Minisymposium 11: Secondary Metabolites

Abs # 23003: Structural studies of plant O-methyltransferases

Presenter:	Zubieta, Chloe , zubieta@embl-grenoble.fr
Authors	Zubieta, Chloe  (B) (A)  Noel, Joseph P. (A)   Dixon, Richard A. (C)   Pichersky, Eran  (D)   Ross, Jeannine R. (A)
Affiliations:	(A): Salk Institute for Biological Studies, CA (B): European Molecular Biology Laboratory, Grenoble (C): Samuel Roberts Noble Foundation, OK (D): Department of Molecular, Cellular, and Developmental Biology, University of Michigan

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 aid in understanding the catalytic mechanism and substrate specificity determinants of this class of proteins. Based on the primary amino acid sequence and demonstrated by structural studies, the OMT class of plant enzymes can be divided into three subfamilies. Representative members of each family of plant OMTs have been structurally characterized and compared. The largest family of plant OMTs methylates hydroxyl groups of phenylpropanoid based small molecules such as flavonoids, chalcones, and pterocarpans. These enzymes use an active site histidine as a general base in the transmethylation reaction. Substrate selection is achieved through a pre-organized active site consisting of specific hydrogen bond donors and acceptors as well as methionine residues positioned to sequester the aromatic moieties of the substrate. Caffeoyl and feruloyl coenzyme-A specific OMTs (CCoAOMTs) from various lignin producing plant species comprise a second group of plant OMTs. These OMTs are most closely related to the mammalian catechol OMTs based on primary, secondary, and tertiary structure. The methyl transfer reaction mechanism is metal dependent, with the active site containing an octahedral metal binding site. The third family of plant OMTs consists primarily of enzymes that convert carboxylic acids to methyl ester derivatives. Salicylic acid carboxyl methyltransferase (SAMT) from Clarkia breweri provides the prototypical structure for this type of OMT. Because the substrate is deprotonated at physiological pH in the case of the carboxylates, transmethylation is achieved through proper positioning of the substrate within the active site with no general base or metal mediated deprotonation necessary. Database searches with the Clarkia SAMT amino acid sequence yield other MTs and numerous putative proteins, most annotated as jasmonic acid (JMT) or SAMT-like methyltransferases. In addition to carboxylate specific MTs, SAMT bears sequence similarity to the alkaloid N-methyltransferases (NMT) involved in caffeine biosynthesis. Thus, the carboxylate OMTs constitute a much larger structural family then originally anticipated, containing not only carboxyl group specific OMTs but also some alkaloid NMTs.