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 (CP47-RC) 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 degreesC, following illumination at -80 degreesC. This band has been determined to be an indicator of the photoaccumulation of QA-. 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 QA-, 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 QA- 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 QA.
Highlights
At the level of the CP47-reaction center (RC) subcore complex leads to destabilization of the bound plastoquinone, which functions as QA
The results clearly show that the quinone within the dimeric form of the complex can act as a secondary electron acceptor indicative of QA activity and because of this exhibits fluorescence characteristics distinctly different from those of the monomeric form of the complex
The real magnitude of Fo was confirmed using K-15, a compound that selectively quenches the part of the photosystem II (PSII) chlorophyll emission that is due to recombinant luminescence [14]
Summary
At the level of the CP47-RC subcore complex leads to destabilization of the bound plastoquinone, which functions as QA. CP43 and CP47 are distinguished by having a large hydrophilic loop linking putative membrane-spanning regions 5 and 6 [3] These loops are almost certainly located on the luminal surface of the complex and may function in water splitting in some way [5]. A method was described for spinach that yielded a preparation of the CP47-RC complex consisting of a mixture of monomeric and dimeric forms [8]. The findings suggest that the monomeric form of the isolated CP47-RC complex was derived by dissociation of the dimer and that the latter conformation is likely to be the in vivo state. We characterize the functional properties of the CP47-RC complex isolated from spinach and, in particular, identify differences between the monomeric and dimeric forms.
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