Quinone redox tuning for regulation and protection of the water splitting enzyme

光合作用是将太阳能转化为生命所需的化学能的过程。这种光被用来分解水，去除其中的一些电子，并利用它们从大气中吸收二氧化碳，以制造生命的基石和燃料。当水以这种方式分裂时，会释放出质子（氢离子）和氧气。氧气在大气中积累，与紫外线反应形成保护臭氧层。氧气也提供了一个允许呼吸发生的反应环境。氧的这两个作用对于多细胞生物的发展都是至关重要的，即我们所知道的生命。最重要的光合酶是光系统II，即水分解酶。正是这种酶改变了地球。水很不活泼，很难分解。一种能够分解水的酶似乎只进化过一次，而所有产生臭氧的光合作用生物，从最古老的蓝藻到橡树，都使用同一种酶。这种复杂的化学反应需要大量的能量，而这些能量来自阳光。光中能量的多少取决于它的颜色，光系统II利用一种叫做叶绿素a的色素吸收红光。叶绿素收集的光中的能量不足以安全地完成PSII的工作，尽管进化为它提供了一套令人印象深刻的化学技巧，旨在保护它免受燃烧，但最终它还是承受了打击。经过大约一百万次反应（大约每半小时，取决于阳光的亮度），它就会被破坏，然后它需要被拆开，被损坏的亚基被新的取代。这种破坏和修复需要消耗能源，在恶劣的条件下，它会限制植物的生长，使作物产量下降。本研究的重点是发现和理解保护光系统II的技巧。我们之前发现了一些有趣的东西。当光存在时，PSII就会受到破坏，系统已经准备好工作了，但它不能用能量做任何有用的事情，因为有些东西阻止了热化学的完成。当这种情况发生时，光产生的电荷再次聚集在一起，形成叶绿素的高能态，称为三重态。三重态叶绿素与正常的氧反应，并将其转化为一种称为单线态氧的超活性形式，这是真正的杀手。这会对光系统II造成损害。原则上，这种损害可能发生在电子不是来自水的情况下，例如在水分解催化剂组装之前，或者由于下游阻塞而没有地方放置电子，例如由于缺乏二氧化碳来固定。但在这两种情况下，燃尽都被最小化了，因为一种叫做QA的成分的反应性被降低了，所以能量以热量的形式被倾倒，而不是进行形成三重态的高能反应。当水分解部分组装好，或者当二氧化碳水平恢复正常时，QA将切换回其高能量功能。我们现在正在密切关注这个链中的下一个组件QB是如何工作的，以及它是否也以不同的方式被调整或控制，或者它是否确实有助于调整它的邻居QA。我们已经有了惊喜，QB的工作方式似乎与一些研究人员认为的非常不同。通过了解PSII损伤和保护机制的细节，可以制定更好的策略，使光合作用更有效，增加粮食产量。最近，其他研究人员设法加速（一种不同的）植物保护性转换，从而改善了作物的生长。所以这个方法是可行的。

Photosynthesis is the process that converts solar energy into the chemical energy that powers life. The light is used to split water, removing some of its electrons and using them to pull down carbon dioxide from the atmosphere to make the building blocks and fuel for life. When water is split in this way, protons (hydrogen ions) and oxygen are released. The oxygen accumulates in the atmosphere, reacting with UV to form the protective ozone layer. The oxygen also provides a reactive environment that allows respiration to occur. Both of these roles of oxygen were crucial for the development of multicellular organisms: life as we know it.The most important photosynthetic enzyme is Photosystem II, the water splitting enzyme. It is the enzyme that changed the planet. Water is very unreactive and splitting it is hard to do. An enzyme capable of splitting water seems to have evolved only once and all O2-producing photosynthesizers, from the most ancient cyanobacterium to the oak tree, use the same enzyme.Such difficult chemistry requires a lot of energy and this comes from sunlight. The amount of energy in light depends on its colour and Photosystem II uses red light absorbed by a pigment called chlorophyll a. The energy available in the light collected by chlorophyll is not enough to do what PSII does safely and although evolution has provided it with an impressive bag of chemical tricks designed to protect it from burning out, in the end it just takes the hit. It is destroyed after about a million reactions (about every half hour, depending on the brightness of the sunlight), and it then needs to be taken apart and the damaged subunits replaced with new ones. This damage and repair costs energy and under severe conditions it can limit plant growth and give smaller crop yields.The present study is focused on discovering and understanding the tricks for protecting Photosystem II. We have previously found some interesting stuff. The damage occurs in PSII when the light is there, the system is ready to work but it can't do anything useful with the energy because something prevents the completion of the hot chemistry. When this happens the light-generated charges come back together again forming a high energy state of chlorophyll called a triplet. The triplet chlorophyll reacts with normal oxygen and turns it into a super-reactive form called singlet oxygen, which is the real killer. This causes the damage to Photosystem II. In principle this damage could happen when electrons don't come from water, for example prior to the assembly of the water splitting catalyst, or when there is nowhere to put the electrons because of a downstream block, for example due to a lack of CO2 to fix. But in both of these cases burnout is minimised because a component called QA has its reactivity tuned down so that the energy is dumped as heat instead of doing the high energy reactions that form the triplet. When the water splitting part is assembled, or when the CO2 levels return to normal, QA is switched back to its high energy function. We are now looking closely at how the next component in the chain, QB, works and if it too is tuned or controlled in a different way or indeed if it helps to tune its neighbour QA. Already we have had surprises and it seems QB works very differently from how some researchers thought. By understanding the details of PSII damage and protection mechanisms, better strategies may be developed for making photosynthesis more efficient and increasing food production. Very recently other researchers got improved crop growth when they managed to accelerate (a different kind of) protective switching in plants. So this approach could just work.