The mechanism of a photoprotective molecular switch in the photosynthetic light-harvesting complex of plants

我们生物圈的生命完全依靠光合作用。数百万年来，氧进化生物一直为包括我们在内的所有异养生物提供呼吸的空气、食物、能量和重要物质的来源。地球上氧气光合作用的成功是一种伟大的生物现象，它首先依赖于光合作用装置分子机制的有效设计和对不断变化的环境条件的适应性。光合膜是所有生物膜中最复杂的，富含各种蛋白质。后者结合了许多重要的辅助因子：叶绿素、类胡萝卜素、脂质、离子和水。蛋白质调节和协调这些辅助因子的功能。这种协调是它们生物功能的核心。光合膜的主要色素-脂蛋白复合物之一是光系统II的光收集复合物（LHCII）。它收集光合膜接收到的大部分光能，并将其转移到反应中心，在那里发生电荷分离，引发一系列事件，导致合成通用生物燃料ATP和NADPH。我们发现LHCII通过控制传递到反应中心的能量发挥重要的调节作用。这是通过将多余的能量耗散成热来实现的。最近我们发现，LHCII目前可用的结构确实与该配合物在相当耗散状态下的结构相对应。这是一个重要的发现，因为它为我们提供了光合膜是如何感知和处理多余光的结构见解，因此植物如何保护自己免受这种类型的压力并存活下来。目前的计划是建立在LHCII结构知识的基础上，旨在回答一些重要的机械问题。我们想找出导致LHCII转换为耗散模式的因素，这种转换在蛋白质组织的什么水平上起作用：单体结构域，三聚体或更多的集体结构域，更高的低聚体单元？伴随耗散态的荧光、组合散射（拉曼）和吸收特征等新的能量参数的性质是什么？激发能耗散的性质是什么？次要配体、叶黄素循环类胡萝卜素、紫黄质和玉米黄质在LHCII开关调控中的作用是什么？该项目将为我们提供关于LHCII天线设计的基本特征的知识，使自然界的光收集过程同时高效和灵活，确保植物的高生产力和对地球光环境的适应性。

The life of our biosphere is entirely dependent upon photosynthesis. Over millions of years oxygen-evolving organisms have been giving all heterotrophic organisms, including us, air to breathe and a source of food, energy and vital materials. The success of oxygenic photosynthesis on this planet is a great biological phenomenon, that relies first of all upon the efficient design and adaptability to the changing environmental conditions of the molecular machinery of the photosynthetic apparatus. The photosynthetic membrane is the most complex of all biological membranes and is the most enriched in various proteins. The latter bind a number of important co-factors: chlorophylls, carotenoids, lipids, ions and water. Protein tunes and co-ordinates the functions of these co-factors. This co-ordination lies at the heart of their biological function. One of the major pigment-lipoprotein complexes of the photosynthetic membrane is the light harvesting complex of photosystem II (LHCII). It collects the most significant part of the light energy received by the photosynthetic membrane and transfers it to the reaction centers, where charge separation occurs to initiate a chain of events leading to a synthesis of the universal biological fuel ATP and NADPH. LHCII was found to play an important regulatory role by controlling the amount of energy delivered to the reaction center. This is being achieved by dissipation of the excess energy into heat. Recently we have discovered that the currently available structure of LHCII does correspond to the structure of this complex in the rather dissipative state. This is an important finding, since it offers us structural insights of how the photosynthetic membrane is sensing and dealing with the excess light, hence how plants can protect themselves against this type of stress and survive it. The current program is built upon the knowledge of LHCII structure and aims to answer a number of important mechanistic questions. We want to find out what factors lead to the switching of LHCII into the dissipative mode, at what level of protein organization does this switch work: domains of the monomer, trimer or more collective, higher oligomer units? What is the nature of the new energetic parameters like fluorescence, combinational scattering (Raman) and absorption features accompanying the dissipative state and what is the nature of the excitation energy dissipation? What is the role of minor ligands, xanthophyll cycle carotenoids, violaxanthin and zeaxanthin in the regulation of the LHCII switch? The project should provide us with the knowledge of the very fundamental features put into the LHCII antenna design, which allow the light harvesting process in nature to be efficient and flexible at the same time, insuring a high level of plant productivity and adaptability to the light environment on our planet.