Structural studies of plant O-methyltransferase families

Plant natural product O-methyltransferases (OMTs) are S-adenosyl-L-methionine (SAM) dependent enzymes that methylate hydroxyl and carboxylate moieties of numerous secondary metabolites. Structural studies of substrate and product complexes of OMT enzymes provide a tool to understand the catalytic mechanism and substrate specificity determinants of this class of proteins. Based on the primary amino acid sequences and demonstrated by the following structural studies, the OMT class of plant enzymes can be subdivided into three families. Representative members of each family of plant OMTs were structurally characterized and compared.; The type 1 family of OMTs methylate hydroxyl moieties of phenylpropanoid based small molecules such as flavonoids, chalcones, and pterocarpans. The high resolution crystal structures of the type 1 OMTs, chalcone O-methyltransferase (ChOMT), isoflavone O-methyltransferase (IOMT) and caffeic acid O-methyltransferase (COMT) from Medicago sativa (alfalfa) in complex with substrates and products provide a structural foundation for understanding the stereochemical principles underlying plant type 1 OMT activity and substrate selectivity. Type 1 OMTs use an active site histidine as a general base in the transmethylation reaction. The phenolic substrate is positioned through specific hydrogen bonds and the aromatic moieties are sequestered by the thioethers of active site methionine residues.; The type 2 family of plant OMTs is comprised of caffeoyl CoA O-methyltransferases (CCOMT) from various lignin producing plant species. The type 2 OMT family is most closely related to the mammalian catechol OMTs based on primary, secondary, and tertiary structure. The crystal structure of CCOMT from alfalfa maintains a different active site topology than the type 1 OMTs as well as a distinct catalytic mechanism. The CCOMT active site contains an octahedral metal binding site with metal binding necessary for catalysis.; The final family of OMTs discussed includes the carboxylate methyltransferases. The crystal structure of a representative member of this family, salicylic acid carboxyl methyltransferase (SAMT) from Clarkia breweri is presented in complex with salicylic acid and the reaction product S-adenosyl-L-homocysteine (SAH). SAMT provides a useful template structure for modeling other type 3 OMTs as demonstrated by mutagenesis studies. The structure based point mutant Y147S converts native SAMT into a bifunctional salicylic acid/jasmonic acid methyltransferase.