The biosynthesis of coumarin in Melilotus alba - …
The results (maximum target sequences: 1000; Supplementary Material ) showed that the hit sequences belonged to the 2OGD family, with maximum scores within the range of 42–111 and minimum E-values within the range of 1 e−27–1 e−2. The highest scoring hits were observed in the Brassicales plants. Although it was necessary to analyze the accumulation of simple coumarins, these clones would show ortho-hydroxylase activity, thus indicating its involvement in simple coumarin formation. Plant species belonged to Spindales, Malvales, Malpigiales, Fabales, Rosales, Fagales, Vitales, Solanales, Lamiales, Gentianales, and Asteriales also showed significantly high scores and low E-values, whereas other plant species with 2OGD sequences were of relatively lower levels of similarity. In plants that accumulate simple coumarins, 2OGDs with higher levels of similarity are likely to exhibit ortho-hydroxylase activity. In Fabales, Lotus japonicus, Glycine max, Vigna unguiculata, and Medicago truncatula harbored ESTs with highly similar sequences. Coumarin is accumulated in Melilotus alba, a Fabales plant (Brown et al., ; Stoker and Bellis, ; Gestetner and Conn, ). These EST sequences in Fabales plants could serve as clues in the search for ortho-hydroxylases in cinnamate (10) from M. alba. In addition, sequences from Euphorbia spp. or Manihot esculenta, which accumulate esculetin (Masamoto et al., ; Bayoumi et al., ; Nazemiyeh et al., ; Shi et al., ), showed high similarities. The biosynthetic pathway of simple coumarins containing esculetin in these plants would be elucidated through the functional analysis of these sequences. Species from the rest of the orders were less similar to the partial sequence of AtF6′H1.
Biosynthesis of coumarins in plants: a major pathway …
Taking advantage of these results, a TBLASTN search (; Altschul et al., ) was performed to explore candidate EST sequences of ortho-hydroxylases involved in the biosynthesis of simple coumarins, using the C-terminal sequences of AtF6′H1 (54 amino acid residues, Supplementary Material ).
Although the details of the biosynthesis of simple coumarins are still unclear, the three examples of ortho-hydroxylases serve as key information for future researches on elucidating the mechanism of coumarin biosynthesis in plants. Substrate specificities of the ortho-hydroxylases from plants that accumulate coumarins will be also clue to know the metabolic grid of coumarin biosynthesis.
Biosynthesis of Coumarin and Furanocoumarin Glycosides
Coumarins are natural plant products that have been the subject of extensive phytochemical and pharmacological research studies in the past few decades. The core structure of coumarins is derived from the respective cinnamates via ortho-hydroxylation of the aromatic ring, trans/cis isomerization, and lactonization. Various substitution patterns of coumarins have been reported, whereas the biosynthesis of coumarins remains elusive. Ortho-hydroxylation is a key step in simple coumarin biosynthesis as a branch point from the lignin biosynthetic pathway. 2-Oxoglutarate-dependent dioxygenases (2OGDs) from plants convert cinnamate derivatives into simple coumarins through the process of ortho-hydroxylation. This review describes the 2OGDs involved in coumarin biosynthesis and their substrate specificities.
of two enzymes involved in coumarin biosynthesis in Melilotus alba.
Chalcone synthase (E3-1), considered to be the rate-limiting enzyme in flavonoid synthesis, catalyzes the formation of the basic C15 skeleton and thus channels hydroxycinnamates into flavonoid biosynthesis. The enzyme condenses 4-coumaroyl- CoA with three molecules of malonyl-CoA to form 'naringenin chalcone' (2',4,4',6'-tetrahydroxychalcone; note the different numbering system compared to that of the other flavonoids). Chalcone synthase is a dimeric protein with Mr 78 0-8 0 and probably two identical subunits. The pH optimum of the reaction is in the range 7.5-8.5 for 4-coumaroyl-CoA. The enzyme from most sources is highly specific for 4-coumaroyl-CoA, although there are examples where other hydroxycinnamoyl-CoAs are accepted.
15/11/2006 · Biosynthesis of coumarins in plants: ..
As in the formation of flavonoidal pterocarpans, dimethylallyltransferase and cyclase activities (cytochrome P-450-monooxygenases) are involved in their biosyntheses. Although the crucial steps in these pathways have not yet been unambiguously elucidated, it is likely that oxidative cyclizations through epoxides lead to the different heterocycles attached to the hydroxycoumarins. A key enzyme in these biosyntheses is marmesin synthase (a cytochrome P-450-monooxygenase) which catalyzes the conversion of demethylsuberosin into marmesin (Fig. 10.8). This product is thought to be the pivotal precursor for both the furano- and the dihydropyranocoumarins .