Telfer Alison
Wolfson Laboratories, Department of Biological Sciences, Imperial College of Science, Technology and Medicine, London SW7 2AY, UK.
Philos Trans R Soc Lond B Biol Sci. 2002 Oct 29;357(1426):1431-39; discussion 1439-40, 1469-70. doi: 10.1098/rstb.2002.1139.
During photosynthesis carotenoids normally serve as antenna pigments, transferring singlet excitation energy to chlorophyll, and preventing singlet oxygen production from chlorophyll triplet states, by rapid spin exchange and decay of the carotenoid triplet to the ground state. The presence of two beta-carotene molecules in the photosystem II reaction centre (RC) now seems well established, but they do not quench the triplet state of the primary electron-donor chlorophylls, which are known as P(680). The beta-carotenes cannot be close enough to P(680) for triplet quenching because that would also allow extremely fast electron transfer from beta-carotene to P(+)(680), preventing the oxidation of water. Their transfer of excitation energy to chlorophyll, though not very efficient, indicates close proximity to the chlorophylls ligated by histidine 118 towards the periphery of the two main RC polypeptides. The primary function of the beta-carotenes is probably the quenching of singlet oxygen produced after charge recombination to the triplet state of P(680). Only when electron donation from water is disturbed does beta-carotene become oxidized. One beta-carotene can mediate cyclic electron transfer via cytochrome b559. The other is probably destroyed upon oxidation, which might trigger a breakdown of the polypeptide that binds the cofactors that carry out charge separation.
在光合作用过程中，类胡萝卜素通常作为天线色素，将单线态激发能传递给叶绿素，并通过类胡萝卜素三线态的快速自旋交换和衰变至基态，防止叶绿素三线态产生单线态氧。现在看来，光系统II反应中心（RC）中存在两个β-胡萝卜素分子已得到充分证实，但它们不会淬灭初级电子供体叶绿素（称为P(680)）的三线态。β-胡萝卜素无法与P(680)足够接近以进行三线态淬灭，因为那样也会允许从β-胡萝卜素到P(+)(680)的极快电子转移，从而阻止水的氧化。它们将激发能传递给叶绿素，尽管效率不高，但表明其与由组氨酸118连接至两个主要RC多肽外围的叶绿素非常接近。β-胡萝卜素的主要功能可能是淬灭电荷复合后产生的单线态氧至P(680)的三线态。只有当水的电子供体受到干扰时，β-胡萝卜素才会被氧化。一个β-胡萝卜素可以通过细胞色素b559介导循环电子传递。另一个可能在氧化时被破坏，这可能会引发结合进行电荷分离的辅因子的多肽的分解。
