Crystal Structure and Catalytic Mechanism of 7-Hydroxymethyl Chlorophyll a Reductase

Xiao Wang; Lin Liu

doi:10.1074/jbc.M116.720342

Crystal Structure and Catalytic Mechanism of 7-Hydroxymethyl Chlorophyll a Reductase*

Xiao Wang^‡,^§ and
Lin Liu^‡¹

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

↵1 To whom correspondence should be addressed: Key Laboratory of Photobiology, CAS Center for Excellence in Molecular Plant Sciences, Institute of Botany, Chinese Academy of Sciences, 20 Nanxincun, Xiangshan, Beijing, 100093, China. Tel.: 86-10-62836483; E-mail: liulin{at}ibcas.ac.cn.

Abstract

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.

Footnotes

↵* This work was supported by National Basic Research Program of China Grant 2011CBA00901, National Natural Science Foundation of China Grant 31370759, and Key Research Program Grant KGZD-EW-T05 from the Chinese Academy of Sciences. The authors declare that they have no conflicts of interest with the contents of this article.