Mechanisms of Photodamage and Protein Degradation During Photoinhibition of Photosystem II

Summary

The primary target of damage during photoinhibition of oxygenic photosynthesis is Photosystem II (PS II). The molecular processes which underlie this vulnerability almost certainly arise from the fact that PS II is unique in that it can generate the very strong oxidants necessary to split water. This presumption has received considerable support from studies using various isolated PS II complexes that offer experimental systems which are amenable for detailed photochemical and biochemical measurements. Such studies have identified two distinct routes for photoinduced damage; designated as acceptor and donor side mechanisms. The acceptor side mechanism involves recombination of the radical pair P680⁺Phe⁻, where P680 is the primary donor of PS II and Phe is pheophytin, the primary acceptor of PS II. The recombination occurs either when the plastoquinone acceptor Q_A is doubly reduced (e.g. in high light) or when back reactions are favored between the partially reduced plastoquinone acceptor Q ⁻_B and the S₂ or S₃ states of the water splitting system (e.g. in low light). The recombination leads to the production of the P680 triplet state which is not quenched by carotenoids but instead leads to the formation of highly toxic singlet oxygen. As a consequence, the D1 protein is modified in such a way as to be triggered for proteolytic degradation. This degradation process involves an initial cleavage in the loop joining transmembrane segments D and E near to the Q_B binding site (C-terminal side of residue 238). Under some circumstances, the D2 protein also undergoes similar degradation with the primary cleavage being in an analogous position to that determined for D1 protein. The donor side mechanism, however, is not dependent on the presence of oxygen and results from damage due to long lived oxidation states (e.g. P680⁺). The high redox potential of these states means that oxidation of pigments (β-carotene and accessory chlorophyll) and amino acids, can occur. Extensive oxidation of this type destabilizes the D1 and D2 protein and the pattern of degradation products observed is different from that generated by the acceptor side mechanism. In the case of the D1 protein, the primary cleavage occurs on the donor side of the membrane in the loop joining transmembrane segments A and B. There is no evidence to suggest that either the acceptor or donor side induced degradation of the D1 and D2 proteins is due to direct photochemical cleavage. Rather, it seems that the detrimental photochemical processes give rise to conformational changes in the D1 and D2 proteins that signal proteolytic reactions. Studies in vivo indicate that in the case of the D1 protein the proteolytic step is normally synchronized with the availability of newly synthesized protein.