Team:Nanjing-China/Design
Overview
To address the principles of our photosynthetic system and silicon encapsulation system, we made some demonstrations and flow charts to explain the background knowledge including CdS-mediated electron transduction, the artificial photosynthetic machinery and aerobic hydrogen production factory.
Induced precipitation of CdS nanoparticles on E.coli cell
In this year’s design our artificial PS system has no need for either chemically synthesized semiconductor nanoparticles or special organisms. The system is constructed on the outer cell membrane of E.coli using fused protein OmpA-PbrR.
Figure 1 The induced precipitation of CdS nanoparticles using fused protein OmpA-PbrR
To construct our light-driven system, we first induce the precipitation of CdS nanoparticles on the cell membrane. Two plasmids encoding the building block OmpA-PbrR and the enzyme HyaABCDEF are co-transferred into E.coli strain. Arabinose is used to induce the expression of OmpA-PbrR fused protein. Then when Cd2+ is added into the media, the ions specifically binds to PbrR leading to a local high concentration of Cd2+ ions. At last when S2- ions are added into the media, E.coli cells form in situ CdS nanoparticles on the cell membrane because of this local high concentration.
PbrR based artificial PS system
Figure 2 Light driven reaction of PbrR based artificial PS system
To address the problem of electron transduction, CdS nanoparticles act as semiconductors imitating the photosynthetic system under illumination. It provided excited electrons to a redox mediator methyl viologen (MV) which then penetrates into E.coli cells and transfer the electrons to hydrogenase Ec-Hyd1, produced from induced-expression under IPTG. Hydrogenase then produces one molecule of H2 out of 2 protons and 2 electrons. The semiconductor regains its lost electron from sacrificial electron donors, labeled ‘X (red/ox)’ in Figure 2.
As a biological system, the PbrR protein bears a high specificity. Our artificial PS system is supposed to self-repair and can be built with a rather low cost. This design is of general applications as OmpA protein is only a surface display machinery for E.coli. Research already shows that TasA and CotC in bacillus as well as GCW21 in yeast functions similarly as surface display proteins. Unlike photosynthetic system constructed in special species, our system can be reconstructed using other surface display machinery in the most common ones like bacillus and yeast. Likewise the downstream enzyme is not restricted to hydrogenase; the principle applies to other enzymes that produce different products.
Construction of in air hydrogen factory
Figure 3 Construction of in air hydrogen factory
To overcome the oxygen sensitivity of hydrogenase, we use silicon encapsulation. Single E.colicells with semiconductors are wrapped into multi layers of PDADMAC and PSS. Cells are then closed with silicification coats into microballs where the exterior consumes oxygen and creates an anaerobic core. Hydrogenase expression is achieved through IPTG induction. When light is provided to this system, hydrogen evolves from the core using our artificial PS system. Evolved hydrogen is then tested through gas chromatography (GC).
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