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Exeter | ERASE

Observing Degradation Products

Summary

In this experiment we demonstrate that our NemA and XenB constructs are both capable of degrading the aromatic ring of TNT forming distinctive degradation products that result in colour change of the sample.

Aim

The aim of our experiment was to qualify the breakdown of TNT by XenB and NemA by observing the characteristic change in sample colour. Specific colour changes observed would be a good indication of TNT aromatic ring breakage. Top 10 will be used as a standard as it does not have amplified NemA or XenB TNT degradation pathways.

TNT Degradation

During the NemA and XenB-catalysed degradation of TNT, a series of nitrite groups as well aromatic ring reduction leads to formation of amino-dimethyl-tetranitrobiphenyl. During this process a hydride-Meisenheimer complex metabolite is formed. This degradation product has a distinct dark-brown colour [Vorbeck et.al 1994]. This degradation product causes reaction mixtures with XenB or NemA, mixed with TNT, to change from colourless to red, then to yellow [Pak 2000]. The resulting yellow colour results from four other degradation product which accumulate following aromatic ring reduction.

Results

In order to validate that our biobrick constructs were working as expected we ran a series of experiments to measure the degradation of TNT in vivo using our NemA and XenB constructs. In order to see the degradation rates over set periods of time one main reaction was left to run for 5 hours while samples were removed and either flash freezed using liquid nitrogen or placed in trichloroacetic acid to stop the enzymatic reaction and calculate the volume of TNT present at that time. We repeated this experiment (degradation experiment 2) in order to validate that the change was repeatable and to also narrow the time in which the degradation products were formed. The concentration of the mixture formed was too low to be analysed by HPLC . When centrifuged the resulting supernatant did not have a colour change. Raman could not be used as the TNT and water mixture is immiscible and forms a layer on top of fluid that is extremely variable in inelastic scattering feedback.

Experiment 1

Experiment 2

**Degradation experiment 2 NemA:**40 minutes after addition of TNT to 0.4ml of overnight culture, NemA had formed a slight brown colour

**Degradation experiment 2 NemA:**80 minutes after addition of TNT to 0.4ml of overnight culture, NemA had formed a dark-red brown colour.

**Degradation experiment 2 XenB:**80 minutes after addition of TNT to 0.4ml of overnight culture, XenB had formed a slight brown colour

**Degradation experiment 2 Top10:**80 minutes after addition of TNT to 0.4ml of overnight culture, Top 10 had not changed colour

Methods

In order to test our constructs in vivo we first had to create cultures with an equal optical density in order that the results produced would be representative of the capability and rate of each enzyme and not the volume of cells. To do this we grew overnight cultures of constructs made with 10ml of lysogeny broth (LB) and 0.01ml of chloramphenicol (CAM) antibiotic and construct glycerol stocks of XenB (001) and NemA (003). The control, Top 10, did not contain any antibiotic. As a final control only LB was placed in the fourth falcon tube. These were then stored in the shaking incubator at 37 degrees at 300 rpm overnight.

Optical density for each of our overnight samples was measure using cuvettes and a light spectrometer.

The following day we removed the overnight cultures from the incubator and measured the optical density. The light spectrometer could only accurately measure with readings less than one, therefore we diluted the 1ml sample removed by adding 0.1ml of overnight culture to 0.9ml of LB. Using the standard curve created through testing of TNT degradation using HPLC we knew that the ideal concentration for TNT detection was between 0.014-0.44mm.

We removed 0.4ml of culture from the overnight cultures and placed them into labelled 2ml Eppendorfs. We then added 0.02ml of aqueous TNT into each Eppendorf and inverted in order to mix. So as to accurately sample the degradation product formation at select time intervals, 0.02ml was removed from each sample and placed in a new Eppendorf tube. These new labelled Eppendorfs were then put into liquid nitrogen in order to snap freeze the samples at that reaction state. Once frozen the samples were placed into a minus 80 degrees C freezer in order to maintain de-activation of the enzymes and prevent natural degradation of TNT. This removal and freezing process was repeated every 20 minutes for 4 hours.

References

Vorbeck, Claudia; Lenke, Hiltrud; Fischer, Peter; Hans-Joachim, Knackmuss (1994) Identification of a Hydride-MeisenheimerComplex as a Metabolite of 2,4,6-Trinitrotoluene by a Mycobacterium Strain ; Journal of Bacteriology
Jeong W. Pak; Kyle L. Knoke; Daniel R. Noguera; Brian G. Fox; Glenn H. Chambliss (2000) Transformation of 2,4,6-Trinitrotoluene by Purified Xenobiotic Reductase B from Pseudomonas fluorescens I-C; Applied and Environmental Microbiology

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