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Within the secondary metabolite class of flavonoids which consist of more than 9000 known structures, flavones define one of the largest subgroups. Their natural distribution is demonstrated for almost all plant tissues. Various flavone aglyca and their O- or C-glycosides have been described in the literature. The diverse functions of flavones in plants as well as their various roles in the interaction with other organisms offer many potential applications, not only in plant breeding but also in ecology, agriculture and human nutrition and pharmacology. In this context, the antioxidative activity of flavones, their use in cancer prevention and treatment as well as the prevention of coronary heart disease should be emphasized. The therapeutic potential of flavones makes these compounds valuable targets for drug design, including recombinant DNA approaches. The biosynthesis of flavones in plants was found to be catalyzed by two completely different flavone synthase proteins (FNS), a unique feature within the flavonoids. The first, FNS I, a soluble dioxygenase, was only described for members of the Apiaceae family so far. The second, FNS II, a membrane bound cytochrome P450 enzyme, has been found in all other flavone accumulating tissues. This phenomenon is particularly of interest from the evolutionary point of view concerning the flavone biosynthesis and functions in plants. Recently, FNS I and FNS II genes have been cloned from a number of plant species. This now enables detailed biochemical and molecular characterizations and also the development of direct metabolic engineering strategies for modifications of flavone synthesis in plants to improve their nutritional and/or biopharmaceutical value.
The flavones can be classified into several subgroups which are mainly indicated either by (i) hydroxylation, (ii) O-methylation, (iii) C-methylation, (iv) isoprenylation, or (v) methylenedioxy substitution. Besides the aglycon structures, O- and C-glycosides are well known. Flavones mostly occur as 7-O-glycosides, but substitution can be found at any other hydroxylated position. Chemically, flavonols are simply 3-hydroxyflavones. However, as flavones and flavonols are biosynthetically distinct flavonoid classes, only flavones will be discussed in the following. Because combinations of various modifications can occur, the number of different flavones is enormous. In 1999, Harborne and Baxter already listed more that 350 flavones and about 500 flavone glycosides indicating the high diversity of this flavonoid class (for an overview of the structures described see: Wollenweber, 1994, Harborne and Baxter, 1999 and Williams and Grayer, 2004). Flavones can be found in all parts of the plants, above- and belowground, in vegetative and generative organs: stem, leaves, buds, bark, heartwood, thorns, roots, rhizomes, flowers, farina, fruit, seeds, and also in root and leaf exudates or resin.
The flavones can be classified into several subgroups which are mainly indicated either by (i) hydroxylation, (ii) O-methylation, (iii) C-methylation, (iv) isoprenylation, or (v) methylenedioxy substitution. Besides the aglycon structures, O- and C-glycosides are well known. Flavones mostly occur as 7-O-glycosides, but substitution can be found at any other hydroxylated position. Chemically, flavonols are simply 3-hydroxyflavones. However, as flavones and flavonols are biosynthetically distinct flavonoid classes, only flavones will be discussed in the following. Because combinations of various modifications can occur, the number of different flavones is enormous. In 1999, Harborne and Baxter already listed more that 350 flavones and about 500 flavone glycosides indicating the high diversity of this flavonoid class (for an overview of the structures described see: Wollenweber, 1994, Harborne and Baxter, 1999 and Williams and Grayer, 2004). Flavones can be found in all parts of the plants, above- and belowground, in vegetative and generative organs: stem, leaves, buds, bark, heartwood, thorns, roots, rhizomes, flowers, farina, fruit, seeds, and also in root and leaf exudates or resin.