Magnesium chelatase catalyzes insertion of a magnesium ion into the center of protoporhyrin IX. This reaction is the first step in the chlorophyll branch. The holoenzyme of magnesium chelatase consists of three subunits: the catalytic subunit ChlH and two AAA family members, ChlI and ChlD. ChlH has a molecular weight of approximate 150 kDa, and cryo-EM study has revealed that ChlH is a cage-like assembly. However, due to low resolution, the structural information about the active site is missing. Lin Liu’s team, in collaboration with Professor Weimin Gong at the University of Science and Technology of China, solved the structure of ChlH from the cyanobacterium Synechocystis sp. PCC 6803 to a resolution of 2.5 angstrom by X-ray crystallography. ChlH has an overall shape of a mallet and is composed of six domains (I - VI). A buried pocket lies at the interface between domains III and V and can accommodate a porphyrin molecule. Amino-acid sequence alignment of ChlHs from different photosynthetic organisms indicates that residues constituting the interface are highly conserved, thus suggesting that this pocket is the active site. A point mutation from Ala to Val at the joint between domains III and V corresponds to the Arabidopsis genomes uncoupled 5 (gun5) mutation, for which ChlH has also been named GUN5. The 2.5-angstrom structure of ChlH/GUN5 provides a solid basis for further elucidation of the catalytic mechanism of magnesium chelatase. This work has been reported in Nature Plants (doi: 10.1038/nplants.2015.125). Lin Liu’s team has also solved the structure of Synechocystis GUN4 in complex with deuteroporphyrin IX and magnesium deuteroporphyrin IX, respectively. GUN4 utilizes a half-open pocket to bind a porphyrin molecule, which might favor efficient delivery of different porphyrin molecules. This work has been published as a cover story in Molecular Plant (doi: 10.1016/j.molp.2015.04.013). Magnesium protoporphyrin IX O-methyltransferase (ChlM) catalyzes the second reaction in the chlorophyll branch. It transfers a methyl group from S-adenosylmethionine (SAM) to the carboxyl group of the C13 propionate side chain of magnesium protoporphyrin IX. The structural and biochemical studies have provided insights into key residues affecting catalysis, and have been published in The Journal of Biological Chemistry (doi: 10.1074/jbc.M114.584920). PhD student Xuemin Chen is the first author of these three papers. These works are supported by the Ministry of Science and Technology of China, the National Natural Science Foundation of China, and the Chinese Academy of Sciences.
Magnesium chelatase catalyzes insertion of a magnesium ion into the center of protoporhyrin IX. This reaction is the first step in the chlorophyll branch. The holoenzyme of magnesium chelatase consists of three subunits: the catalytic subunit ChlH and two AAA family members, ChlI and ChlD. ChlH has a molecular weight of approximate 150 kDa, and cryo-EM study has revealed that ChlH is a cage-like assembly. However, due to low resolution, the structural information about the active site is missing. Lin Liu’s team, in collaboration with Professor Weimin Gong at the University of Science and Technology of China, solved the structure of ChlH from the cyanobacterium Synechocystis sp. PCC 6803 to a resolution of 2.5 angstrom by X-ray crystallography. ChlH has an overall shape of a mallet and is composed of six domains (I - VI). A buried pocket lies at the interface between domains III and V and can accommodate a porphyrin molecule. Amino-acid sequence alignment of ChlHs from different photosynthetic organisms indicates that residues constituting the interface are highly conserved, thus suggesting that this pocket is the active site. A point mutation from Ala to Val at the joint between domains III and V corresponds to the Arabidopsis genomes uncoupled 5 (gun5) mutation, for which ChlH has also been named GUN5. The 2.5-angstrom structure of ChlH/GUN5 provides a solid basis for further elucidation of the catalytic mechanism of magnesium chelatase. This work has been reported in Nature Plants (doi: 10.1038/nplants.2015.125). Lin Liu’s team has also solved the structure of Synechocystis GUN4 in complex with deuteroporphyrin IX and magnesium deuteroporphyrin IX, respectively. GUN4 utilizes a half-open pocket to bind a porphyrin molecule, which might favor efficient delivery of different porphyrin molecules. This work has been published as a cover story in Molecular Plant (doi: 10.1016/j.molp.2015.04.013). Magnesium protoporphyrin IX O-methyltransferase (ChlM) catalyzes the second reaction in the chlorophyll branch. It transfers a methyl group from S-adenosylmethionine (SAM) to the carboxyl group of the C13 propionate side chain of magnesium protoporphyrin IX. The structural and biochemical studies have provided insights into key residues affecting catalysis, and have been published in The Journal of Biological Chemistry (doi: 10.1074/jbc.M114.584920). PhD student Xuemin Chen is the first author of these three papers. These works are supported by the Ministry of Science and Technology of China, the National Natural Science Foundation of China, and the Chinese Academy of Sciences.