Structural insights into the catalytic mechanism of Synechocystis magnesium protoporphyrin IX O-methyltransferase (ChlM)

J Biol Chem. 2014 Sep 12;289(37):25690-8. doi: 10.1074/jbc.M114.584920. Epub 2014 Jul 30.


Magnesium protoporphyrin IX O-methyltransferase (ChlM) catalyzes transfer of the methyl group from S-adenosylmethionine to the carboxyl group of the C13 propionate side chain of magnesium protoporphyrin IX. This reaction is the second committed step in chlorophyll biosynthesis from protoporphyrin IX. Here we report the crystal structures of ChlM from the cyanobacterium Synechocystis sp. PCC 6803 in complex with S-adenosylmethionine and S-adenosylhomocysteine at resolutions of 1.6 and 1.7 Å, respectively. The structures illustrate the molecular basis for cofactor and substrate binding and suggest that conformational changes of the two "arm" regions may modulate binding and release of substrates/products to and from the active site. Tyr-28 and His-139 were identified to play essential roles for methyl transfer reaction but are not indispensable for cofactor/substrate binding. Based on these structural and functional findings, a catalytic model is proposed.

Keywords: Chlorophyll; Chloroplast; Enzyme Mechanism; Enzyme Structure; Photosynthesis; Protein Structure.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Bacterial Proteins / chemistry*
  • Bacterial Proteins / metabolism
  • Catalysis
  • Chlorophyll / biosynthesis*
  • Chlorophyll / chemistry
  • Crystallography, X-Ray
  • Kinetics
  • Methyltransferases / chemistry*
  • Methyltransferases / metabolism
  • Photosynthesis*
  • Protein Conformation
  • S-Adenosylmethionine / chemistry
  • Substrate Specificity
  • Synechocystis / chemistry*


  • Bacterial Proteins
  • Chlorophyll
  • S-Adenosylmethionine
  • Methyltransferases
  • chlM protein, Synechocystis sp.

Associated data

  • PDB/4QDJ
  • PDB/4QDK