The photosynthetic reaction centre (RC) is central to the conversion of solar energy into chemical energy and is a model for bio-mimetic engineering approaches to this end. We describe bio-engineering of a Photosystem II (PSII) RC inspired peptide model, building on our earlier studies. A non-photosynthetic haem containing bacterioferritin (BFR) from Escherichia coli that expresses as a homodimer was used as a protein scaffold, incorporating redox-active cofactors mimicking those of PSII. Desirable properties include: a di-nuclear metal binding site which provides ligands for bivalent metals, a hydrophobic pocket at the dimer interface which can bind a photosensitive porphyrin and presence of tyrosine residues proximal to the bound cofactors, which can be utilised as efficient electron-tunnelling intermediates. Light-induced electron transfer from proximal tyrosine residues to the photo-oxidised ZnCe6(•+), in the modified BFR reconstituted with both ZnCe6 and Mn(II), is presented. Three site-specific tyrosine variants (Y25F, Y58F and Y45F) were made to localise the redox-active tyrosine in the engineered system. The results indicate that: presence of bound Mn(II) is necessary to observe tyrosine oxidation in all BFR variants; Y45 the most important tyrosine as an immediate electron donor to the oxidised ZnCe6(•+) and that Y25 and Y58 are both redox-active in this system, but appear to function interchangebaly. High-resolution (2.1Å) crystal structures of the tyrosine variants show that there are no mutation-induced effects on the overall 3-D structure of the protein. Small effects are observed in the Y45F variant. Here, the BFR-RC represents a protein model for artificial photosynthesis.

Photosystem II (PSII) is the chlorophyll/protein complex in green plants that catalyzes the oxidation of water to molecular oxygen. We have utilized bacterioferritin (BFR), an iron storage protein found in Escherichia coli, as a protein scaffold to build in PSII cofactors in a simpler in vitro model system. Previously, we have shown that the native heme in BFR can be replaced with the chlorophyll analog zinc-chlorin (ZnCe6) and that the intrinsic di-iron site can bind two manganese ions. Upon flash excitation of the ZnCe6 modified BFR, not only is the dinuclear manganese complex oxidized but also a tyrosine residue. There are seven tyrosine residues in each BFR monomeric subunit. We mutated the three tyrosine residues within electron tunnelling distance of the ZnCe6. Here we present evidence based on electron paramagnetic resonance and fluorescence spectroscopy that one is not oxidized while the other two seem to be oxidized in parallel. Localization of this photoactive tyrosine is the first step in creating a linear electron flow in BFR like in PSII.

This paper presents an overview of the prospects for bio-solar energy conversion. The Global Artificial Photosynthesis meeting at Lord Howe Island (14–18 August 2011) underscored the dependence that the world has placed on non-renewable energy supplies, particularly for transport fuels, and highlighted the potential of solar energy. Biology has used solar energy for free energy gain to drive chemical reactions for billions of years. The principal conduits for energy conversion on earth are photosynthetic reaction centres – but can they be harnessed, copied and emulated? In this communication, we initially discuss algal-based biofuels before investigating bio-inspired solar energy conversion in artificial and engineered systems. We show that the basic design and engineering principles for assembling photocatalytic proteins can be used to assemble nanocatalysts for solar fuel production.

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