Abstract
Water oxidation and concomitant dioxygen formation by the manganese-calcium cluster of oxygenic photosynthesis has shaped the biosphere, atmosphere, and geosphere. It has been hypothesized that at an early stage of evolution, before photosynthetic water oxidation became prominent, light-driven formation of manganese oxides from dissolved Mn(2+) ions may have played a key role in bioenergetics and possibly facilitated early geological manganese deposits. Here we report the biochemical evidence for the ability of photosystems to form extended manganese oxide particles. The photochemical redox processes in spinach photosystem-II particles devoid of the manganese-calcium cluster are tracked by visible-light and X-ray spectroscopy. Oxidation of dissolved manganese ions results in high-valent Mn(III,IV)-oxide nanoparticles of the birnessite type bound to photosystem II, with 50-100 manganese ions per photosystem. Having shown that even today’s photosystem II can form birnessite-type oxide particles efficiently, we propose an evolutionary scenario, which involves manganese-oxide production by ancestral photosystems, later followed by down-sizing of protein-bound manganese-oxide nanoparticles to finally yield today’s catalyst of photosynthetic water oxidation.
Highlights
Native PSII-enriched thylakoid membrane particles were prepared from fresh market spinach following our established procedures[35]
Their typical O2-evolution activity was ~1200 μmol O2 mg−1 chlorophyll h−1, which proved the full integrity of the PSII proteins and the water-oxidizing Mn4CaO5 complex
We have shown earlier that this type of PSII preparation contains ~200 chlorophyll molecules per PSII reaction center[73,74]
Summary
Native PSII-enriched thylakoid membrane particles were prepared from fresh market spinach following our established procedures[35]. Their typical O2-evolution activity (as determined by polarography with a Clark-type electrode at 27 °C) was ~1200 μmol O2 mg−1 chlorophyll h−1, which proved the full integrity of the PSII proteins and the water-oxidizing Mn4CaO5 complex. PSII membranes were dissolved at 200 μg chlorophyll mL−1 in a high-salt buffer (30 mL) containing 20 mM TEMED (N,N,N’,N’-tetramethylethylenediamine) as a reductant for the PSII-bound Mn(III,IV) ions, 20 mM MES (2-(N-morpholino)ethane-sulfonic-acid) buffer (pH 6.5), and a high-salt concentration (500 mM MgCl2) and incubated in the dark on ice for 10 min. The Mn-depleted PSII showed zero O2-evolution activity as revealed by polarography
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