioreactor. Cultivation results of E. coli CS7 in bioreactor in MM-Glucose and MM-Glycerol are shown in figure 3.58 to 3.61.
E. coli CS7 cultures in MM-Glucose was induced with 0.25 mM at 13.5 h and glucose feeding was started after 18.5 h when the initial glucose in MM was completely consumed. During the fed batch cultivation (16.5 h to 27 h) the growth rate (µ) was approx. constant (0.17 to 0.18 h-1). The cultures reached stationary phase at 45 h and cultivation was stopped after 48 h. E. coli CS7 cultures in MM-Glycerol grew slower than those in MM-Glucose (maximum µ of 0.19 h-1 and 0.26 h-1 resp.). Glycerol feeding was started at 27 h when all the initial glycerol in MM-Glycerol was consumed. E. coli CS7 in MM-Glucose and MM-Glycerol reached approximately identical cell densities OD600nm of 22 to 23 at the end of cultivation (48 h and 62 h respectively). Refer to figure 3.58.
MGGBQ production pattern in MM-Glucose and MM-Glycerol differed from each other.
MGGBQ production started only after induction with IPTG in both cases. 25 % of the final amount of reduced MGGBQ produced in MM-Glucose was already produced after 16h. After starting of fed batch cultivation, the MGGBQreduced yield (in µg/g CDW), quadrupled between 16 and 22 h. The MGGBQ reduced yield decreased further with MGGBQ oxidized yield remaining constant. One reason for the decrease in reduced MGGBQ was due to the conversion of it into -tocotrienol (0.45 + 0.05 µg/l or 0.15 + 0.02 µg/g CDW) which could be seen in 27 h sample in figure 3.60. The MGGBQ reduced yield gradually decreased from 27 h to 43.75 h before it was almost exhausted at 48 h. During this time MGGBQ oxidized and -tocotrienol also increased gradually to reach 586 µg/g CDW and 3.3 µg/ g CDW respectively. MGGBQ
reduced production in E. coli CS7 in MM-Glycerol started at 16 h and increased at slow rate from 67.5 µg /g CDW to 182.5 µg /g CDW at 23.5 h. MGGBQ oxidized was produced between 23.5 h and 27 h which resulted in decrease in MGGBQ reduced yield. After 27 h the yield of MGGBQ reduced, increased more than 8 times, i.e.from 114 µg /g CDW to 907 µg /g CDW at 47 h. Further it decreased to approx. half with increase in -tocotrienol concentration. The difference to that in MM-Glucose was that the oxidized MGGBQ yield didn’t increase above 165 µg / g CDW. The -tocotrienol production increased from 4.8 + 0.4 µg/l to 26.6 + 1.8 µg/l from 30.25 h to 54 h and didn’t increase further till 60 h despite the presence of approx. 400 µg /g CDW of MGGBQreduced (Refer figure 3.61). Once again it showed that cyclase activity was limiting at least in case of MM-Glycerol where reduced MGGBQ was available and
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tocotrienol production almost stagnated after 62 h. At this point it was not clear why the E.
coli CS7 cultures didn’t carry out the cyclization reaction. When E. coli CS7 cultivation in MM-Glucose and MM-Glycerol were compared produced approximately the same amounts of reduced MGGBQ and -tocotrienol.
Figure 3.58: Cell growth and glucose/glycerol concentration curve during bioreactor cultivation of E. coli CS7 in minimal medium at 30°C. (A) Glucose. (B) Glycerol. Arrows in (A) and (B) represents the time of induction with 0.25 mM IPTG. Vertical lines show the time at which fed batch process was start with feeding of carbon source glucose or glycerol.
0 5 10 15 20 25 30
0 10 20 30 40 50 60 70
Time [h]
OD600nm [-]
0 1 2 3 4 5 6 7
Glucose [g/l]
OD Glucose
0 5 10 15 20 25 30
0 10 20 30 40 50 60 70
Time [h]
OD600nm [-]
0 1 2 3 4 5 6 7
Glycerol [g/l]
OD Glycerol
(B) (A)
0 5 10 15 20 25 30
0 10 20 30 40 50 60 70
Time [h]
OD600nm [-]
0 1 2 3 4 5 6 7
Glucose [g/l]
OD Glucose
0 5 10 15 20 25 30
0 10 20 30 40 50 60 70
Time [h]
OD600nm [-]
0 1 2 3 4 5 6 7
Glycerol [g/l]
OD Glycerol
(B)
0 5 10 15 20 25 30
0 10 20 30 40 50 60 70
Time [h]
OD600nm [-]
0 1 2 3 4 5 6 7
Glycerol [g/l]
OD Glycerol
(B) (A)
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Figure 3.59: Glucose/Glycerol Feed Rate (F) during bioreactor cultivation of E. coli CS7 in minimal medium at 30°C
0.00 0.20 0.40 0.60 0.80 1.00 1.20
0 10 20 30 40 50 60
Time [h]
"F" Feed rate [g/h]
Glucose Glycerol
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Figure 3.60: Reduced and Oxidized MGGBQ yields achieved in E. coli CS7 during bioreactor cultivation in minimal medium at 30 °C. (A) Glucose as sole carbon and energy source
(B) Glycerol as sole carbon and energy source.
0 400 800 1200 1600 2000
0 13.5 15.5 16 18.5 23.5 27 30.3 37 39.8 43.8 47 54 60
Time [h]
Reduced and Oxidized MGGBQ [µg/g CDW]
Reduced Oxidized 0
400 800 1200 1600 2000
0 11.5 16 19 22 27 29 30.3 37 39.8 43.8 47
Time [h]
Reduced and Oxidized MGGBQ [µg/g CDW]
Reduced Oxidized
(B) (A)
0 400 800 1200 1600 2000
0 13.5 15.5 16 18.5 23.5 27 30.3 37 39.8 43.8 47 54 60
Time [h]
Reduced and Oxidized MGGBQ [µg/g CDW]
Reduced Oxidized 0
400 800 1200 1600 2000
0 11.5 16 19 22 27 29 30.3 37 39.8 43.8 47
Time [h]
Reduced and Oxidized MGGBQ [µg/g CDW]
Reduced Oxidized
(B) (A)
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Figure 3.61: -Tocotrienol production in E. coli CS7 strain during cultivation in bioreactor at 30
°C in minimal medium (A) Glucose as carbon, energy source (B) Glycerol as carbon, energy source.
0 5 10 15 20 25 30
0.0 11.50
16.00 19 22.00
27.00 29.00
30.25 37.00
39.75 43.75
47.00 Time [h]
-Tocotrienol [µg/l or µg/ g CDW]
µg/g CDW µg/ l
0 5 10 15 20 25 30
0.00 13.50
15.50 16.00
18.50 23.50
27.00 30.25
37.00 39.75
43.75 47.00
60.00 Time [h]
-Tocotrienol [µg/l or µg/ g CDW]
µg/ g CDW µg/l
-tocotrienol in MM-Glucose
-tocotrienol in MM-Glycerol
(A)
(B)
0 5 10 15 20 25 30
0.0 11.50
16.00 19 22.00
27.00 29.00
30.25 37.00
39.75 43.75
47.00 Time [h]
-Tocotrienol [µg/l or µg/ g CDW]
µg/g CDW µg/ l
0 5 10 15 20 25 30
0.00 13.50
15.50 16.00
18.50 23.50
27.00 30.25
37.00 39.75
43.75 47.00
60.00 Time [h]
-Tocotrienol [µg/l or µg/ g CDW]
µg/ g CDW µg/l
-tocotrienol in MM-Glucose
-tocotrienol in MM-Glycerol
(A)
(B)
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Figure 3.62A: Product yield calculated per gram of carbon source utilized during bioreactor cultivation of E. coli CS7 in minimal medium using either glucose or glycerol at 30 °C.
(A) Total MGGBQ yield in µg/g Glucose or g Glycerol consumed (B) -Tocotrienol yield in µg/g Glucose or g Glycerol consumed
0 100 200 300 400 500 600 700
0 10 20 30 40 50 60 70
Time [h]
Total MGGBQ [µg/g Glucose] or [µg/g Glycerol]
µg/g Glucose µg/ g Glycerol
0.00 0.20 0.40 0.60 0.80 1.00 1.20
0 10 20 30 40 50 60 70
Time [h]
d-Tocotrienol [µg/g Glucose] or [µg/g Glycerol]
µg/g Glucose µg/ g Glycerol
(A)
(B)
0 100 200 300 400 500 600 700
0 10 20 30 40 50 60 70
Time [h]
Total MGGBQ [µg/g Glucose] or [µg/g Glycerol]
µg/g Glucose µg/ g Glycerol
0.00 0.20 0.40 0.60 0.80 1.00 1.20
0 10 20 30 40 50 60 70
Time [h]
d-Tocotrienol [µg/g Glucose] or [µg/g Glycerol]
µg/g Glucose µg/ g Glycerol
(A)
(B)
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