EXC 314/1: Elucidation of the photo-assembly mechanism of the native and modified water oxidation catalysts in photosystem II Biocatalytic coupling of photosystem I with FDH and CODH supercomplexes

Oxygen photosynthesis in green plants, algae and cyanobacteria is catalysed by two large membrane protein complexes Photosystem I (PSI) and Photosystem II (PSII). Both complexes contain a reaction centre (RC) that conducts a light-driven charge transfer across the thylakoid membrane, forming a radical pair P+A- of an oxidized primary donor P and a reduced acceptor A in picoseconds. The strong oxidant P+ is able to abstract electrons from water in the oxygen-evolving complex (OEC), a protein-bound Mn4CaO5 cluster. The OEC passes through five intermediate states (S-states) corresponding to the successive abstraction of four electrons from H2O. In this way, the four-electron reaction 2 H2O  O2 + 4e- + 4 H+ is coupled to the one-electron reaction in the RC. Despite the recent progress in the structure elucidation of the dimeric PSII core complex (dPSIIcc), the mechanism of the light-driven assembly of the Mn4CaO5 cluster has remained elusive. Very recently, we obtained a crystal structure of PSII fully depleted of the Mn4CaO5 cluster at 2.55 Å resolution (apo-PSII). This structure can serve as a basis for understanding the mechanism of OEC assembly. Within the framework of the UniSysCat cluster the fundamental understanding of the dynamic water oxidation reaction in PSII under physiological conditions is a crucial prerequisite for the design of artificial water-oxidizing catalysts. Hence, a systematic investigation of the photo-assembly of the Mn4CaO5 clusters into apo-dPSII single crystals is planned and the dynamic light-induced structure of the Mn4CaO5 cluster in PSII is to be decoded. This would provide import information for the synthesis of artificial water-splitting catalysts.

Learning from nature, light-to-charge carrier converting proteins from oxygenic photosynthesis of plants and cyanobacteria are of high interest for the construction of new functional devices. One of the most promising light-converting complexes is photosystem I (PSI) because of its high quantum efficiency (~100%), fast and stable charge separation and a proper spectral overlap with our sun. PSI from the thermophilic cyanobacterium Thermosynechococcus elongatus (T. elongatus) is a trimeric pigment-protein supercomplex consisting of 12 different protein subunits, harbouring 96 chlorophylls a (Chl a) and 22 carotenoids per monomer. Most Chls serve as light-harvesting antenna pigments and 6 Chls form an electron transport chain. PSI catalyses the light-driven transfer of an electron from reduced cytochrome c (Cyt cred) at the luminal side to oxidized ferredoxin (Fdox) at the stromal side. The charge separation in PSI occurs between the Chl a / Chl a´ heterodimer, named P700 and the primary acceptor named A0. The electron is subsequently transferred to a bound phyllochinone and then serially through three [4Fe-4S] clusters, FX, FA and FB, to a soluble [2Fe-2S] Fd. As part of the UniSysCat cluster the main objective of this project, in close collaboration with the other working groups is the production of basic chemicals with light as an energy source. To achieve these long-term goals an artificial photosynthesis system will be established through coupling photochemical, PSI, and catalytic modules, e.g. formate dehydrogenase (FDH) or carbon monoxide dehydrogenase (CODH), for the construction of a light-driven formate or CO evolving device. To transfer electrons efficiently and with high quantum yield, we will use a molecular wire that covalently attaches the FB cluster of PSI to an iron-sulfur cluster of FDH or CODH.