Investigation of Water Oxidizing Catalysis for Renewable Energy

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The burning of fossil fuels releases CO2 which is almost certainly responsible for anthropogenic climate change. Therefore, we must find alternative 'carbon-neutral' sources of energy as a matter of urgency. By far the largest potential source of renewable energy is sunlight. Harnessing this energy is one of the great challenges that our civilization faces, but using it is problematic. Existing silicon solar cells are expensive and inefficient, and do not produce fuel. Plants have hit on the perfect solution; they use the energy of sunlight to oxidise water (H2O), liberating O2, protons and electrons. The electrons and protons are used to fix carbon dioxide as organic sugars, which may then be used for biosynthesis or as fuel for respiration. The total process is known as photosynthesis. Plants can be grown to generate so-called 'biofuels' such as biodiesel, but this is inefficient, and competes with food production. What is needed is an artificial photosynthetic system, that, like plants, converts sunlight, water and CO2 into fuel, but is cheap, efficient and can be deployed over large areas. The proposed project is to create a vital component of a future solar energy conversion system. There are many components to the photosynthetic apparatus, but the main one of interest to this project is an enzyme called photosystem II (PSII). PSII is responsible for the light-driven water splitting reaction of photosynthesis. At its core, PSII has a cluster of one calcium and four manganese ions, which catalyse the water splitting reaction. This cluster is known as the oxygen evolving centre (OEC). The precise structure of the OEC and the mechanism of its action are still unknown, but both of these must be understood if a synthetic light-driven water oxidase is to be constructed. Building such a system is a vital prerequisite for the efficient large scale use of solar energy. The OEC in PSII is difficult to study, as PSII is a large complex containing many protein molecules and cofactors as well as the OEC. Therefore I propose to use small proteins as scaffolds for manganese ions, and so construct an OEC analogue that is uncoupled from the PSII enzyme and can be studied much more easily, and is a realistic prototype for future devices. Apart from PSII, there are many known enzymes which contain two manganese ions at their active sites, but PSII is unique in having four manganese ions at one site. I would like to take one of these simpler manganese enzymes and engineer it to bind more manganese ions, to mimic PSII. This can be accomplished by recombinant DNA technology. A DNA molecule with a sequence encoding the designed enzyme is constructed and then introduced into a harmless bacterium. The bacterium is then induced to produce the modified enzyme, which is then extracted and purified for further study. This technique has the advantage that DNA molecules are easy to manipulate, and specific sequences can be produced quickly and cheaply, allowing many designs of enzyme to be tried in a short time. Having produced a modified protein molecule that binds multiple manganese ions, the three dimensional structure can be determined. The protein will be probed for enzyme activity similar to that of PSII. I will try to catalyse the oxidation of water or other substrates using powerful oxidants as a substitute for light. In plants, chlorophyll is used as the main photosensitive pigment, but chlorophyll is usually unstable in artificial systems. Instead I will couple stable synthetic pigments to the protein and try to generate oxidative reactions using light. The results of these experiments can be related to the three dimensional structure of the enzyme and then used to to inform modifications in the design of the engineered proteins, which will then be subjected to further rounds of experimentation and design. This 'evolution by artificial selection', can be iterated until the desired goal of a soluble water oxidase is realised.