Summary

Initially some trials of shaking flask cultivations were made with different parameters to identify the best conditions for the production of the His-tagged laccase LttH from [http://www.dsmz.de/catalogues/details/culture/DSM-7039.html?tx_dsmzreso Thermus thermophilus HB27] named TTHL. Due to the absence of enzyme activity of the enzyme in the cell lysate a purification method was established (using Ni-NTA-His tag resin). Using E. coli KRX containing BioBrick <partinfo>BBa_K863010</partinfo>, TTHL could not be detected by SDS-PAGE (molecular weight of 53 kDa) or by activity test. Therefore a new BioBrick <partinfo>BBa_K863012</partinfo> was constructed and expressed in E. coli Rosetta-Gami 2. With this expression system the TTHL could be detected by SDS-PAGE and purified by using a small scale Ni-NTA column. The fractionated samples were tested regarding their activity. TTHL was shown to oxidize ABTS. After measuring activity of TTHL a scale up of the fermentation was successfully implemented up to 6 L. A further scale up to 12 L with a optimized medium (HSG) and a labscale Ni-NTA-Purification were implemented to enable additional experiments to characterize TTHL. A total specific enzyme activity of 15 U mg^-1 was determined for TTHL at pH 4 at 25°C with ABTS as a substrate.

Cultivation, Purification and SDS-PAGE

Shaking Flask Cultivation

• flask design: shaking flask without baffles
• medium: LB-Medium
• antibiotics: 60 µg mL^-1 chloramphenicol and 300 µg mL^-1 ampicillin
• temperature: 37 °C
• cultivation time: 24 h

The reproducibility of the measured data and results were investigated for the shaking flask cultivation, but not yet for the bioreactor cultivation.

Fermentation of E. coli KRX with <partinfo>BBa_K863012</partinfo>

Figure 1: Fermentation of E. coli Rosetta-Gami 2 with <partinfo>BBa_K863012</partinfo> (TTHL) in a Bioengineering NFL22. Conditions: 6 L of autoinduction medium + 60 µg/mL chloramphenicol at 37 °C, pH 7. Agitation increased when pO₂ was below 30 % and OD₆₀₀ was measured each hour.

Purification of TTHL

The cells were harvested by centrifugation and resuspended in Ni-NTA-equilibrationbuffer, mechanically disrupted by high pressure homogenization and centrifuged. After preparing the cell paste the TTHL could not be purified with the 15 mL column, due to a not available column. For this reason a small scale purification (6 mL) of the supernatant of the homogenisation was made with a 1 mL Ni-NTA-column.

SDS-PAGE of purified TTHL

Figure 2: SDS-PAGE of purified E. coli Rosetta-Gami 2 containing <partinfo>BBa_K863012</partinfo> lysate (fermented in 6 L Bioengineering NFL22). The flow-through, wash and elution fraction 1 to 5 are shown. The arrow marks the TTHL band with a molecular weight of 53 kDa.

Figure 2 shows the SDS-PAGE of the purified E. coli Rosetta-Gami 2 lysates fermented in 6 L Bioengineering NFL22 fermenter. Additionally the flow-through, wash and all elution fractions (1 to 5) are shown. TTHL has a molecular weight of 53 kDa and the corresponding band is marked with a red arrow. The TTHL band can be found in fractions 1 to 3, but not in the other two elution fractions. Furthermore there are some other non-specific bands, which could not be identified. To improve the purification an 15 mL column was implemented.

Since Regionals: 12 L Fermentation of E. coli Rosetta Gami 2 with [http://partsregistry.org/wiki/index.php?title=Part:BBa_K863012 BBa_K863012]

Figure 3: Fermentation of E. coli Rosetta-Gami 2 with <partinfo>BBa_K863012</partinfo> (TTHL) in a Bioengineering NFL22. Conditions: 12 L of HSG autoinduction medium + 60 µg/mL chloramphenicol at 37 °C, pH 7. Agitation increased when pO₂ was below 50 % and OD₆₀₀ was measured each hour.

Since Regionals: Purification of TTHL since Regionals

The harvested cells were resuspended in Ni-NTA- equilibration buffer and mechanically disrupted by homogenization. The cell debris were removed by centrifugation and microfiltration via [http://www.millipore.com/catalogue/module/C7493 Millipore Pellicon XL 50]. The supernatant of the cell lysate was concentrated with [http://www.millipore.com/catalogue/module/C7493 Millipore Pellicon XL 50] with 10 kDa and loaded on the Ni-NTA column (15 mL Ni-NTA resin) with a flow rate of 1 mL min^-1 cm^-2. Then the column was washed with 10 column volumes (CV) Ni-NTA equilibration buffer. The bound proteins were eluted by an increasing Ni-NTA elution buffer step elution from 5 % (equates to 25 mM imidazole) with a length of 50 mL, to 50 % (equates to 250 mM imidazole) with a length of 80 mL and finally to 100 % (equates to 500 mM imidazole) with a length of 80 mL. This strategy was chosen to improve the purification caused by a step by step increasing Ni-NTA-elution buffer concentration. The elution was collected in 10 mL fractions. In Figure 3 only the UV-detection signal of the wash step and the elution are shown, this is because of the high UV-detection signal of the loaded samples and to simplify the illustration of the detected product peak. A typical chromatogram of purified laccases is illustrated here. The chromatogram of the TTHL elution is shown in Figure 4:

Figure 4: Chromatogram of wash and elution fractions from FLPC Ni-NTA His-tag Purification of TTHL produced by 12 L fermentation of E. coli Rosetta Gami 2 with <partinfo>BBa_K863012</partinfo>. TTHL was eluted by a concentration of 50 % (equates to 250 mM imidazole) with a maximal UV-detection signal of 123 mAU.

Contrary to our expectations, the chromatogram shows one distinguished peak. This peak was detected at a Ni-NTA-equilibration buffer concentration of 100 % (equates to 500 mM imidazole) and resulted from the elution of bound protein. Earlier measurements of other bacterial laccases showed that the elution of these laccases begins with a elution buffer concentration of 50 % equates to 250 mM imidazole. One explanation of this result could be a low concentration of the produced TTHL. Consequently all elution fractions were analyzed by SDS-PAGE to detect TTHL. In the chromatogram no further peaks were detected. The following increasing UV detection signal by increasing concentration of the elution buffer results from the rising imidazole concentration of the Ni-NTA elution buffer. The corresponding SDS-PAGES are shown in Figure 5.

Since Regionals: SDS-PAGE of protein purification

Figure 5: SDS-PAGE of purification from the 12 L fermentations from 10/11 ([http://partsregistry.org/wiki/index.php?title=Part:BBa_K863012 BBa_K863012]). Purification of the supernatant via microfiltration, diafiltration and Ni-NTA column (step gradient with 5 %, 50 % and 100 % elution buffer).

In Figure 5 the SDS-PAGE of the Ni-NTA purification of the lysed E.coli Rosetta-Gami 2 culture containing [http://partsregistry.org/wiki/index.php?title=Part:BBa_K863012 BBa_K863012] is illustrated. It shows the permeate and retentate of microfiltration and diafiltration respectively, several fractions of flow-through, wash and the elutions with different buffer concentrations respectively. The selected samples were taken where peaks were seen in the chromatogram. The His-tagged TTHL has a molecular weight of 53 kDa. Apparently the concentration of TTHL is too low to see a band.

Activity analysis of [http://partsregistry.org/wiki/index.php?title=Part:BBa_K863012 TTHL]

Initial activity tests of purified fractions

There was no activity measurable after cultivation and purification of [http://partsregistry.org/wiki/index.php?title=Part:BBa_K863022 BBa_K863022] under the control of a T7 promoter. Activity tests of [http://partsregistry.org/wiki/index.php?title=Part:BBa_K863012 TTHL] under a constitutive promoter did reveal TTHL laccases capable of oxidizing ABTS. Fractions 1 to 5 of the purification above were rebuffered with deionized H₂O and incubated with 0.4 mM CuCl₂ for 2 hours. Activity measurements were performed using 140 µL sample, 0.1 mM ABTS and 100 mM sodium acetate buffer (pH 5) to a final volume of 200 µL. The change in optical density at 420 nm was detected, reporting the oxidization of ABTS through laccases. Fractions 1 to 5 show activity (Figure 6). Fraction 2 seems to contain most of TTHL showing the highest activity compared to the other fractions: 40 % of the used ABTS has been oxidized after 2 hours. Based on these results protein concentrations have to be determined and the activity of the TTHL laccase can be characterized in further experiments including pH optimum and activity in regard of temperature shifts.

Figure 6: Activity test of [http://partsregistry.org/wiki/index.php?title=Part:BBa_K863012 TTHL] fractions resulting from the purification. Reaction setup includes 140 µL fraction sample (CuCl₂ incubated), 0.1 mM ABTS and 100 mM sodium actetate buffer (pH 5) to a final volume of 200 µL. Measurements were done at 25 °C and over a time period of 5 hours. [http://partsregistry.org/wiki/index.php?title=Part:BBa_K863012 TTHL] shows activity in oxidizing ABTS except fractions 1 seems to have no active [http://partsregistry.org/wiki/index.php?title=Part:BBa_K863012 TTHL]. (n=4)

Since Regionals: Initial activity tests of purified fractions

The purificated fractions of the cultivation after the Regional Jamborees in Amsterdam were tested concerning their protein concentration. After re-buffering the protein concentration was determined again and all fraction were incubated with 0.4 mM CuCl₂. For the initial activity test the protein amount was adjusted for comparison. The fractions were measured in Britton-Robinson buffer at pH 5 with 0.1 mM ABTS. Fraction 50 % 1 showed the highest activity (Fig. 7). Regarding the protein amount of this fraction and the statement, that 90 % of this are TTHL laccase, fraction 50 % 1 contains 4,03 µg mL^-1. To ensure enough protein for further experiments, the second best fraction, which is fraction 5 % 3 was added to fraction 50 % 1. The first number indicates the percentage of used elution buffer, whereas the second number stands for the fraction number of this elution. In total, both fraction contain 4,4 µg mL^-1.

Figure 7: Activity assay of each purified fraction of the new cultivation with TTHL. Samples were re-buffered into H₂O and the protein amount in each fraction has been adjusted. The measurements were done using the standard activity assay protocol over night. The first number indicates the percentage of used elution buffer, whereas the second number stands for the fraction number of this elution.

Since Regionals: [http://partsregistry.org/wiki/index.php?title=Part:BBa_K863012 TTHL] activity depending on different ABTS concentrations

In order to find the substrate saturation, laccase activity was measured with ABTS concentrations ranging from 0.1 mM to 8 mM. 616 ng TTHL laccase were used for measurements with ABTS concentrations of 0.1 mM to 5 mM, 308 ng TTHL laccase were used for measurements with ABTS concentrations of 5 mM to 8 mM. Measurements were done in Britton-Robinson buffer (pH 5) at 25 °C for 30 minutes taking the OD₄₂₀ every 5 minutes. Comparing the graphs in Figure 8 and Figure 9, the substrate saturation is not reached with 5 mM ABTS. An application of 8 mM shows less oxidized ABTS as measurements with 7 mM ABTS. Further experiments were done with 7 mM ABTS.

Figure 8: Activity assay to determine the substrate saturation with ABTS as a substrate. Measurements were done with 616 ng TTHL laccase in Britton-Robinson buffer (pH 5) at 25 °C. ABTS concentrations ranged from 0.1 mM to 5 mM.

Figure 9: Activity assay to determine the substrate saturation with ABTS as a substrate. Measurements were done with 308 ng TTHL laccase in Britton-Robinson buffer (pH 5) at 25 °C. ABTS concentrations ranged from 5 mM to 8 mM. An ABTS concentration of 7 mM was determined as substrate saturated.

Since Regionals: [http://partsregistry.org/wiki/index.php?title=Part:BBa_K863012 TTHL] pH optimum

Figure 10: Microtiter plate of the measurements for pH optimum determination. The more intensive the blue color the more ABTS got oxidized. At pH 5 the darkest color has been reached.

The pH of the medium containing the enzyme is very important for its activity. The pH optimum of TTHL is at pH 5. This is the result of activity measurements using Britton-Robinson buffer with differently adjusted pHs. TTHL laccase was re-buffered into H₂O and incubated with 0.4 mM CuCl₂. The range from pH 4 to pH 9 was tested under substrate saturation at 25 °C for 30 minutes. At pH 5 ABTS gets oxidized the fastest (see Fig. 10 and 11). At higher and lower pHs than pH 5, the activity of TTHL is decreased considerably. The resulting Units mg^-1 support the observed data (see Fig. 12). At pH 5 TTHL shows a specific enzyme activity of ~15 U mg^-1. The higher the pH, the less U mg-1 can be calculated for TTHL. At pH 4 and 6 the activity is decreased to 42 % and at pH 7 even to 14 % in comparison to pH 5. But still TTHL is active at this pH allowing an application of this laccase in a waste water treatment plant where the average pH is a pH of 6.9. However, a combination with a more effective enzyme should be considered.

Figure 11: Oxidized ABTS by TTHL laccases at different pH adjustments. The experimental setup included CuCl₂ incubated TTHL laccase (308 ng), Britton Robinson buffer adjusted to the tested pHs and 5 mM ABTS. Measurements were done at 25 °C for 30 minutes. The most amount of oxidized ABTS can be detected at pH 5.

Figure 12: Calculated specific enzyme activity of TTHL at different pH conditions. The highest specific enzyme activity for ABTS is under pH 5 conditions. The higher the pH, the less ABTS gets oxidized. One unit is defined as the amount of laccase that oxidizes 1 μmol of ABTS substrate per minute.

Since Regionals: [http://partsregistry.org/wiki/index.php?title=Part:BBa_K863012 TTHL] activity at different temperatures

Figure 13: Standard activity test for TTHL measured at 10 °C and 25 °C resulting in a decreased activity at 10 °C. As a negative control the impact of 0.4 mM CuCl₂ in oxidizing ABTS at 10 °C and 25 °C were analyzed.

Figure 14: Deriving from the obtained values of oxidized ABTS in time at 10 °C and 25 °C the specific enzyme activity was calculated. For the temperatures a difference of 2 U mg^-1 could be detected. One unit is defined as the amount of laccase that oxidizes 1 μmol of ABTS substrate per minute.

To investigate the activity of TTHL at temperatures that will apply at a waste water treatment plant throughout the year, activity tests as described above were performed at 10 °C and 25 °C. The measurements were conducted for 30 minutes. The obtained results reveal an activity decrease of about 35 % of TTHL at 10 °C in comparison to 25 °C (see Fig. 13). The obtained results were used to calculate the specific enzyme activity which was at 13 and 15 U mg^-1, respectively (see Fig. 14). The negative control without TTHL laccase but 0.4 mM CuCl₂ at 10 °C and 25 °C show a negligible oxidation of ABTS. The low difference observed between the two samples is great news for the possible application in waste water treatment plants.

Immobilization

Figure 15: The percentage of laccases immobilized to CPC-Beads. 99 % of ECOL, 97 % of BPUL and 79 % of BHAL and TTHL laccases were bound to the beads.

Figure 16: Illustration of ABTS oxidation by TTHL with time compared to the negative control. The increase in ABTS oxidized proves laccase activity.