Wang Xiao, Liu Lin
From the Key Laboratory of Photobiology, CAS Center for Excellence in Molecular Plant Sciences, Institute of Botany, Chinese Academy of Sciences, Beijing 100093 and the University of Chinese Academy of Sciences, Beijing 100049, China.
From the Key Laboratory of Photobiology, CAS Center for Excellence in Molecular Plant Sciences, Institute of Botany, Chinese Academy of Sciences, Beijing 100093 and
J Biol Chem. 2016 Jun 17;291(25):13349-59. doi: 10.1074/jbc.M116.720342. Epub 2016 Apr 12.
7-Hydroxymethyl chlorophyll a reductase (HCAR) catalyzes the second half-reaction in chlorophyll b to chlorophyll a conversion. HCAR is required for the degradation of light-harvesting complexes and is necessary for efficient photosynthesis by balancing the chlorophyll a/b ratio. Reduction of the hydroxymethyl group uses redox cofactors [4Fe-4S] cluster and FAD to transfer electrons and is difficult because of the strong carbon-oxygen bond. Here, we report the crystal structure of Arabidopsis HCAR at 2.7-Å resolution and reveal that two [4Fe-4S]clusters and one FAD within a very short distance form a consecutive electron pathway to the substrate pocket. In vitro kinetic analysis confirms the ferredoxin-dependent electron transport chain, thus supporting a proton-activated electron transfer mechanism. HCAR resembles a partial reconstruction of an archaeal F420-reducing [NiFe] hydrogenase, which suggests a common mode of efficient proton-coupled electron transfer through conserved cofactor arrangements. Furthermore, the trimeric form of HCAR provides a biological clue of its interaction with light-harvesting complex II.
7-羟甲基叶绿素a还原酶（HCAR）催化叶绿素b向叶绿素a转化过程中的后半段反应。HCAR是光捕获复合物降解所必需的，并且通过平衡叶绿素a/b比例对高效光合作用至关重要。羟甲基的还原利用氧化还原辅因子[4Fe-4S]簇和黄素腺嘌呤二核苷酸（FAD）来转移电子，由于碳-氧键很强，这一过程具有挑战性。在此，我们报道了拟南芥HCAR在2.7埃分辨率下的晶体结构，并揭示在非常短的距离内的两个[4Fe-4S]簇和一个FAD形成了一条通向底物口袋的连续电子通路。体外动力学分析证实了依赖铁氧化还原蛋白的电子传递链，从而支持质子激活的电子转移机制。HCAR类似于古菌F420还原[NiFe]氢化酶的部分重构，这表明通过保守的辅因子排列实现高效质子耦合电子转移的常见模式。此外，HCAR的三聚体形式为其与光捕获复合物II的相互作用提供了生物学线索。
