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Crystal structure of the catalytic subunit of magnesium chelatase

Abstract

Tetrapyrroles, including haem and chlorophyll, play vital roles for various biological processes, such as respiration and photosynthesis, and their biosynthesis is critical for virtually all organisms. In photosynthetic organisms, magnesium chelatase (MgCh) catalyses insertion of magnesium into the centre of protoporphyrin IX, the branch-point precursor for both haem and chlorophyll, leading tetrapyrrole biosynthesis into the magnesium branch1,2. This reaction needs a cooperated action of the three subunits of MgCh: the catalytic subunit ChlH and two AAA+ subunits, ChlI and ChlD (refs 35). To date, the mechanism of MgCh awaits further elucidation due to a lack of high-resolution structures, especially for the 150 kDa catalytic subunit. Here we report the crystal structure of ChlH from the photosynthetic cyanobacterium Synechocystis PCC 6803, solved at 2.5 Å resolution. The active site is buried deeply inside the protein interior, and the surrounding residues are conserved throughout evolution. This structure helps to explain the loss of function reported for the cch and gun5 mutations of the ChlH subunit, and to provide the molecular basis of substrate channelling during the magnesium-chelating process. The structure advances our understanding of the holoenzyme of MgCh, a metal chelating enzyme other than ferrochelatase.

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Figure 1: Crystal structure of the ChlH subunit.
Figure 2: Dimer in crystal.
Figure 3: The internal pocket.
Figure 4: Model of the PPO–ChlH complex.

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Acknowledgements

We thank M.-Z. Wang at the Institute of Biophysics of the Chinese Academy of Sciences and the staff of beamline BL17U at the Shanghai Synchrotron Radiation Facility for technical support. This work was supported by the National Basic Research Program of China Grant 2011CBA00901, the Key Research Program KGZD-EW-T05 and the Hundred Talents Program of the Chinese Academy of Sciences.

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Contributions

X.C. cloned the construct, purified, crystallized, collected, processed and refined data; H.P., Y.F., X.W. and S.Z. purified and collected data; Y.L., M.Z. and H.D. analysed data; W.G. and L.L. designed the study and wrote the paper.

Corresponding authors

Correspondence to Weimin Gong or Lin Liu.

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The authors declare no competing financial interests.

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Chen, X., Pu, H., Fang, Y. et al. Crystal structure of the catalytic subunit of magnesium chelatase. Nature Plants 1, 15125 (2015). https://doi.org/10.1038/nplants.2015.125

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