Multifunctional 5-hydroxyconiferaldehyde O-methyltransferases (CAldOMTs) in plant metabolism

Lignin, flavonoids, melatonin, and stilbenes are plant specialized metabolites with diverse physiological and biological functions, supporting plant growth and conferring stress resistance. Their biosynthesis requires O-methylations catalyzed by 5-hydroxyconiferaldehyde O-methyltransferase (CAldOMT; also called caffeic acid O-methyltransferase, COMT). CAldOMT was first known for its roles in syringyl (S) lignin biosynthesis in angiosperm cell walls and later found to be multifunctional. This enzyme also catalyzes O-methylations in flavonoid, melatonin, and stilbene biosynthetic pathways. Phylogenetic analysis indicated the convergent evolution of enzymes with OMT activities towards the monolignol biosynthetic pathway intermediates in some gymnosperm species that lack S-lignin and Selaginella moellendorffii, a lycophyte which produces S-lignin. Furthermore, neofunctionalization of CAldOMTs occurred repeatedly during evolution, generating unique O-methyltransferases (OMTs) with novel catalytic activities and/or accepting novel substrates, including lignans, 1,2,3-trihydroxybenzene, and phenylpropenes. This review summarizes multiple aspects of CAldOMTs and their related proteins in plant metabolism and discusses their evolution, molecular mechanism, and roles in biorefineries, agriculture, and synthetic biology.

Diversification of Chemical Structures of Methoxylated Flavonoids and Genes Encoding Flavonoid-O-Methyltransferases

The O-methylation of specialized metabolites in plants is a unique decoration that provides structural and functional diversity of the metabolites with changes in chemical properties and intracellular localizations. The O-methylation of flavonoids, which is a class of plant specialized metabolites, promotes their antimicrobial activities and liposolubility. Flavonoid O-methyltransferases (FOMTs), which are responsible for the O-methylation process of the flavonoid aglycone, generally accept a broad range of substrates across flavones, flavonols and lignin precursors, with different substrate preferences. Therefore, the characterization of FOMTs with the physiology roles of methoxylated flavonoids is useful for crop improvement and metabolic engineering. In this review, we summarized the chemodiversity and physiology roles of methoxylated flavonoids, which were already reported, and we performed a cross-species comparison to illustrate an overview of diversification and conserved catalytic sites of the flavonoid O-methyltransferases.

Rational Design of Resveratrol O-methyltransferase for the Production of Pinostilbene

Pinostilbene is a monomethyl ether analog of the well-known nutraceutical resveratrol. Both compounds have health-promoting properties, but the latter undergoes rapid metabolization and has low bioavailability. O-methylation improves the stability and bioavailability of resveratrol. In plants, these reactions are performed by O-methyltransferases (OMTs). Few efficient OMTs that monomethylate resveratrol to yield pinostilbene have been described so far. Here, we report the engineering of a resveratrol OMT from Vitis vinifera (VvROMT), which has the highest catalytic efficiency in di-methylating resveratrol to yield pterostilbene. In the absence of a crystal structure, we constructed a three-dimensional protein model of VvROMT and identified four critical binding site residues by applying different in silico approaches. We performed point mutations in these positions generating W20A, F24A, F311A, and F318A variants, which greatly reduced resveratrol's enzymatic conversion. Then, we rationally designed eight variants through comparison of the binding site residues with other stilbene OMTs. We successfully modified the native substrate selectivity of VvROMT. Variant L117F/F311W showed the highest conversion to pinostilbene, and variant L117F presented an overall increase in enzymatic activity. Our results suggest that VvROMT has potential for the tailor-made production of stilbenes.

Function Analysis of Caffeoyl-CoA O-Methyltransferase for Biosynthesis of Lignin and Phenolic Acid in Salvia miltiorrhiza

In this study, we cloned a full-length cDNA and the genomic DNA sequence of SmCCoAOMT (GenBank ID JQ007585) from Salvia miltiorrhiza. The 744-bp open-reading frame encodes a protein of 247 amino acids that shares 95 % similarity with one in Vitis vinifera. Real-time quantitative PCR analysis revealed that SmCCoAOMT is most highly expressed in the stems and can be induced by methyl jasmonate (MeJA) and XC-1 treatment. To evaluate its function in vivo, we generated RNA interference transgenic plants through Agrobacterium tumefaciens-mediated gene transfer. Compared with untransformed control plants, the transgenics had significantly less lignin and the expression of lignin-biosynthetic genes SmCCR and SmCOMT was depressed. In 90-day-old roots from plants of transgenic line M5, accumulations of rosmarinic acid and salvianolic acid B (Sal B) were greatly reduced by 0.89- and 0.69-fold, respectively. This low-Sal B phenotype was stable in the roots, with the level of accumulation being approximately 43.58 mg g^-1 dry weight, which was 52 % of the amount measured in the untransformed control. Our results suggest that SmCCoAOMT is involved in lignin biosynthesis and affects the accumulation of phenolic acids. This study also provides potential guidance for using lignin-related genes to genetically engineer Salvia miltiorrhiza.

Methylation of flavonoids: Chemical structures, bioactivities, progress and perspectives for biotechnological production

Among the natural products, flavonoids have been particularly attractive, highly studied and become one of the most important promising agent to treat cancer, oxidant stress, pathogenic bacteria, inflammations, cardio-vascular dysfunctions, etc. Despite many promising roles of flavonoids, expectations have not been fulfilled when studies were extended to the in vivo condition, particularly in humans. Instability and very low oral bioavailability of dietary flavonoids are the reasons behind this. Researches have demonstrated that the methylation of these flavonoids could increase their promise as pharmaceutical agents leading to novel applications. Methylation of the flavonoids via theirs free hydroxyl groups or C atom dramatically increases their metabolic stability and enhances the membrane transport, leading to facilitated absorption and highly increased oral bioavailability. In this paper, we concentrated on analysis of flavonoid methoxides including O- and C-methoxide derivatives in aspect of structure, bioactivities and description of almost all up-to-date O- and C-methyltransferases' enzymatic characteristics. Furthermore, modern biological approaches for synthesis and production of flavonoid methoxides using metabolic engineering and synthetic biology have been focused and updated up to 2015. This review will give a handful information regarding the methylation of flavonoids, methyltransferases and biotechnological synthesis of the same.

Structure-function analyses and molecular modeling of caffeic acid-O-methyltransferase and caffeoyl-CoA-O-methyltransferase: revisiting the basis of alternate methylation pathways during monolignol biosynthesis

Ten protein sequences, each of caffeic acid-O-methyltransferase (COMT) and caffeoyl-coenzyme A-O-methyltransferase (CCoAOMT), catalyzing methylation of precursors of monolignol from selected dicots and monocots have been analyzed and compared on the basis of their amino acid sequence, motifs/domains, three-dimensional (3D) structure, and substrate binding. The isoelectric points of all the COMT and CCoAOMT sequences analyzed were found to vary in the pH range of 5 to 6. Molecular weight analyses suggested CCoAOMT to be smaller monomeric proteins (27-29 kDa) as compared with those of COMTs (39-40 kDa), which were dimeric. On the basis of phylogenetic analysis, COMT and CCoAOMT were clustered into two major groups, each of which could be further divided into two subgroups of monocots and dicots. Modeling and superimposition of COMT and CCoAOMT sequences of alfalfa (Medicago sativa) revealed that both were quite different at the 3D levels, although they had similarity in the core region. Molecular docking of 16 putative substrates (intermediates of monolignol biosynthesis pathway) revealed that both enzymes interact with all 16 substrates in a similar manner, with thiol esters being the most potent and binding of these putative substrates to CCoAOMT being more efficient.