Arnoux Pascal, Morosinotto Tomas, Saga Giorgia, Bassi Roberto, Pignol David
Commissariat à l'Energie Atomique, Direction des Sciences du Vivant, Institut de Biologie Environementale et de Biotechnologie, Laboratoire de Bioénergétique Cellulaire, Saint-Paul-lez-Durance, F-13108, France.
Plant Cell. 2009 Jul;21(7):2036-44. doi: 10.1105/tpc.109.068007. Epub 2009 Jul 28.
Plants adjust their photosynthetic activity to changing light conditions. A central regulation of photosynthesis depends on the xanthophyll cycle, in which the carotenoid violaxanthin is converted into zeaxanthin in strong light, thus activating the dissipation of the excess absorbed energy as heat and the scavenging of reactive oxygen species. Violaxanthin deepoxidase (VDE), the enzyme responsible for zeaxanthin synthesis, is activated by the acidification of the thylakoid lumen when photosynthetic electron transport exceeds the capacity of assimilatory reactions: at neutral pH, VDE is a soluble and inactive enzyme, whereas at acidic pH, it attaches to the thylakoid membrane where it binds its violaxanthin substrate. VDE also uses ascorbate as a cosubstrate with a pH-dependent Km that may reflect a preference for ascorbic acid. We determined the structures of the central lipocalin domain of VDE (VDEcd) at acidic and neutral pH. At neutral pH, VDEcd is monomeric with its active site occluded within a lipocalin barrel. Upon acidification, the barrel opens up and the enzyme appears as a dimer. A channel linking the two active sites of the dimer can harbor the entire carotenoid substrate and thus may permit the parallel deepoxidation of the two violaxanthin beta-ionone rings, making VDE an elegant example of the adaptation of an asymmetric enzyme to its symmetric substrate.
植物会根据光照条件的变化来调整其光合活性。光合作用的核心调控依赖于叶黄素循环，在强光下，类胡萝卜素紫黄质会转化为玉米黄质，从而激活将多余吸收能量以热的形式耗散以及清除活性氧的过程。紫黄质脱环氧化酶（VDE）是负责玉米黄质合成的酶，当光合电子传递超过同化反应能力时，它会被类囊体腔的酸化激活：在中性pH值下，VDE是一种可溶且无活性的酶，而在酸性pH值下，它会附着在类囊体膜上并结合其紫黄质底物。VDE还将抗坏血酸用作共底物，其Km值依赖于pH，这可能反映了对抗坏血酸的偏好。我们测定了VDE中心脂笼结构域（VDEcd）在酸性和中性pH值下的结构。在中性pH值下，VDEcd是单体，其活性位点被封闭在脂笼桶内。酸化后，桶打开，酶呈现为二聚体。连接二聚体两个活性位点的通道可以容纳整个类胡萝卜素底物，因此可能允许两个紫黄质β-紫罗兰酮环同时进行脱环氧化，这使得VDE成为不对称酶适应其对称底物的一个绝佳例子。
