Introduction

The Ribulose 1,5-bisphosphate Carboxylase Oxygenase (RuBisCO) is the most important enzyme in the Calvin cycle. It binds gaseous carbon dioxide to ribulose-1,5-bisphosphate (Ru-BP) generating two molecules of 3-phosphoglycerate (3-PGA). Therefore it is responsible for the fixation of carbon dioxide. 3-PGA is further converted in the Calvin cycle to glycerinaldehyde-3-phosphate. This is an essential intermediate in the central metabolism, as it plays a central role in glycolysis and gluconeogenesis. RuBisCO enzymes are chracterised as enzymes with slow reaction rates with a k_cat of approximately 20 (Sage, 2002). Furthermore they catalyse a side reaction with oxygen instead of of carbon dioxide, deteriorating the catalytic efficiency. The inclusion of the RuBisCO in a carboxysome, would significantly improve the efficiency of carbon fixation.
It was our aim to enable carbon fixation in E. coli for generating an autotrophic organism. Implementation of the Calvin cycle in this heterotrophic model organism should be associated by the expression of the carboxysome. We would like to use a carboxysome to generate a higher efficiency. As a carbon source for our experimtens with E. coli, we choose the pentose xylose. Thereby could be ensured, that the glycolysis for the generation of energy could be avoided. Xylose is metabolized by the cells to ribulose-5-phosphate. This is the substrate for the phosphoribulokinase A from S. elongatus, which is recombinant expressed from E. coli. The PrkA attaches a phosphate group to ribulose-5-phosphate generating ribulose-1,5-bisphophate. This again is used by the RuBisCO to produce 3-phosphoglycerate. 3-phosphoglycerate can enter the glycolyses and pyruvat as a product is build up. The reaction mechanism is illustrated in figure 1.

Figure 1: Pathway of the D-xylose consumption in E. coli for the fixation of carbon dioxide by the RuBisCO from Halothiobacillus neapolitnaus. For this approach the substrate ribulose 1,5-bisphosphate needs to be accumulated in the cell. This is realzied be the PrkA from Snyechoccous elongatus.

Our first aim was to proof the carbon dioxide fixation by the RuBisCO. The functionality of the RuBisCo is essential for our project, as this is basic of carbon dioxide fixation. Expression of the RuBisCO together with the PrkA, which is generating the substrate for the RuBisCo, could give the proof of carbon dioxide fixation in E. coli. Therefore, we investigated the functionality of the RuBisCO in vitro perfoming a RuBisCO activity assay.
Furthermore, the carbon dioxide fixation should be measured by cultivation of E. coli KRX in a bioreactor under high carbon dioxide concentrations measuring the content of CO₂ in the exhaust air. The wildtype KRX was compared with the KRX conaining ptac_Hneap RuBisCO together with the PrkA.

RuBisCO activity assay

For the verification of RuBisCo expression, we analyzed protein expression of E. coli KRX containing the construct T7_Hneap RuBisCo and in a second verification, protein expression of BBa_K1465213. Therefore, cultivation was carried out as described in Cultivation for Expression of recombinant proteins. Samples were generated using the protocol for Fast Cell Lysis for SDS-PAGE. The results are shown in figure 2 and 3.

Figure 2: Proteinexpression of T7_Hneap RuBisCO induced with 0.1 % rhamnose in the time curve of the cultivation .

Figure 3: Proteinexpression of BBa_K1465213 induced with 0.5 mM IPTG in the time curve of the cultivation .

In both SDS-PAGEs, there is a clearly increasing band over the duration of the cultivation. Analysis with MALDI-TOF proofed the assumption, that the band somewhat smaller than 55 kDa is attributed to the large subunit of the RuBisCO from Halothiobacillus neapolitanus. The analysis was done via tryptic digestion in silico and alignment of the identified peptides with the in silico peptides. Expression of the RuBisCO under control of the T7 promoter showed seven identical peptides and the sequence coverage was 15.2 % (MS) and 15.2 % (MS/MS). RuBisCO expression under control of the p_tac promoter gave six identical peptides and the sequence coverage was 13.5 % (MS) and 13.5 % (MS/MS). The small unit of the RuBisCO could only be identified via MALDI-TOF expressed under the control of the T7 promoter. The analysis was performed as described above, showing three identical peptides and a sequence coverage of 35.5 % (MS) and 12.7 (MS/MS). The small subunit with a size of 12.8 kDa was hard to find in the SDS-PAGE. This results correspond with the verification of protein expression from the plasmid pHnCBS1D, which we used as the basic for purifying of carboxysomes. Expression of this plasmid gave only in one of three experiments the proof of expression the small subunit. This could be due to the size of the protein, which may prevent a fine separation in SDS-PAGE.

For the verification of RuBisCO activity, we performed an in vitro assay measuring variances for ribulose-1,5-bisphosphate and 3-phosphoglycerate, substrate and product of the RuBisCo, in the cell extract of KRX wildtype and KRX carrying the construct T7_Hneap RuBisCO. The methode for the in vitro assay is described


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and the measuring via HPLC in the protocol for HPLC.
The first measurement with HPLC was made to identify substrate and product of the RuBisCO, Ru-BP and 3-PGA. Therefore a standard containing just one of the substances were measured (figure 4). The substances are clearly separable with a retention time of 14.4 min for Ru-BP and 12,6 min for 3-PGA.

Figure 4: Standards for the RuBisCO activity assay. The first measurement was ribulose-1,5-bisphosphate, the second was 3-phosphoglycerate.

To show that Ru-BP does not occur in the cell extract of E. coli wildtype, we performed the assay with just the cell extract of the wildtype without addition of Ru-BP. As a control, we had a seond attachment containing the cell extract and Ru-BP was added. The samples were taken after 30 min, to demonstrate, that there is no unspecific degradation of Ru-BP. The result is shown in figure 5. Unexspected there is in both attachments a small peak from 3-PGA. In both replicates, the peak was slightly taller in the cell extract when Ru-BP was added. The production of 3-PGA is a specific reactions, which is only catalzed by the RuBisCO.

Figure 5: HPLC measurement of cell extract from E. coli KRX wildtype. In the first measurement, ribulose-1,5-bisphosphate was added. The second measurement was performed with the cell extract without adding ribulose-1,5-bisphosphate. Samples were taken after 30 min.

In the third experiment we performed the assay with the cell extract of the wildtype and KRX carrying the construct T7_Hneap RuBisCO taking samples every 5 min to demonstrate the time course of the reaction. Two biological replicates each were performed, which showed both the same results. In figure 6 the progress of the reaction is shown.

Figure 6: RuBisCO activity assay. The cell extract from E. coli KRX T7_Hneap RuBisCO and E. coli KRX wildtype was examined for RuBisCO activity. The substrate for the RuBisCO, ribulose-1,5-bisphosphate was added and the time curve of ribulose-1,5-bisphosphate and the product of the enzymatic reaction, 3-phosphoglycerate, was measured via HPLC in 5 minutes intervalls.

In the attachment containing the wildtype the peak for Ru-BP remains constant over the time. A exeption is the sample taken after 10 min. The peak is smaller in this measurement, indicating that the concentration is lower. This is inconsistent upon consideration the sample taken after 15 min, where the peak reaches the same intensity than reached in the sample 1 and 2. The attachment containing the cell extract of KRX T7_Hneap RuBisCO shows a clearly decreas of the substrate Ru-BP and a increase of 3-phosphoglycerate over the time course (figure 6). It seems, that the reaction is not completed after 15 min, because there is still some Ru-BP detectable. This may be referable to the small reaction rates of the RuBisCO (Sage, 2002).
It can be concluded, that the RuBisCO is active, showing the conversion of Ru-BP to 3-PGA over the measured time course. Therefore, we sucessfully proofed the activity of the RuBisCO.

Cultivation

Bild Carbonat-Gleichgewicht
Bild Reaktor Schema
Bild Reaktor
Kalbriergerade

Figure x: Calibration 10%.

Figure x: Calibration 4%.

Figure x: Calibration by linear fit of the output signal of the qubit analyzer to determine the carbon dioxide fixation.

x = y - 1555,34754 / 4,10739

Figure x: Comparision of the calibration by the measured carbon dioxide using the Qubit Analyzer and calculation from the measured flow rates of the system.

Kultivierung

References

Sage, Rowan F. „Variation in the K(cat) of Rubisco in C(3) and C(4) Plants and Some Implications for Photosynthetic Performance at High and Low Temperature“. Journal of Experimental Botany 53, no. 369 (2002): 609–20.
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