Biohybrids for Solar Chemicals and Fuels: Whole-cell Photocatalysis by Non-photosynthetic Organisms

Lead Research Organisation: University of Cambridge
Department Name: Chemistry

Abstract

From news headlines to social media streams, we are constantly reminded of the need to help secure a healthy planet for future generations. The abundance of solar panels in urban and rural landscapes immediately illustrates how, nationally and as individuals, we are embracing technology to do this. Solar panels absorb sunlight and convert its energy to electricity. This helps to mitigate against climate change by lowering our use of fossil carbon reserves and harnessing sunlight is very sensible; in just two hours the sun provides Earth with sufficient energy to meet its present annual energy demand. However, solar panels are not without problems since they are constructed with toxic materials and suffer from intermittent output, which may not match patterns of energy demand. Developing more sustainable routes to harness the energy of sunlight is a global challenge. To help meet this challenge, and inspired by photosynthesis in green plants, we aim to combine the best of natural and synthetic approaches to solar energy conversion for the sustainable production of chemicals including fuels.

Photosynthesis is Nature's way of harnessing solar energy. Using abundant and non-toxic elements assembled as environmentally benign proteins, green plants absorb sunlight to drive the synthesis of fats and sugars. Amazingly, the only side-product of this reaction is the oxygen we need to breath. Fats and sugars provide fuels that we, like plants, can use for energy whether it is day or night, sunny or cloudy. Natural photosynthesis is a self-sustaining, renewable model for solar energy conversion but it is not without bottlenecks. The light-harvesting systems absorb only a small fraction of the solar spectrum and are easily damaged - during the day they are usually replaced every 30 minutes. By contrast, synthetic light-harvesting materials like those in solar panels, are more robust than natural photosystems. In addition 'rainbow absorbers' can harness much more of the solar spectrum.

Our research will combine robust, synthetic light-harvesting materials with non-photosynthetic bacteria in a powerful, sustainable solution to delivering complex chemical transformations. We aim to develop systems producing fuels, for example, ethanol (from a major underutilised by-product of biodiesel production), hydrogen (from water) and formate (from carbon dioxide a greenhouse gas). Importantly our biohybrids will simultaneously produce two different fuels but no side products so they are true mimics of natural photosynthesis in their chemical efficiency.

How will we do this? We will use detailed knowledge of the structure of a protein conducting electrons from the inside to the outside of Shewanella bacteria. This knowledge allows us to engineer the external surface of the Shewanella bacteria for selective labelling with light-harvesting electrocatalysts. Then, powered by the energy of sunlight, electrons will move between enzyme catalysts inside the bacteria and the synthetic catalyst outside the bacteria in order to couple the production of one fuel inside the bacterium and a different fuel outside the bacterium. Our approach allows us to use enzymes to deliver complex transformations without expensive purification that can result in fragile systems. By using bacteria there is also the possibility that the performance of our systems will benefit from natural processes of enzyme self-repair and regeneration.

Technical Summary

We propose to tap into sunlight, an underutilised source of clean power, and address the direct exchange of electrons between bacterial cells and inorganic photocatalysts for the biophotocatalytic production of solar chemicals including fuels. Current state-of-the-art technology, referred to as "bionic leaf", relies on the transfer of energy via intermediates such as hydrogen. By contrast, recent results from the applicants indicate that cytochromes purified from the extracellular respiratory machinery of Shewanella oneidensis MR-1 (MR-1) enable direct exchange of solar energy with synthetic photosensitisers. The MR-1 extracellular respiratory machinery exchanges energy/electrons across the bacterial outer membrane. To this end, we propose a novel synthetic biology approach in which bespoke photocatalysts are directly coupled to the extracellular cytochromes of MR-1 in vivo. Modular biohybrid assemblies will be produced that use intracellular redox transformations to sustain light-driven extracellular catalysis thereby closing the redox loop and enabling self-sustaining production of solar fuels. Two proof-of-principle light-driven reactions are proposed: (a) MR-1 catalysed reductions, such as protons to hydrogen or carbon dioxide to formate, coupled to inorganic photo-oxidation of industrially-relevant alcohols to aldehydes, and (b) MR-1 catalysed oxidation of glycerol (a major underutilised by-product of biodiesel production) to ethanol coupled to the inorganic photo-reduction of carbon dioxide to formate.

Planned Impact

Societal Impact:

Renewable energy is recognised as a top national strategic priority (UK White Paper on Energy). Several incidents have demonstrated the fragility of the global energy supply such as 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 GLOBAL importance. The Paris Agreement in 2015 has set out a global strategy to minimise the impact of climate change by reducing greenhouse gas emissions. A solution to this problem has to be sought by combining a multitude of complementary 'alternative' energy sources; this research will contribute to this progress. Specifically, we aim to use synthetic biology, protein engineering, (bio)nanotechnology and chemistry to develop bacterial biohybrids that harvest solar energy for novel and innovative approaches to produce value added chemicals and fuels. For example, with our industrial partner, we will explore MR-1 oxidation of glycerol to ethanol and CO2 coupled to external light-driven inorganic reduction of CO2 to formate. This carbon-neutral reaction photocatalysis would add considerable value to glycerol, a low-commodity chemical, through the production of two fuels. In the US, approximately 1 kg of crude glycerol is produced for every 10 kg of biodiesel which equates to the production of approximately 0.8 x 10^9 kg of glycerol in 2016 .

Technological Impact:

The state-of-the-art in solar-driven microbial catalytic systems, 'bionic leaf' technology, first produces hydrogen and, in a second step, uses this hydrogen as an energy vector to drive downstream bioproduction of higher value compounds, e.g. reduction of CO2 to fusel alcohols by Ralstonia. This proposal presents fundamental research that aims to advance the state-of-the-art by directly coupling the photocatalyst and microbial catalytic systems. We envisage that successful completion of this project will demonstrate proof-of-principle for a disruptive technology contributing to the future design of hybrid bacterial-inorganic (photo-)catalytic systems for chemical conversions including those requiring NAD(P)+/NADPH recycling.

Conjugation between inorganic materials and biomacromolecules has wide-ranging relevance to technology, including bioenergy (as proposed here), health technology, e.g. drug delivery and the development of novel probes for cellular localisation and trafficking. We envisage that our research will also be of immediate impact in the development of emerging technologies for electrically interfacing living systems and abiotic materials. In this area, our research could impact the development of artificial vision and light-dependent sensing/signalling pathways.

Impact through Collaborations and Training:

This project will consolidate the recently formed partnership between Butt, Clarke (UEA), Jeuken (Leeds) and Reisner (Cambridge). The project will extent the partnership to Gralnick (USA), who will bring expertise in engineering Shewanella for targeted biotransformations, and the Molecular Foundry, who will bring expertise for the analysis of protein-nanoparticle conjugates. Within this project we will also provide top-quality cross-disciplinary training for three BBSRC PDRAs and a research technician, to provide expertise in the development of alternative energy biotechnologies, an area of critical scientific, technological and economic importance for the future.

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