Structural Insights into the Catalytic Mechanism of Synechocystis Magnesium Protoporphyrin IX O-Methyltransferase (ChlM) [Plant Biology]

July 30th, 2014 by Chen, X., Wang, X., Feng, J., Chen, Y., Fang, Y., Zhao, S., Zhao, A., Zhang, M., Liu, L.

Magnesium protoporphyrin IX O-methyltransferase (ChlM) catalyzes transfer of the methyl group from S-adenosylmethionine (SAM) 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 SAM and S-adenosylhomocysteine (SAH) at resolutions of 1.6 Å 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. Tyr28 and His139 were identified to play essential roles for methyl transfer reaction but not indispensable for cofactor/substrate binding. Based on these structural and functional findings, a catalytic model is proposed.