Factors Controlling the Redox Potential of ZnCe₆ in an Engineered Bacterioferritin Photochemical 'Reaction Centre'

Photosystem II (PSII) of photosynthesis has the unique ability to photochemically oxidize water. Recently an engineered bacterioferritin photochemical 'reaction centre' (BFR-RC) using a zinc chlorin pigment (ZnCe₆) in place of its native heme has been shown to photo-oxidize bound manganese ions through a tyrosine residue, thus mimicking two of the key reactions on the electron donor side of PSII. To understand the mechanism of tyrosine oxidation in BFR-RCs, and explore the possibility of water oxidation in such a system we have built an atomic-level model of the BFR-RC using ONIOM methodology. We studied the influence of axial ligands and carboxyl groups on the oxidation potential of ZnCe₆ using DFT theory, and finally calculated the shift of the redox potential of ZnCe₆ in the BFR-RC protein using the multi-conformational molecular mechanics-Poisson-Boltzmann approach. According to our calculations, the redox potential for the first oxidation of ZnCe₆ in the BRF-RC protein is only 0.57 V, too low to oxidize tyrosine. We suggest that the observed tyrosine oxidation in BRF-RC could be driven by the ZnCe₆ di-cation. In order to increase the efficiency of tyrosine oxidation, and ultimately oxidize water, the first potential of ZnCe₆ would have to attain a value in excess of 0.8 V. We discuss the possibilities for modifying the BFR-RC to achieve this goal.