Comparison of the functional properties of the monomeric and dimeric forms of the isolated CP47-reaction center complex

M. Bianchetti, D. Zheleva, Z. Deak, S. Zharmuhamedov, V. Klimov, J. Nugent, I. Vass, J. Barber

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Abstract

Chlorophyll fluorescence, thermoluminescence, and EPR spectroscopy have been used to investigate the functional properties of the monomeric and dimeric forms of the photosystem II CP47-reaction center (CP47RC) subcore complex that was isolated (Zheleva, D., Sharma, J., Panico, M., Morris, H. R., and Barber, J. (1998) J. Biol. Chem 273, 16122-16127). Chlorophyll fluorescence yield changes induced either by the initiation of continuous actinic light or by repetitive light flashes indicated that the dimeric, but not the monomeric, form of the CP47-RC complex showed secondary electron transport properties indicative of QA reduction. Thermoluminescence measurements also clearly distinguished the monomer from the dimer in that the latter showed a Zv band, which appeared at -55 °C, following illumination at -80 °C. This band has been determined to be an indicator of the photoaccumulation of Q(A)·-. The ability of the dimeric CP47-RC to show secondary electron transport properties was clearly demonstrated by EPR studies. The dimer was characterized by organic radical signals at about g = 2 induced either by illumination or by the addition of dithionite. The dithionite-induced signal was attributed to Q(A)·-, but there was no indication of any interaction with non-heme iron. The signal induced by light was more complex, being composed not only of the Q(A)·- radical but also of radicals generated on the donor side. Difference analyses indicated that one of these radicals is likely to be due to a D1 tyrosine 161 or D2 tyrosine 161. In contrast, the monomeric CP47-RC complex did not show similar EPR- detectable radicals and instead was dominated by a high yield of the spin- polarized triplet signal generated by recombination reactions between the oxidized primary reductant, pheophytin, and the primary donor, P680. It is also concluded from EPR analyses that both the monomeric and dimeric forms of the CP47-RC subcore complex contain one cytochrome b559 per reaction center. Overall the results suggest that photosystem II normally functions as a dimer complex and that monomerization at the level of the CP47-RC subcore complex leads to destabilization of the bound plastoquinone, which functions as Q(A).

Original languageEnglish
Pages (from-to)16128-16133
Number of pages6
JournalJournal of Biological Chemistry
Volume273
Issue number26
DOIs
Publication statusPublished - Jun 26 1998

ASJC Scopus subject areas

  • Biochemistry
  • Molecular Biology
  • Cell Biology

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