Predominant Protection of D2-Protein against Photodestruction in Isolated D1/D2/Cytochrome b₅₅₉ by K15, a Phenolic-Type Inhibitor of Electron Transfer in Photosystem 2

O. V. Pobeguts^*, T. N. Smolova, S. K. Zharmukhamedov, and V. V. Klimov

Institute of Basic Biological Problems, Russian Academy of Sciences, Pushchino 142290, Moscow Region, Russia; fax: (27) 39-0532; E-mail: lwomain@issp.serpukhov.su

^* To whom correspondence should be addressed.

Received June 23, 2003; Revision received January 25, 2004

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A protective action of K15 (4-[methoxy-bis(trifluoromethyl)methyl]-2,6-dinitrophenylhydrazone methyl ketone), an inhibitor of electron transport in photosystem 2 (PS 2), against photoinactivation of the PS 2 reaction center (RC) D1/D2/cytochrome b₅₅₉ complex, isolated from pea chloroplasts, by red light (0.7 mmol photons/sec per m²) has been investigated under aerobic conditions. The inhibitor K15 causing cyclic electron transfer around PS 2 and thus prohibiting stabilization of separated charges has been shown to effectively protect RC both against the loss of photochemical activity (measured as reversible photoinduced absorbance changes related to photoreduction of pheophytin) and aggregation and degradation of the proteins D2 and D1 during photoinactivation. Comparison of the protective action of K15 and of another inhibitor of electron transfer in PS 2, diuron, against light-induced destruction of proteins D1 and D2 shows that diuron stabilizes protein D1 and K15 stabilizes protein D2. The preferential protection of D2 against photoinduced destruction revealed in our work is in accord with the concept of a specific binding of K15 with this protein. It is proposed that this binding site may be that of the primary quinone electron acceptor Q_A located on the D2 protein (in contrast to diuron, which is known to replace the secondary electron acceptor Q_B from its binding site on D1).

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KEY WORDS: photosystem 2, pigment-protein complex, reaction center, photoinactivation, photosystem 2 electron transport inhibitor

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