The single chlorophyll a molecule in the cytochrome b6 f complex:: Unusual optical properties protect the complex against singlet oxygen
The single chlorophyll a molecule in the cytochrome b6 f complex:: Unusual optical properties protect the complex against singlet oxygen
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DOI：
10.1529/biophysj.104.058693
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发表时间：
2005-06-01
影响因子：
3.4
通讯作者：
Savikhin, S
中科院分区：
文献类型：
--
作者：
Dashdorj, N;Zhang, HM;Savikhin, S
氧光合作用的细胞色素b(6)f复合物介导光系统I和II反应中心之间的电子传递，并促进耦合质子跨膜易位。细胞色素b(6)f复合物的高分辨率x射线晶体结构（Kurisu et al., 2003; Stroebel et al., 2003）明确表明，Chl - a分子是细胞色素b(6)f复合物的内在成分。虽然目前尚不清楚Chl a的功能作用（Kuhlbrandt, 2003），但已知受激Chl a分子由于能量从Chl a的激发态转移到氧分子而产生有毒的单线态氧。为了防止光收集配合物中单线态氧的形成，类胡萝卜素通常位于Chl a分子的4埃范围内，有效地猝灭Chl a的三重态激发态。然而，在细胞色素b(6)f配合物中，β -胡萝卜素距离Chl a太远（>= 14埃），无法有效猝灭Chl a的三重态激发态。在本研究中，我们提出在该复合体中，这种保护至少部分是通过局部蛋白质结构的特殊安排实现的，这种安排使Chl a的单线态激发态寿命缩短了20-25倍，从而显著减少了Chl a三重态的形成。基于光学超快吸收差分实验和基于结构的计算，提出了Chl - a单线态激发态寿命缩短的原因是与附近酪氨酸残基的电子交换转移。据我们所知，这种对单线态氧的保护机制在其他含叶绿素的蛋白质复合体中尚未见报道。也有报道称细胞色素b(6)f复合物中的Chl a分子在激发态下不改变取向。
The cytochrome b(6)f complex of oxygenic photosynthesis mediates electron transfer between the reaction centers of photosystems I and II and facilitates coupled proton translocation across the membrane. High-resolution x-ray crystallographic structures (Kurisu et al., 2003; Stroebel et al., 2003) of the cytochrome b(6)f complex unambiguously show that a Chl a molecule is an intrinsic component of the cytochrome b(6)f complex. Although the functional role of this Chl a is presently unclear (Kuhlbrandt, 2003), an excited Chl a molecule is known to produce toxic singlet oxygen as the result of energy transfer from the excited triplet state of the Chl a to oxygen molecules. To prevent singlet oxygen formation in light-harvesting complexes, a carotenoid is typically positioned within similar to 4 angstrom of the Chl a molecule, effectively quenching the triplet excited state of the Chl a. However, in the cytochrome b(6)f complex, the beta-carotene is too far (>= 14 angstrom) from the Chl a for effective quenching of the Chl a triplet excited state. In this study, we propose that in this complex, the protection is at least partly realized through special arrangement of the local protein structure, which shortens the singlet excited state lifetime of the Chl a by a factor of 20-25 and thus significantly reduces the formation of the Chl a triplet state. Based on optical ultrafast absorption difference experiments and structure-based calculations, it is proposed that the Chl a singlet excited state lifetime is shortened due to electron exchange transfer with the nearby tyrosine residue. To our knowledge, this kind of protection mechanism against singlet oxygen has not yet been reported for any other chlorophyll-containing protein complex. It is also reported that the Chl a molecule in the cytochrome b(6)f complex does not change orientation in its excited state.