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 nm and 814 nm and three slow-decaying bands at 810 nm, 825 nm, 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 sulphoquinovosyldiacylglycerol 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 + .Keywords β-carotene; carotenoid cation radical; chlorophyll cation radical; photosystem II; secondary electrontransfer pathwayIn photosystem II, light energy is transferred from the antenna Chls in the Chl-binding proteins CP43 and CP47 into the reaction center where the primary photochemical reactions are initiated. The primary electron donor Chl is most likely chlorophyll B A (see Figure 1) (1,2). The excited state of this chlorophyll drives charge separation, producing a reduced pheophytin (Pheo A − ) and the chlorophyll cation radical species, P 680 + . The charge separation, P 680 + Pheo − , is stabilized by transfer of the electron to the protein-bound primary quinone, Q A . Under normal conditions P 680 + is reduced by an electron from the oxygen-evolving complex (OEC) † This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences (DE-FG02-01ER15281), and an NIH predoctoral traineeship grant T32 GM008283 (C.A.T.). NSF Grant CHE-0215926 provided funds to purchase the ELEXSYS E500 EPR spectrometer.*To whom correspondence should be addressed. Phone: (203) NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript which consists of a redox-active tyrosine, Y Z , and a tetranuclear Mn cluster (Mn 4 ) leading to the production of molecular oxygen from...

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