Research Department of Genetics, Evolution and Environment, Darwin Building, University College London, Gower Street, London, WC1E 6BT, UK.
Physiol Plant. 2017 Sep;161(1):28-44. doi: 10.1111/ppl.12577. Epub 2017 May 31.
In oxygenic photosynthesis there are two 'light states' - adaptations of the photosynthetic apparatus to spectral composition that otherwise favours either photosystem I or photosystem II. In chloroplasts of green plants the transition to light state 2 depends on phosphorylation of apoproteins of a membrane-intrinsic antenna, the chlorophyll-a/b-binding, light-harvesting complex II (LHC II), and on the resulting redistribution of absorbed excitation energy from photosystem II to photosystem I. The transition to light state 1 reverses these events and requires a phospho-LHC II phosphatase. Current structures of LHC II reveal little about possible steric effects of phosphorylation. The surface-exposed N-terminal domain of an LHC II polypeptide contains its phosphorylation site and is disordered in its unphosphorylated form. A molecular recognition hypothesis proposes that state transitions are a consequence of movement of LHC II between binding sites on photosystems I and II. In state 1, LHC II forms part of the antenna of photosystem II. In state 2, a unique but as yet unidentified 3-D structure of phospho-LHC II may attach it instead to photosystem I. One possibility is that the LHC II N-terminus becomes ordered upon phosphorylation, adopting a local alpha-helical secondary structure that initiates changes in LHC II tertiary and quaternary structure that sever contact with photosystem II while securing contact with photosystem I. In order to understand redistribution of absorbed excitation energy in photosynthesis we need to know the structure of LHC II in its phosphorylated form, and in its complex with photosystem I.
在含氧光合作用中有两种“光状态”——光合作用器官对光谱组成的适应，这种适应要么有利于光系统 I，要么有利于光系统 II。在绿色植物的叶绿体中，向光状态 2 的转变依赖于膜内在天线的脱辅基蛋白的磷酸化，该天线是叶绿素-a/b 结合的光捕获复合物 II（LHC II），以及由此导致的从光系统 II 到光系统 I 的吸收激发能的重新分布。向光状态 1 的转变逆转了这些事件，并需要一个磷酸-LHC II 磷酸酶。目前的 LHC II 结构几乎没有揭示磷酸化可能产生的空间效应。LHC II 多肽的暴露于表面的 N 端结构域包含其磷酸化位点，并且在未磷酸化形式下处于无序状态。一个分子识别假说提出，状态转变是 LHC II 在光系统 I 和 II 的结合位点之间移动的结果。在状态 1 中，LHC II 构成光系统 II 天线的一部分。在状态 2 中，磷酸化-LHC II 的独特但尚未确定的 3D 结构可能将其连接到光系统 I。一种可能性是，LHC II N 端在磷酸化后变得有序，采用局部α-螺旋二级结构，从而引发 LHC II 三级和四级结构的变化，切断与光系统 II 的接触，同时确保与光系统 I 的接触。为了理解光合作用中吸收激发能的再分配，我们需要知道磷酸化形式下的 LHC II 的结构，以及与光系统 I 形成复合物的 LHC II 的结构。
