Flavonol 3-O-glycoside hydroxycinnamoyltransferases from Scots pine (Pinus sylvestris L.)

Protein Engineering of PhUGT, a Donor Promiscuous Glycosyltransferase, for the Improved Enzymatic Synthesis of Antioxidant Quercetin 3-O-N-Acetylgalactosamine

Quercetin 3-O-N-acetylgalactosamine (Q3GalNAc), a derivative of dietary hyperoside, had never been enzymatically synthesized due to the lack of well-identified N-acetylgalactosamine-transferase (GalNAc-T). Herein, PhUGT, an identified flavonoid 3-O-galactosyltransferase from Petunia hybrida, was demonstrated to display quercetin GalNAc-T activity, transferring a N-acetylgalactosamine (GalNAc) from UDP-N-acetylgalactosamine (UDP-GalNAc) to the 3-OH of quercetin to form Q3GalNAc with a low conversion of 11.7% at 40 °C for 2 h. Protein engineering was thus performed, and the resultant PhUGT variant F368T got an enhanced conversion of 75.5% toward UDP-GalNAc. The enzymatically synthesized Q3GalNAc exhibited a comparable antioxidant activity with other quercetin 3-O-glycosides. Further studies revealed that PhUGT was a donor promiscuous glycosyltransferase (GT), recognizing seven sugar donors. This finding overturned a previous notion that PhUGT exclusively recognized UDP-galactose (UDP-Gal). The reason why PhUGT was mistaken for a UDP-Gal-specific GT was demonstrated to be a shorter reaction time, in which many quercetin 3-O-glycosides, except hyperoside, could not be effectively synthesized. The fact that the microbial cell factory expressing PhUGT could yield an array of Q3Gs further confirmed the donor promiscuity of PhUGT. This study laid a foundation for the scale production of Q3GalNAc and provided a potent biocatalyst capable of glycodiversifying quercetin as well.

Quantitative Changes of Flavonol Glycosides from Pine Needles by Cultivar, Harvest Season, and Thermal Process

Five flavonol glycosides including quercetin 3-O-β-D-glucoside (QG), kaempferol 3-O-β-D-glucoside (KG), quercetin 3-O-(6″-O-acetyl)-β-D-glucoside (QAG), kaempferol 3-O-(6″-O-acetyl)-β-D-glucoside (KAG), and quercetin 3-O-(3″-O-p-coumaroyl)-β-D-glucoside (QCG) were isolated and purified from red pine (Pinus densiflora Sieb. et Zucc.) nee-dles, and identified by nuclear magnetic resonance and mass spectrometer spectral analyses. In addition, the quantification of the five flavonol glycosides in pine needles was performed by high-performance liquid chromatography analysis according to cultivar, growing district, harvest season, and thermal processing. The red pine needles had higher amounts of the five flavonol glycosides than the black pine needles except for QCG. There were no large differences in flavonoid composition and content among pine needles grown in three different areas. Levels of the five flavonol glycosides in red pine needles harvested during Spring ranged from 6.13 to 27.03 mg/100 g dry weight. Levels of two flavonol glycosides, QG and KG, gradually decreased with increasing harvest time, whereas the acylated flavonol glycoside, QCG, a predominant flavo-noid in pine needles, increased gradually with increasing harvest time. Two acetyl flavonol glycosides, QAG and KAG, increased steadily through Spring to Autumn, and then decreased gradually by Winter. Meanwhile heat treatments, such as roasting and steaming, increased the five flavonol glycosides during heating for 3 min, but then slowly decreased these when heating for 10 min. Microwave processing increased to some extent the five flavonol glycosides when heating for 3 min, and remained unchanged during the 10 min heating. These results suggest that the pretreated red pine needles with enhanced flavonoid content may be useful as potential sources for nutraceuticals and cosmeceuticals.

Current understanding of the pathways of flavonoid biosynthesis in model and crop plants

Flavonoids are a signature class of secondary metabolites formed from a relatively simple collection of scaffolds. They are extensively decorated by chemical reactions including glycosylation, methylation, and acylation. They are present in a wide variety of fruits and vegetables and as such in Western populations it is estimated that 20-50 mg of flavonoids are consumed daily per person. In planta they have demonstrated to contribute to both flower color and UV protection. Their consumption has been suggested to presenta wide range of health benefits. Recent technical advances allowing affordable whole genome sequencing, as well as a better inventory of species-by-species chemical diversity, have greatly advanced our understanding as to how flavonoid biosynthesis pathways vary across species. In parallel, reverse genetics combined with detailed molecular phenotyping is currently allowing us to elucidate the functional importance of individual genes and metabolites and by this means to provide further mechanistic insight into their biological roles. Here we provide an inventory of current knowledge of pathways of flavonoid biosynthesis in both the model plant Arabidopsis thaliana and a range of crop species, including tomato, maize, rice, and bean.

The O-methyltransferase PMT2 mediates methylation of pinosylvin in Scots pine

Heartwood extractives are important determinants of the natural durability of pine heartwood. The most important phenolic compounds affecting durability are the stilbenes pinosylvin and its monomethylether, which in addition have important functions as phytoalexins in active defense. A substantial portion of the synthesized pinosylvin is 3-methoxylated but the O-methyltransferase responsible for this modification has not been correctly identified. We studied the expression of the stilbene pathway during heartwood development as well as in response to wounding of xylem and UV-C treatment of needles. We isolated and enzymatically characterized a novel O-methyltransferase, PMT2. The methylated product was verified as pinosylvin monomethylether using ultra performance liquid chromatography-tandem mass spectrometry and high performance liquid chromatography analyses. The PMT2 enzyme was highly specific for stilbenes as substrate, in contrast to caffeoyl-CoA O-methyltransferase (CCoAOMT) and PMT1 that were multifunctional. Expression profile and multifunctional activity of CCoAOMT suggest that it might have additional roles outside lignin biosynthesis. PMT1 is not involved in the stilbene pathway and its biological function remains an open question. We isolated a new specific O-methyltransferase responsible for 3-methoxylation of pinosylvin. Expression of PMT2 closely follows stilbene biosynthesis during developmental and stress induction. We propose that PMT2 is responsible for pinosylvin methylation in Scots pine (Pinus sylvestris), instead of the previously characterized methyltransferase, PMT1.

Ectopic expression of snapdragon transcription factors facilitates the identification of genes encoding enzymes of anthocyanin decoration in tomato

Given the potential health benefits of polyphenolic compounds in the diet, there is a growing interest in the generation of food crops enriched with health-protective flavonoids. We undertook a series of metabolite analyses of tomatoes ectopically expressing the Delila and Rosea1 transcription factor genes from snapdragon (Antirrhinum majus), paying particular attention to changes in phenylpropanoids compared to controls. These analyses revealed multiple changes, including depletion of rutin and naringenin chalcone, and enhanced levels of anthocyanins and phenylacylated flavonol derivatives. We isolated and characterized the chemical structures of the two most abundant anthocyanins, which were shown by NMR spectroscopy to be delphinidin-3-(4'''-O-trans-p-coumaroyl)-rutinoside-5-O-glucoside and petunidin-3-(4'''-O-trans-p-coumaroyl)-rutinoside-5-O-glucoside. By performing RNA sequencing on both purple fruit and wild-type fruit, we obtained important information concerning the relative expression of both structural and transcription factor genes. Integrative analysis of the transcript and metabolite datasets provided compelling evidence of the nature of all anthocyanin biosynthetic genes, including those encoding species-specific anthocyanin decoration enzymes. One gene, SlFdAT1 (Solyc12g088170), predicted to encode a flavonoid-3-O-rutinoside-4'''-phenylacyltransferase, was characterized by assays of recombinant protein and over-expression assays in tobacco. The combined data are discussed in the context of both our current understanding of phenylpropanoid metabolism in Solanaceous species, and evolution of flavonoid decorating enzymes and their transcriptional networks in various plant species.

Environmental and developmental effects on the biosynthesis of UV-B screening pigments in Scots pine (Pinus sylvestris L.) needles

The major UV-B screening pigments of the epidermal layer of Scots pine (Pinus sylvestris) needles are flavonol 3-o-glycosides (F3Gs) esterified with hydroxycinnamic acids at positions 3" and 6". Acylation is the last step in biosynthesis and is catalysed by position-specific hydroxycinnamoyl transferases (3" and 6"HCT). The UV-B dependence of these enzyme activities was studied in primary needles of Scots pine seedlings grown under different UV-B conditions in environmentally controlled sun simulators. 6"HCT activity was induced upon UV-B irradiation while 3"HCT activity was not induced but showed high constitutive values. To investigate the biosynthesis of diacylated F3Gs during needle development under natural conditions, the HCT activities and metabolite contents were analysed in needles of field-grown mature pine trees. Accumulation of diacylated compounds as well as of 6"HCT activity occurred transiently in the first year of needle development only. In contrast, 3"HCT activity exhibited broad maxima in two consecutive years during needle growth. The data suggest that acylated F3Gs are first formed as soluble compounds which are then translocated into the cell wall to be bound by their hydroxycinnamoyl residues.

Acyltransferases in plants: a good time to be BAHD

Acylation is a common and biochemically significant modification of plant secondary metabolites. Plant BAHD acyltransferases constitute a large family of acyl CoA-utilizing enzymes whose products include small volatile esters, modified anthocyanins, as well as constitutive defense compounds and phytoalexins. The catalytic versatility of BAHD enzymes makes it very difficult to make functional predictions from primary sequence alone. Recent advances in genome sequencing and the availability of the first crystal structure of a BAHD member are, however, providing insights into the evolution and function of these acyltransferases within the plant kingdom.