Schmitz Jessica, Srikanth Nishtala V, Hüdig Meike, Poschmann Gereon, Lercher Martin J, Maurino Veronica G
Institute of Developmental and Molecular Biology of Plants, Plant Molecular Physiology and Biotechnology Group, Heinrich Heine University, Universitätsstraße 1, and Cluster of Excellence on Plant Sciences (CEPLAS), 40225, Düsseldorf, Germany.
Institute for Computer Science and Department of Biology, Heinrich Heine University, Universitätsstraße 1, and Cluster of Excellence on Plant Sciences (CEPLAS),, 40225, Düsseldorf, Germany.
Photosynth Res. 2017 May;132(2):183-196. doi: 10.1007/s11120-017-0355-1. Epub 2017 Feb 28.
Like other oxygenic photosynthetic organisms, diatoms produce glycolate, a toxic intermediate, as a consequence of the oxygenase activity of Rubisco. Diatoms can remove glycolate through excretion and through oxidation as part of the photorespiratory pathway. The diatom Phaeodactylum tricornutum encodes two proteins suggested to be involved in glycolate metabolism: PtGO1 and PtGO2. We found that these proteins differ substantially from the sequences of experimentally characterized proteins responsible for glycolate oxidation in other species, glycolate oxidase (GOX) and glycolate dehydrogenase. We show that PtGO1 and PtGO2 are the only sequences of P. tricornutum homologous to GOX. Our phylogenetic analyses indicate that the ancestors of diatoms acquired PtGO1 during the proposed first secondary endosymbiosis with a chlorophyte alga, which may have previously obtained this gene from proteobacteria. In contrast, PtGO2 is orthologous to an uncharacterized protein in Galdieria sulphuraria, consistent with its acquisition during the secondary endosymbiosis with a red alga that gave rise to the current plastid. The analysis of amino acid residues at conserved positions suggests that PtGO2, which localizes to peroxisomes, may use substrates other than glycolate, explaining the lack of GOX activity we observe in vitro. Instead, PtGO1, while only very distantly related to previously characterized GOX proteins, evolved glycolate-oxidizing activity, as demonstrated by in gel activity assays and mass spectrometry analysis. PtGO1 localizes to mitochondria, consistent with previous suggestions that photorespiration in diatoms proceeds in these organelles. We conclude that the ancestors of diatoms evolved a unique alternative to oxidize photorespiratory glycolate: a mitochondrial dehydrogenase homologous to GOX able to use electron acceptors other than O.
与其他产氧光合生物一样，由于核酮糖-1,5-二磷酸羧化酶/加氧酶（Rubisco）的加氧酶活性，硅藻会产生乙醇酸这种有毒中间体。硅藻可通过排泄以及作为光呼吸途径的一部分进行氧化来去除乙醇酸。三角褐指藻编码两种被认为参与乙醇酸代谢的蛋白质：PtGO1和PtGO2。我们发现这些蛋白质与其他物种中负责乙醇酸氧化的实验表征蛋白质——乙醇酸氧化酶（GOX）和乙醇酸脱氢酶的序列有很大差异。我们表明PtGO1和PtGO2是三角褐指藻中与GOX同源的仅有的序列。我们的系统发育分析表明，硅藻的祖先在与绿藻的首次次生内共生过程中获得了PtGO1，而绿藻可能先前已从变形菌中获得了该基因。相比之下，PtGO2与嗜热栖热放线菌中一种未表征的蛋白质直系同源，这与其在与产生当前质体的红藻的次生内共生过程中获得该蛋白一致。对保守位置氨基酸残基的分析表明，定位于过氧化物酶体的PtGO2可能使用除乙醇酸以外的底物，这解释了我们在体外观察到的缺乏GOX活性的现象。相反，虽然PtGO1与先前表征的GOX蛋白的亲缘关系非常远，但它进化出了乙醇酸氧化活性，这在凝胶活性测定和质谱分析中得到了证明。PtGO1定位于线粒体，这与之前关于硅藻光呼吸在这些细胞器中进行的观点一致。我们得出结论，硅藻的祖先进化出了一种独特的替代方式来氧化光呼吸乙醇酸：一种与GOX同源的线粒体脱氢酶，它能够使用除氧气以外的电子受体。
