Dual role of triplet localization on the accessory chlorophyll in the photosystem II reaction center: Photoprotection and photodamage of the D1 protein

Infrared absorption and electron spin resonance studies have shown that the excited triplet state of chlorophyll formed by radical pair recombination in the PSII reaction center is mainly localized on the accessory chlorophyll, which is most probably located in the D1 protein (Chl(1)). This triplet localization plays two contrasting roles, depending on the redox state of Q(A), in the process of acceptor-side photoinhibition of PSII. In the early stage of photoinhibition, in which singly reduced Q(A) is reversibly stabilized, the triplet state of Chl(1) ((3)Chl(1)(*)) is rapidly quenched (t(1/2) = 2-20 mus) by the interaction with Q(A)(-), preventing formation of harmful singlet oxygen. In the next inhibitory stage, in which Q(A) is doubly reduced and then irreversibly released from the Q(A) pocket, the lifetime of (3)Chl(1)(*) becomes longer by more than two orders of magnitude (t(1/2) = 1-3 ms). As a result, singlet oxygen is produced around Chl(1), in the D1 protein, causing damage preferably to the D1 protein, which induces subsequent proteolytic degradation. Thus, (3)Chl(1)(*) functions as a switch to change from the protective to the degradative phase of the PSII reaction center by sensing either reversible or irreversible inhibited state at the QA site.