Difference between revisions of "Team:Nanjing-China/Background"

This year, our team Nanjing-China is aimed at nitrogen fixation. Nitrogen fixation is the process that coverts free nitrogen into compound form.

•What is nitrogen fixation?

Nitrogen → ammonia (NH₃) or other molecules available to living organisms.

What is nitrogen fixation for?

-global food supply
-reduce the use of chemical nitrogen fertilizers 

Various ways are discovered to fix nitrogen in numerous strains. Because pathways that generate high-energy electrons are evolved to being in their metabolism system. What’s more, plants cannot fix nitrogen as they do to carbon for lack of an efficient enzyme system, nitrogenase. But some microorganisms do. Together, the energy in these electrons can be utilized by nitrogenase to convert free nitrogen into ammonia.

Nitrogenase is a huge system varied by the essential metallic co-enzyme it requires such as Mo-Fe, V-Fe and Fe nitrogenase. Wherein, Mo-Fe nitrogenase is better understood than other types. Therefore, in our project, we choose the best known to perform the needed reaction. Meanwhile, the gene clusters that code nitrogenase are different in distinct species. It is reported by previous studies that there is a small nitrogen fixation gene cluster consisting of nine relative genes from Paenibacillus. It is quite simple and has been proved to be functional after transferred into E. coli cells, besides its expression does not lead to obvious negative feedback regulation.

As illustrated in Fig.5 above, nitrogen is fixed by Mo-Fe nitrogenase. Most of the nitrogenase is Mo-dependent, which exists mainly in bacteria and archaea. Such nitrogenase is made up with two components, Mo-Fe protein and Fe protein. At room temperature and on one atmosphere, it costs at least 16mol ATP to reduce 1 mol dinitrogen to ammonia by the nitrogenase. The process marches as high-energy electrons passed by Fe protein to Mo-Fe protein. After the binding to such electrons, Mo-Fe protein is able to reduce free nitrogen. However, the cost of the reaction is not so economical. In 2016, our team established a system to produce hydrogen driven by light which is considerably cost-effective. Inspired by our previous project, this year, we choose to alter ATP with solar energy. There has always been an interest in harvesting the most important renewable energy source, the solar energy, which meanwhile is the hardest to capture. The significant breakthrough reported by Katherine et al. showed that certain semiconductor, cadmium sulfide (CdS) nanocrystals, function to photosensitize the Mo-Fe protein, replacing ATP hydrolysis by light harvesting to obtain electrons for the fulfill of the reduction of N2 into NH3. The results contributed a lot to the development of our system. Lead-specific binding protein is applied to biosynthesize such semiconductors.

Herein, we aim to establish a sound and ideal whole-cell photocatalytic nitrogen fixation system consisting the following elements: (i) a biocompatible and highly efficient light-harvesting inorganic semiconductor; (ii) active engineered E. coli cells as biocatalysts. The engineered E. coli cells that express nitrogenase as well as have the capability of in situ biosynthesis of CdS nanocrystals for the existence of the surface-displayed heavy lead-specific binding proteins is developed. Such system is able to reduce N2 to NH3 driven by light instead of ATP-hydrolysis with considerably high efficiency. The whole-cell system will be more biocompatible and cost-effective than any other ones.

References

1.             Wang, L., et al., A minimal nitrogen fixation gene cluster from Paenibacillus sp. WLY78 enables expression of active nitrogenase in Escherichia coli. PLoS Genet, 2013. 9(10): p. e1003865.
2.             Fixen, K.R., et al., Light-driven carbon dioxide reduction to methane by nitrogenase in a photosynthetic bacterium. Proc Natl Acad Sci U S A, 2016. 113(36): p. 10163-7.
3.             Brown, K.A., et al., Light-driven dinitrogen reduction catalyzed by a CdS:nitrogenase MoFe protein biohybrid. Science, 2016. 352(6284): p. 448-50.
4.             Kuypers, M.M.M., H.K. Marchant, and B. Kartal, The microbial nitrogen-cycling network. Nat Rev Microbiol, 2018. 16(5): p. 263-276.
5.             Wei, W., et al., A surface-display biohybrid approach to light-driven hydrogen production in air. Sci Adv, 2018. 4(2): p. eaap9253.
6.             Howard, J.B. and D.C. Rees, Structural basis of biological nitrogen fixation. Chemical Reviews, 1996. 96(7): p. 2965-2982.