Crystal Structure and Functional Characterization of Photosystem II-Associated Carbonic Anhydrase CAH3 in Chlamydomonas reinhardtii
Crystal Structure and Functional Characterization of Photosystem II-Associated Carbonic Anhydrase CAH3 in Chlamydomonas reinhardtii
复制标题
DOI：
10.1104/pp.114.253591
复制
发表时间：
2015-03-01
期刊：
影响因子：
7.4
通讯作者：
Sauer-Eriksson, A. Elisabeth
中科院分区：
文献类型：
--
作者：
Benlloch, Reyes;Shevela, Dmitriy;Sauer-Eriksson, A. Elisabeth
在氧气光合作用中，光能以化学能的形式储存，将二氧化碳和水转化为碳水化合物。在光系统II （PSII）中，水的光驱动氧化为随后的二氧化碳固定提供了电子和质子。最近的研究表明，在高等植物中，HCO3-通过作为PSII产生的质子的移动受体而增加PSII的活性。在绿藻莱茵衣藻（Chlamydomonas reinhardtii）中，一种腔碳酸酐酶CrCAH3被认为可能通过从CO2中快速重组HCO3-来提高PSII的质子脱除。在本研究中，我们通过膜入口质谱和x射线晶体学研究了PSII和CrCAH3之间的相互作用。膜入口质谱测定表明，在光照条件下，CrCAH3在类囊体腔中普遍存在的微酸性pH值下最活跃。在2.6埃和2.7埃的分辨率下分别测定了与乙酰唑胺或磷酸离子配合物中CrCAH3的晶体结构。CrCAH3是pH为4.1的二聚体，通过交换n端臂来稳定，这是以前在α型碳酸酐酶中未观察到的特征。该结构包含一个二硫键，用二硫苏糖醇对CrCAH3功能进行氧化还原滴定表明该酶可能具有氧化还原调节作用。通过比较野生型和不含CrCAH3的PSII制剂的闪致析氧模式，证实了CrCAH3和CO2/HCO3-对PSII活性的刺激作用。我们发现CrCAH3具有独特的结构特征，使该酶在低pH和低CO2浓度下最大化PSII活性。
In oxygenic photosynthesis, light energy is stored in the form of chemical energy by converting CO2 and water into carbohydrates. The light-driven oxidation of water that provides the electrons and protons for the subsequent CO2 fixation takes place in photosystem II (PSII). Recent studies show that in higher plants, HCO3- increases PSII activity by acting as a mobile acceptor of the protons produced by PSII. In the green alga Chlamydomonas reinhardtii, a luminal carbonic anhydrase, CrCAH3, was suggested to improve proton removal from PSII, possibly by rapid reformation of HCO3- from CO2. In this study, we investigated the interplay between PSII and CrCAH3 by membrane inlet mass spectrometry and x-ray crystallography. Membrane inlet mass spectrometry measurements showed that CrCAH3 was most active at the slightly acidic pH values prevalent in the thylakoid lumen under illumination. Two crystal structures of CrCAH3 in complex with either acetazolamide or phosphate ions were determined at 2.6- and 2.7-angstrom resolution, respectively. CrCAH3 is a dimer at pH 4.1 that is stabilized by swapping of the N-terminal arms, a feature not previously observed in alpha-type carbonic anhydrases. The structure contains a disulfide bond, and redox titration of CrCAH3 function with dithiothreitol suggested a possible redox regulation of the enzyme. The stimulating effect of CrCAH3 and CO2/HCO3- on PSII activity was demonstrated by comparing the flash-induced oxygen evolution pattern of wild-type and CrCAH3-less PSII preparations. We showed that CrCAH3 has unique structural features that allow this enzyme to maximize PSII activity at low pH and CO2 concentration.