Multiple Redox-Active Chlorophylls in the Secondary Electron-Transfer Pathways of Oxygen-Evolving Photosystem II

Abstract

Photosystem II (PS II) is unique among photosynthetic reaction centers in having secondary electron donors that compete with the primary electron donors for reduction of P_(680^+). We have characterized the photooxidation and dark decay of the redox-active accessory chlorophylls (Chl) and β-carotenes (Car) in oxygen-evolving PS II core complexes by near-IR absorbance and EPR spectroscopies at cryogenic temperatures. In contrast to previous results for Mn-depleted PS II, multiple near-IR absorption bands are resolved in the light-minus-dark difference spectra of oxygen-evolving PS II core complexes including two fast-decaying bands at 793 and 814 nm and three slow-decaying bands at 810, 825, and 840 nm. We assign these bands to chlorophyll cation radicals (Chl^+). The fast-decaying bands observed after illumination at 20 K could be generated again by reilluminating the sample. Quantization by EPR gives a yield of 0.85 radicals per PS II, and the yield of oxidized cytochrome b_(559) by optical difference spectroscopy is 0.15 per PS II. Potential locations of Chl^+ and Car^+ species, and the pathways of secondary electron transfer based on the rates of their formation and decay, are discussed. This is the first evidence that Chls in the light-harvesting proteins CP43 and CP47 are oxidized by P_(680^+) and may have a role in Chl fluorescence quenching. We also suggest that a possible role for negatively charged lipids (phosphatidyldiacylglycerol and sulfoquinovosyldiacylglycerol identified in the PS II structure) could be to decrease the redox potential of specific Chl and Car cofactors. These results provide new insight into the alternate electron-donation pathways to P_(680^+).

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