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

Xuemin Chen,Xiao Wang,Juan Feng,Yuhong Chen,Ying Fang,Shun Zhao,Aiguo Zhao,Min Zhang,Lin Liu

doi:10.1074/jbc.m114.584920

Abstract

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.

Highlights

Magnesium protoporphyrin IX methyltransferase (ChlM) catalyzes the second step in the magnesium branch of tetrapyrrole biosynthesis
The structures of Synechocystis ChlM (SyChlM) bound to SAM and SAH were determined at resolutions of 1.6 and 1.7 Å, respectively (Table 1)
Structural Comparison— the structure of SyChlM is highly similar to a few members of the Class I SAM-dependent methyltransferases (Table 3), none of them catalyzes carboxyl methylation

Summary

Background

Magnesium protoporphyrin IX methyltransferase (ChlM) catalyzes the second step in the magnesium branch of tetrapyrrole biosynthesis. 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. To understand the molecular basis underlying the catalytic and regulatoporphyrin IX O-methyltransferase; SyChlM, Synechocystis ChlM; SAM, S-adenosylmethionine; SAH, S-adenosylhomocysteine; ITC, isothermal titration calorimetry; r.m.s.d., root mean square deviation. Based on the SyChlM structure and supported by biochemical experiments on site-directed mutants, we identified two residues that are required for the methyl transfer reaction. These findings suggest a possible molecular basis for ChlM catalytic and regulatory mechanism

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION