Structural analysis of coniferyl alcohol 9-O-methyltransferase from Linum nodiflorum reveals a novel active-site environment.
Coniferyl alcohol 9-O-methyltransferase from Linum nodiflorum (Linaceae) catalyzes the unusual methylation of the side-chain hydroxyl group of coniferyl alcohol. The protein was heterologously expressed in Escherichia coli as a hexahistidine derivative and purified for crystallization. Diffracting crystals were obtained of the pure protein and of its selenomethionine derivative, as well as of complexes with coniferyl alcohol and with S-adenosyl-L-homocysteine together with coniferyl alcohol 9-O-methyl ether (PDB entries 4ems, 4e70 and 4evi, respectively). The X-ray structures show that the phenylpropanoid binding mode differs from other phenylpropanoid O-methyltransferases such as caffeic acid O-methyltransferase. Moreover, the active site lacks the usually conserved and catalytic histidine residue and thus implies a different reaction mode for methylation. Site-directed mutagenesis was carried out to identify critical amino acids. The binding order of coniferyl alcohol and S-adenosyl-L-methionine was investigated by isothermal titration calorimetry experiments.