title: Crystal Structure and Catalytic Mechanism of 7-Hydroxymethyl Chlorophyll a Reductase.
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.
authors.affiliation: 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.
authors.forename: Xiao
authors.initials: X
authors.lastname: Wang
authors.affiliation: From the Key Laboratory of Photobiology, CAS Center for Excellence in Molecular Plant Sciences, Institute of Botany, Chinese Academy of Sciences, Beijing 100093 and liulin@ibcas.ac.cn.
authors.forename: Lin
authors.initials: L
authors.lastname: Liu
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000737:chemistry* / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000737:chemistry* / Q000378:metabolism*; D002734:Chlorophyll / Q000378:metabolism; D000077194:Chlorophyll A; D018360:Crystallography, X-Ray; D004579:Electron Transport; D005182:Flavin-Adenine Dinucleotide / Q000378:metabolism; D062105:Molecular Docking Simulation; D010088:Oxidoreductases / Q000737:chemistry* / Q000378:metabolism*; D011487:Protein Conformation; D055503:Protein Multimerization; D016415:Sequence Alignment
chemical_list: D029681:Arabidopsis Proteins; D002734:Chlorophyll; D005182:Flavin-Adenine Dinucleotide; C581015:7-hydroxymethyl chlorophyll a reductase, Arabidopsis; D010088:Oxidoreductases; D000077194:Chlorophyll A
keywords: chlorophyll cycle; chlorophyll degradation; crystal structure; electron transfer; flavin adenine dinucleotide (FAD); iron-sulfur protein; proton-coupled electron transfer; reductase