Mechanisms of Photodamage and Protein Degradation During Photoinhibition of Photosystem II
Mechanisms of Photodamage and Protein Degradation During Photoinhibition of Photosystem II
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DOI：
10.1007/0-306-48135-9_4
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发表时间：
1996
期刊：
影响因子：
--
通讯作者：
B. Andersson;J. Barber
中科院分区：
文献类型：
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作者：
B. Andersson;J. Barber
含氧光合作用光抑制期间受损的主要目标是光系统 II (PS II)。造成这种脆弱性的分子过程几乎肯定是由于 PS II 的独特性，因为它可以产生分解水所需的非常强的氧化剂。这一假设得到了使用各种分离的 PS II 复合物的研究的大力支持，这些复合物提供了适合详细光化学和生化测量的实验系统。这些研究已经确定了光诱导损伤的两种不同途径：指定为接受方和捐助方机制。受体侧机制涉及自由基对 P680+Phe− 的重组，其中 P680 是 PS II 的主要供体，Phe 是脱镁叶绿素，PS II 的主要受体。当质体醌受体 QA 双重还原时（例如在高光下）或当部分还原的质体醌受体 QB- 和水分解系统的 S2 或 S3 状态之间有利于逆反应时（例如在低光下），就会发生重组。重组导致 P680 三重态的产生，该三重态不会被类胡萝卜素猝灭，而是导致形成剧毒的单线态氧。结果，D1 蛋白被修饰以触发蛋白水解降解。该降解过程涉及连接靠近 QB 结合位点（残基 238 的 C 端侧）的跨膜片段 D 和 E 的环中的初始裂解。在某些情况下，D2 蛋白也会经历类似的降解，其初级切割的位置与 D1 蛋白所确定的位置类似。然而，供体侧机制并不依赖于氧的存在，而是由长寿命氧化态（例如 P680+）造成的损害造成的。这些状态的高氧化还原电位意味着可能发生色素（β-胡萝卜素和辅助叶绿素）和氨基酸的氧化。这种类型的广泛氧化使 D1 和 D2 蛋白不稳定，观察到的降解产物模式与受体侧机制产生的模式不同。对于 D1 蛋白，初次裂解发生在连接跨膜片段 A 和 B 的环中膜的供体侧。没有证据表明受体或供体侧诱导的 D1 和 D2 蛋白降解是由于直接光化学裂解所致。相反，有害的光化学过程似乎会引起 D1 和 D2 蛋白的构象变化，从而发出蛋白水解反应的信号。体内研究表明，对于 D1 蛋白，蛋白水解步骤通常与新合成蛋白的可用性同步。
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 QAis doubly reduced (e.g. in high light) or when back reactions are favored between the partially reduced plastoquinone acceptor QB−and the S2or S3states 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 QBbinding 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.