Advancing Biotechnologies for Fuel Generation: Exploiting Transmembrane Cytochromes for Solar Energy Conversion

Reports concerning dwindling reserves of fossil fuels and concerns over fuel security are frequent news headlines. The rising costs of fuel are a daily reminder of the challenges faced by a global society with ever increasing energy demands. In this context it is perhaps surprising that so many of the renewable energy supplies available to us, namely, sunlight, winds and waves, remain largely untapped resources. This is mainly due to the challenges that exist in converting these energy forms into fuels from which energy can be released 'on demand' when we wish to play computer games, drive a car and so on. However, during plant photosynthesis fuels are made naturally from the energy in sunlight. Light absorption by the green chlorophyll pigments generates an energised electron that is directed, along chains of metal centres, to catalysts that make sugars. These sugars fuel us, and all animals, when their energy is released following digestion of a meal. However, using farmed plants to produce biofuels is controversial as agriculture is also required to feed the world. As a consequence, and inspired by natural processes, we propose to build a system for artificial photosynthesis. In essence, we wish to place tiny solar-panels on microbes in order to harness sunlight to drive the production of hydrogen - a fuel from which the technologies to release energy on demand are well-advanced. We will use dyes and semi-conductor particles as mechanically and chemically robust materials to capture the energy in sunlight and generate energised electrons. We will couple these particles to biology's version of conducting wires. These wires are made from heme proteins that span membranes that provide Nature's solution to compartmentalising water-filled chambers (i.e., the inside of the bacterium). The heme-wires are produced naturally by 'rock-breathing' microorganisms and after these wires have transferred the energised electrons across the membrane they will drive enzyme catalysis to produce hydrogen Our novel bio-mimetic photocatalysts will establish new principles for the design of homogeneous photocatalysts with spatially segregated sites for fuel-evolution and the supply of electrons that is needed to sustain this process. We imagine that our photocatalysts will proove versatile and that with slight modification they will be able to harness solar energy for the manufacture of drugs and fine chemicals.

Most artificial homogeneous photosynthetic systems suffer a major drawback: a short-lived charge separated state, which is due to the failure to spatially decouple the reductive and the oxidative sites required to sustain charge separation. Inspired by membrane bilayers as Nature's solution to spatially decouple reduction and photoexcitation/oxidation, we propose to utilise synthetic biology to develop a novel approach that adopts the principles of natural photosynthesis; light harvesting, charge separation and catalysis. Our photocatalysts will exploit the outer-membrane spanning, cytochrome-based electron-transfer conduits produced naturally by Shewanella oneidensis MR-1. We will establish methods to attach photosensitisers, e.g., dye-sensitised TiO2 and CdS nanoparticles, to the external cytochrome of this conduit in a manner that allows for a rapid charge separation across the membrane creating a long-lived charge separated state. We will develop methods to deliver electrons from the internal face of the conduit to redox catalysts. As a proof-of-principle, the conduit will be coupled to hydrogen-evolving catalysts that will include a [NiFeSe]-hydrogenase, a synthetic cobaloxime catalyst that evolves hydrogen in pH neutral conditions, and colloidal platinum, well-known for its hydrogen evolving properties. Voltammetric and spectroscopic methods together with quantification of hydrogen evolution by gas-chromatography will define the solar conversion efficiencies, electron transfer rate and catalytic properties of these systems. Conditions will then be established to combine systems with the desired properties as hybrid photocatalysts in the bilayers of liposomes and also in S. oneidensis MR-1. Two methods will be employed to deliver the electrons required to sustain hydrogen evolution, sacrificial electron donors such as triethanolamine and electrodes. The latter is explored as it offers opportunities for simultaneous production of electricity.

Societal impact

The aim of this project is to use biotechnological, biophysical, (bio)nanotechnological and synthetic biological approaches to study and exploit Shewenella sp. and Shewanella proteins. In particular, we aim to exploit Shewenalla sp. and their respiratory proteins to harvest solar energy and produce carbon-neutral fuels such as hydrogen. A renewable energy cycle is recognized as a top national strategic priority in the UK (UK White Paper on Energy). In the last 18 months, several incidents have demonstrated the fragility of the global energy supply: the sharp rise in oil prices following the outbreak of conflicts and civil wars in the Middle-East and the ecological and humanitarian threat of a nuclear meltdown in Fukushima, Japan. The search for alternative energy sources is therefore of major importance to THE GLOBAL SOCIETY. A solution to this problem has to be sought by combining a multitude of 'alternative' energy sources; this research will contribute to this progress.

A new academic partnership & training of new leaders in the energy sector

This project will establish a new academic partnership between Butt, Clarke, Richardson at Univ. East Anglia, Jeuken at Univ. Leeds and Reisner at Univ. Cambridge. The strong ties through this BBSRC project will allow us to form a nucleus around which future networks and collaborations will be built. Within this project we will also provide top-quality cross-disciplinary training for three BBSRC PDRAs, to provide expertise in the development of alternative energy biotechnologies, an area of critical scientific, technological and economic importance for the future.

Contribution to technology of alternative energy sources

Of particular interest for the studies proposed here are the multi-heme proteins in Shewanella which mediate electron transfer to the outside of the cell or to inorganic substrates. Shewanella serve as an important model system for mediator-less microbial fuel cells that run on waste carbon sources (such as in waste water) to produce electricity or hydrogen. Research into the electron transport of Shewanella will increase our understanding of their capabilities in microbial fuel cells. At this stage, these are basic research aims, with academic beneficiaries. However, after successful completion of this project, we propose that our work will contribute to the future design of such microbial fuel cells, in particular where future work aims to genetically or synthetically modify the microbes to enhance electron transfer rates to the anode (i.e., increase electrical current). Furthermore, this BBSRC proposal explores a novel and innovative approach in which the natural electron transfer pathway is reversed. Instead of generating electricity by respiring hydrogen or a carbon source, we propose to use solar energy to produce hydrogen. Although the overall concept of this proposal is the harvesting of solar energy and the storage of energy (in the form of hydrogen), the fact that electricity can be used by microbes to make 'higher-energy' organic molecules, including hydrogen and a variety of hydrocarbons, is of major economic value. Microbes as catalysts are ideal as they are relatively cheap to make and maintain (i.e., they grow and regenerate). This makes them ideal catalyst to synthesise organic molecules, such as formate, using electricity and CO2.