Difference between revisions of "Team:NPU-China/Demonstrate"

Latest revision as of 02:22, 2 November 2017

1. Core-part：the activity of rate limiting enzyme ceaS2 has been improved

Acrylic acid is a byproduct of CEAS2 enzyme, the catalytic effect of wild type ceaS2 enzyme is very weak.
We used the AEMD platform to analyze the ceaS2 enzyme and screened the 38 mutants in the range of 5 Å around the active site to carry out molecular cloning of point mutation, and then tested the acrylic acid yield by HPLC after whole cell catalysis. Because there are a large number of mutants, we divided them into five batches to carry out the reaction, the results are as follows:

In the figure, the horizontal axis stands for each different point mutation. We selected two reaction times 21h and 42h, the vertical axis is acrylic acid production (mg / L).
Due to the differences in wild type between different batches, we will normalize all the data in order to facilitate the analysis of the catalytic effect of each mutation point compared to the respective WT, that is, to compare each mutation point to The batch WT yield multiple is a new indicator, the result is as follows:

The horizontal axis in the figure is the position of each mutational site, and the vertical axis is the multiple of the acrylic acid yield of each mutational site compared to each corresponding batch of the wild type. It can be seen that there were 11 mutational sites, whose yields were higher than the wild type ceaS2, in the 38 mutant programs, and the F438M mutant had the highest yield of 11 times the wild type. The effect was significant.

2.System：S. cerevisiae is more suitable for chassis cells than E. coli

Acrylic acid has strong chemical reactivity and is very destructive to cell membrane. Therefore, the chassis cells’ tolerance to acrylic acid is a "roof" factor that restricts high yield of acrylic acid.
We chose E. coli and S. cerevisiae, the two most convenient model chassis organisms in prokaryotic and eukaryotic organisms. In order to investigatethe chassis cells’ tolerance to acrylic acid, we set up a cytotoxicity test where the two chassis cells grew in different concentrations of acrylic acid medium, and the bacteria OD changes were monitored.The results are as follows:

Fig1. OD of E.coli MG1655 under acrylic acid of different concentration and time

Fig2. OD of S. cerevisiaeBY4741 under acrylic acid of different concentration and time

Two kinds of chassis cells have different tolerance to acrylic acid. Here we selected 500mg / L and 1000mg / L two kinds of acrylic acid concentration to analyze:

Fig3. A comparison of OD of BY4741 and MG1655 under 500mg/L acrylic acid

Fig4. A comparison of OD of BY4741 and MG1655 under 1000mg/L acrylic acid

As can be seen from the results, when the concentration of acrylic acid reached 500mg / L, E. coli bacterial growth was inhibited or even declined while S. cerevisiae normally grew and entered a stable period. And when the concentration of acrylic acid reached 1000 mg / L, the growth of S. cerevisiae was then inhibited.

Conclusion: S. cerevisiae has a better tolerance to acrylic acid toxicity than E. coli, and may be more suitable for use as chassis cells, and our results of the pathway further confirm this conclusion.

3.Pathway：Successfully build a new acrylic acid synthesis pathway and increase acrylic acid production

In order to increase the ability of the chassis cells convert ing glycerol to DHAP or 4G3P, we designed a new GlyDH-DAK glycerol metabolic pathway. To maintain the supply of the reducing power of GlyDH enzymes, the NOX-CAT reducing power module was also introduced, which eventually forms the acrylic synthesis pathway — GDNCC Pathways.
First, we introduced new pathways into two chassis cells through two or three plasmid vectors.

pET-28a-ceaS2; pCDFDuet-gld-DAK; pETDuet-NOX-CAT; YCplac33-LEU-ceaS2; YCplac33-LEU-ceaS2-NOX; YCplac33-URA-gld-DAK

Fig5 1:E.gld+DAK;2:S-ceaS2;3,E.NOX-CAT;4.S.NOX-ceaS2;5:DAK;6:NOX;7,ceaS2; 8:gld;9:s.gld-DAK;10:CAT

We also used the whole cell catalytic reaction and HPLC determination method to determine the amount of acrylic acid produced. For E. coli, yields of using new and old synthetic pathways of acrylic acid are as follows:
Conditions: reaction time 42h, PH8.0, glycerol concentration 1%

It can be seen that the acrylic acid yield is increased by 3 times after the introduction of the GlyDH enzyme and the DAK enzyme compared to the introduction of only the ceaS2 enzyme in old pathway. And the acrylic acid yield is increased by 8 times compared to the old one after the addition of the reducing power module. The new pathway does enhance the ability of E. colisynthesizing acrylic acid.

As for S. cerevisiae, since S. cerevisiae itself has a higher activity of hydrogen peroxide reductase, the reducing power module onlyhas NOX enzyme. Theacrylic acid yields ofapplying new and old synthetic pathways are as follows:
Conditions: reaction time 72h, PH8.0, glycerol concentration 2% Normalized the results based on the acrylic acid yield of BY4741-ceas2 as the indicator.

It can be seen that, similar to the results of E. coli, the introduction of new pathways does improve the ability of S. cerevisiae synthesizing acrylic acid.
Compared the old pathway introduced only ceaS2 enzyme, acrylic acid production was increased by 3 times after introduction of GlyDH enzymes and DAK enzymes. And the yield of acrylic acid was increased by 5 times compared to the old pathway after the addition of the reducing power module.
We also used CRISPR-CAS9 to optimize the bypass metabolic pathway of the S. cerevisiae.

Colonial verification results show that we have successfully knocked out the S. cerevisiae's DLD genes:

Fig 6 S.C BY4741DLD1gene Agarose gel figure of colonies verification after CRISPR knockout.

WT is the corresponding nucleic acid stripe of wild-type S.C BY4741; M is a GeneRuler 1 kb DNA ladder; lanes 1, 2, 3 are three selected nucleic acid stripes of monoclonal colonies.
We also tested the acrylic acid synthesis ability of the transformed strain. The results are as follows: Conditions: reaction time 72h, PH8.0, glycerol concentration 2% Normalized the results based on the acrylic acid yield of BY4741-ceas2 as the indicator.

It can be seen that the optimization of bypass metabolic flux is conducive to the concentration of metabolic flux and improving the yield of acrylic acid. Of coursewe also found in the process of the experiment that after knocking out the 9 genes, S. cerevisiae colony growth became very slow, indicating that a more tender method should be adopted, such as RNAi, to inhibit the bypass pathway.

4.Product：Multi - Conditional Optimization of Acrylic Cell Factory Catalytic Reaction Process

There are several important conditions for whole cell reaction: enzyme induction temperature, carbon source, Buffer, PH, reaction time. We set different control experiments with E.coli BL21 (DE3) as the chassis cells. The results are as follows: 4.1 The effects of different induction temperatures on the amount of acrylic acid were investigated. The results are as follows: Induction time: 14h

It can be seen that when the induction temperature was 30 ℃, the enzyme expression and activity were the highest, and the yield of acrylic acid was the best.

4.2 the results of production of acrylic acid with different carbon sources Condition: PH7.4 Reaction time: 16h Glucose concentration: 4g/L Glycerol concentration: 1%

It can be seen that the yield of acrylic acid was higher when the glycerol was used as the carbon source, because the carbon flow rate of the glycerol metabolic pathway was more concentrated, thus turning more carbon source into acrylic acid. Plus, the glycerol itself owning a higher reducing powermay also be one of the reasons. 4.3 The effects of different pH on the amount of acrylic acid were investigated. The results are as follows: Reaction conditions: 12h reaction time, 1% concentration of substrate glycerol

It can be drawn that PH8.0 was most suitable for acrylic acid production; the reason may be that alkaline environment made E.coli more resistant to acrylic acid. 4.4 The effect of different Buffer on the amount of acrylic acid were investigated. The results are as follows:

It can be seen that the DHa or G3P activity of the two substrates of ceaS2 enzyme was higher under PBS buffer condition. 4.5 The effects of different reaction time on the amount of acrylic acid were investigated. The results are shown as follows

It can be drawn that the yield of acrylic acid reached a higher level after the whole cell catalytic reaction endured for 16h. The sampling point should be set after 16h.

5.Conclusion

Due to the time limit of the experiment, we did not have enough time to replace the optimal mutation site into the existing cell factory. At present, the highest yield of acrylic acid that we have acquired is 211.655 mg / L, which is 200 times than that of GAACF1.0.

Fig7. Yeast strain: BY4741-ceaS2-gld-DAK; Condition of whole cell catalysis: PH: 7.4; Concentration of the substrate glycerol: 2%.

The chromatogam of the sample by HPLC shows the yield is up to 211.655 mg / L according to the standard curve. 211.655mg/L , currently this is the highest yield of acrylic acid biosynthesis, where glycerol serves as the carbon source. As an undergraduate team, in just a few months, we have tried our best to create an efficient acrylic cell factory. We were surprised by the huge increase in GAACF 2.0 production, which is only the production of wild-type ceaS2. Because it is a continuing project, we are planning to screen for more active mutants on the basis of several productive mutations using HTS for point saturation mutations and high-throughput screening. And then, we will transform them into the existing chassis organism. We believe that we will create a new technology for acrylic acid production which has more industrialization prospect!