Role of Atypical D1 Proteins in Photosystem II

在光合作用中，光被用来除去水中的氢以提供氧气。然后，氢当量被用来将二氧化碳化学还原为有机分子。水氧化反应是由一种称为光系统II(PSII)的酶催化的。光很难催化这一反应，到目前为止，唯一已知的有效体系是自然体系。该反应发生在一个含有4个锰离子和1个钙离子的金属团簇上。这一簇由PSII中一种高度保守的蛋白质“D1蛋白”结合并稳定。PSII复合体包含20多个其他蛋白质亚基。然而，在活跃的叶片中，D1蛋白每30分钟左右就会迅速降解一次。这被认为是由于水分解反应产生的活性氧副产物破坏了蛋白质。存在一种复杂的修复系统，可以用新鲜的D1蛋白再生PSII。蓝藻有几个不同的D1基因，适合不同的情况。一些是在强光下合成的，另一些是在低氧环境中合成的。一些具有光合作用的蓝藻也能固定空气中的氮。这可能是一个问题，因为固氮酶被氧气不可逆转地抑制。这些生物要么在物理上将固氮酶从光合作用中分离出来，要么只在PSII不活跃的时候在夜间固定氮素。最近发现了一类D1基因，在与锰簇结合的位置上，相对于“典型的D1”发生了突变。这些突变是这样的，这些“无赖”的D1被认为不能放氧，但有足够的官能团使金属结合成为可能。我们认为，非典型的D1序列可能是用非催化的D1来灭活PSII的一种机制，然后当需要再次活性时，它会被取代。还有一类更非典型的d1(“超级无赖d1”)与蓝藻有关，它通过产生红移的叶绿素，叶绿素f来适应远红光。这种叶绿素在红外线中有峰值吸收，但似乎仍然能够利用这些较低能量的光子来驱动水的氧化。超流氓基因d1在远红光条件下上调1000倍，因此可能在适应远红光中起作用。非典型d1序列在系统发育上较早，因此使人联想到祖先d1，因此可能提供有关氧合光合作用进化的信息。如果在底物氧化中起作用，变异体PSII可以使用水以外的底物。并再次为光合作用的进化提供了洞察力。这样的反应中心将是一个新的发现。我们将在体内和体外研究非典型D1蛋白在PSII中的功能。对于活体研究，我们将在各种条件下用非典型的D1培养蓝藻，包括昼夜明暗节律。我们将研究非典型D1与正常D1的表达模式，以及与其他因素的关系，如光暗、固氮和外部碳源。在体外研究中，我们将用无赖和超无赖的D1纯化PSII。我们将研究它们的功能，如金属含量、氧化底物和转移电子的能力。我们将评估所有PSII亚基在修改后的反应中心中的存在。通过体内和体外方法的结合，我们将了解非典型D1序列的生物学功能是什么，以及如何在生化和生物物理水平上实现这一功能。

In photosynthesis, light is used to remove the hydrogen from water to give oxygen. The hydrogen equivalents are then used to chemically reduce carbon dioxide to organic molecules. The water oxidation reaction is catalysed by an enzyme known as photosystem II (PSII). It is hard to catalyse this reaction with light, so far the only known effective system is the natural one. The reaction takes place at a metallocluster containing 4 manganese ions and a calcium ion. This cluster is bound and stabilised by a highly conserved protein known as the "D1 protein" in PSII. The PSII complex contains more than 20 other protein subunits. However, in an active leaf, the D1 protein is degraded extremely quickly, every 30 minutes or so. This is thought to be because of the generation of reactive oxygen side products from the water-splitting reaction which damage the protein. A sophisticated system of repair exists to regenerate PSII with fresh D1 protein. Cyanobacteria have several different D1 genes suited to different situations. Some are synthesized in response to high light, others in low oxygen environments. Some photosynthetic cyanobacteria can also fix nitrogen from the air. This can be a problem as the nitrogenase enzyme is irreversibly inhibited by oxygen. These organisms either physically separate the nitrogenase from photosynthesis, or only fix nitrogen at night when PSII is inactive.There is a recently discovered a class of D1 genes that are mutated relative to the "canonical D1" at the sites binding the manganese cluster. The mutations are such that these "rogue" D1 are not thought to be capable of oxygen evolution, however, there are sufficient functional groups for metal binding to be a possibility. We believe that the atypical D1 sequences might be a mechanism to inactivate PSII with a non-catalytic D1, which is then replaced when activity is required again. There is a further class of even more atypical D1 ("super-rogue D1") that is associated with cyanobacteria that adapt to far-red light by making a red-shifted chlorophyll, chlorophyll f. This chlorophyll has a peak absorption in the infra-red, yet seems to still be capable of using these lower energy photons to drive water oxidation. The super-rogue D1 is 1000-fold up-regulated in far-red light conditions, so probably has a role in adaptation to far-red light.The atypical D1 sequences are phylogenetically early, so are reminiscent of an ancestral D1, so could provide information on the evolution of oxygenic photosynthesis. If functional in substrate oxidation, the variant PSII may use a substrate other than water. Of interest in itself and again providing insight into the evolution of photosynthesis. Such a reaction centre would be a novel finding.We will investigate the function of the atypical D1 proteins in PSII both in vivo and in vitro. For in vivo studies we will culture cyanobacteria with atypical D1 under a variety of conditions, including circadian light-dark rhythms. We will investigate the expression pattern of the atypical D1 in comparison to normal D1, in relation to other factors, such as light-dark, nitrogen fixation, and external carbon sources. For in vitro studies we will purify PSII with the rogue and super-rogue D1. We will investigate their function, such as metal content, ability to oxidise substrates and transfer electrons. We will assess the presence of all of the PSII subunits in the modified reaction centres. With a combination of the in vivo and in vitro approaches we will learn what the biological function of the atypical D1 sequences is, and how, at a biochemical and biophysical level, it is accomplished.