Datensatz

Zusammenfassung

Natural water oxidation, carried out by an inorganic Mn₄CaO₅ cluster embedded in the enzyme photosystem II of photosynthetic organisms,
underpins all oxygenic life on earth [1]. Among the many poorly
understood aspects of this process, which serves as the ultimate blueprint
for synthetic efforts towards development of synthetic water splitting
catalysts, is the role of calcium: why does the catalyst depend critically
on calcium for its function, and why is natural water oxidation inhibited
by very similar cations, even though they may be structurally incorporated in the catalytic cluster? We address these questions by combining
recent results from spectroscopy (EPR/ENDOR), information from
kinetics measurements, and extensive theoretical modelling of photosystem II and its oxygen evolving complex [1–4]. Our results suggest that the calcium ion satisfies not one but several diverse requirements, which are electronic as much as structural in nature. Most importantly,
calcium simultaneously modulates the properties of not only the Mn₄CaO₅ cluster itself, but also of the redox-active tyrosine residue that mediates electron transfer from the water oxidation site to the photodriven charge separation site of the enzyme.