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

Tracewell, Cara A. and Brudvig, Gary W. (2008) Multiple Redox-Active Chlorophylls in the Secondary Electron-Transfer Pathways of Oxygen-Evolving Photosystem II. Biochemistry, 47 (44). pp. 11559-11572. ISSN 0006-2960. PMCID PMC2674297. doi:10.1021/bi801461d.

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[img] PDF (The temperature dependence of the formation of Chl^+ and Car^+ measured by near IR spectroscopy and the pH dependence (at 20 K) of the formation of Chl^+ and Car^+ are described. The EPR spectra of the near-IR radical species formed as a function of ...) - Supplemental Material
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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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Additional Information:© 2008 American Chemical Society. Received August 3, 2008; Revised Manuscript Received September 14, 2008. Publication Date (Web): October 14, 2008. 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. We thank Russ Bernardo for the construction of clamps for the cryostat in the near-IR spectrometer and the Teflon plunger for the cuvettes, Natasha Keith for reconstructing and greatly enhancing the nitrogen-purge plexiglas box for transferring samples into the cryostat, James McEvoy for careful reading and comments on the manuscript, and Phil Romero for assistance in measuring the edge-to-edge distances of cofactors in the crystal structure of PS II.
Funding AgencyGrant Number
Department of Energy (DOE)DE-FG02-01ER15281
NIH Predoctoral FellowshipT32 GM008283
Issue or Number:44
PubMed Central ID:PMC2674297
Record Number:CaltechAUTHORS:20161014-160236704
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Official Citation:Multiple Redox-Active Chlorophylls in the Secondary Electron-Transfer Pathways of Oxygen-Evolving Photosystem II Cara A. Tracewell and Gary W. Brudvig Biochemistry 2008 47 (44), 11559-11572 DOI: 10.1021/bi801461d
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:71124
Deposited By: George Porter
Deposited On:17 Oct 2016 19:37
Last Modified:11 Nov 2021 04:39

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