Ruban Alexander V, Berera Rudi, Ilioaia Cristian, van Stokkum Ivo H M, Kennis John T M, Pascal Andrew A, van Amerongen Herbert, Robert Bruno, Horton Peter, van Grondelle Rienk
School of Biological and Chemical Sciences, Queen Mary University of London, Mile End Road, London E1 4NS, UK.
Nature. 2007 Nov 22;450(7169):575-8. doi: 10.1038/nature06262.
Under conditions of excess sunlight the efficient light-harvesting antenna found in the chloroplast membranes of plants is rapidly and reversibly switched into a photoprotected quenched state in which potentially harmful absorbed energy is dissipated as heat, a process measured as the non-photochemical quenching of chlorophyll fluorescence or qE. Although the biological significance of qE is established, the molecular mechanisms involved are not. LHCII, the main light-harvesting complex, has an inbuilt capability to undergo transformation into a dissipative state by conformational change and it was suggested that this provides a molecular basis for qE, but it is not known if such events occur in vivo or how energy is dissipated in this state. The transition into the dissipative state is associated with a twist in the configuration of the LHCII-bound carotenoid neoxanthin, identified using resonance Raman spectroscopy. Applying this technique to study isolated chloroplasts and whole leaves, we show here that the same change in neoxanthin configuration occurs in vivo, to an extent consistent with the magnitude of energy dissipation. Femtosecond transient absorption spectroscopy, performed on purified LHCII in the dissipative state, shows that energy is transferred from chlorophyll a to a low-lying carotenoid excited state, identified as one of the two luteins (lutein 1) in LHCII. Hence, it is experimentally demonstrated that a change in conformation of LHCII occurs in vivo, which opens a channel for energy dissipation by transfer to a bound carotenoid. We suggest that this is the principal mechanism of photoprotection.
在阳光过剩的条件下，植物叶绿体膜中发现的高效光捕获天线会迅速且可逆地转变为光保护猝灭状态，在这种状态下，潜在有害的吸收能量以热的形式耗散，这一过程通过叶绿素荧光的非光化学猝灭或qE来衡量。尽管qE的生物学意义已得到确立，但其涉及的分子机制尚不清楚。主要光捕获复合体LHCII具有通过构象变化转变为耗散状态的内在能力，有人认为这为qE提供了分子基础，但尚不清楚这种事件是否在体内发生，以及在这种状态下能量是如何耗散的。利用共振拉曼光谱法鉴定出，向耗散状态的转变与LHCII结合的类胡萝卜素新黄质的构型扭曲有关。应用这项技术研究分离的叶绿体和完整叶片，我们在此表明，新黄质构型的相同变化在体内也会发生，其程度与能量耗散的幅度一致。对处于耗散状态的纯化LHCII进行飞秒瞬态吸收光谱研究表明，能量从叶绿素a转移到一个低能类胡萝卜素激发态，该激发态被鉴定为LHCII中的两种叶黄素之一（叶黄素1）。因此，实验证明LHCII的构象变化在体内发生，这为通过转移到结合的类胡萝卜素进行能量耗散开辟了一条通道。我们认为这是光保护的主要机制。
