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。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，质子（氢离子）和氧气被释放出来。氧气在大气中积累，与紫外线反应形成保护臭氧层。氧气还提供了允许呼吸发生的反应环境。氧的这两种作用对多细胞生物的发展至关重要：我们所知道的生命。最重要的光合酶是光系统II，水分解酶。它是改变地球的酶。水是非常不活泼和分裂它是很难做到的。一种能够分解水的酶似乎只进化了一次，而所有产生氧气的光合作用生物，从最古老的蓝藻到橡树，都使用相同的酶。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。光的能量取决于它的颜色，光系统II使用被称为叶绿素a的色素吸收的红光。叶绿素收集的光中可用的能量不足以安全地做PSII所做的事情，尽管进化为它提供了一系列令人印象深刻的化学技巧，旨在保护它免受燃烧，但最终它只是受到了打击。它在大约一百万次反应后被破坏（大约每半小时一次，取决于阳光的亮度），然后需要将其拆开，用新的亚基替换受损的亚基。这种损伤和修复需要消耗能量，在恶劣条件下，它会限制植物生长，降低作物产量。我们以前发现了一些有趣的东西。当光存在时，PSII中发生损坏，系统准备工作，但它不能用能量做任何有用的事情，因为某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，将其转化为一种叫做单线态氧的超活性形式，这是真实的杀手。这会破坏光系统II。原则上，当电子不是来自水时，例如在组装水裂解催化剂之前，或者当由于下游阻塞而无处放置电子时，例如由于缺乏CO2来固定，这种损害可能发生。但在这两种情况下，燃尽都被最小化，因为一种叫做QA的成分的反应性被降低了，这样能量就以热量的形式被释放出来，而不是进行形成三重态的高能反应。当水分解部件组装好，或者当CO2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进行了调整或控制，或者它是否确实有助于调整其邻居QA。我们已经有了惊喜，似乎QB的工作方式与一些研究人员的想法非常不同。通过了解PSII损伤和保护机制的细节，可以开发更好的策略，使光合作用更有效，增加粮食产量。最近，当其他研究人员设法加速植物中的（另一种）保护性转换时，作物生长得到了改善。所以这种方法可能会奏效。

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.