Biosynthesis

The biosynthesis of phenolic compounds occurs via various pathways. In recent years, considerable progress has been made in elucidating the biosynthesis of these compounds due to our rapidly developing knowledge of enzymology, and the improved quantification of substrates and products by high-performance liquid chromatography, mass spectrometry, and nuclear magnetic resonance spectroscopy. All flavonoids derive their carbon skeleton from two basic compounds, malonyl-CoA and the CoA ester of a hy-

droxycinnamic acid (10). The origins of the direct flavonoid precursors, 4-coumaroyl-CoA and malonyl-CoA are derived from carbohydrates (Fig. 2). Malonyl-CoA is synthesized from the glycolysis intermediate acetyl-CoA and carbon dioxide, the reaction being catalyzed by acetyl-CoA caboxylase. The synthesis of 4-coumaroyl-CoA involves the shikimate/arogenate pathway, which is the main route to the aromatic amino acids, phenylalanine, tryptophan, and tyrosine in higher plants (10,11). The condensation of three molecules of malonyl-CoA with 4-coumaroyl-CoA to the C15 chalcone is catalyzed by chalcone synthase. Transformation by chalcone isomerase produces naringenin (flavo-none) and other flavonoids via various steps. Dihydroflavo-nols formed by direct hydroxylation of flavanones in the 3 position are biosynthetic intermediates in the formation of flavonols, catechins, proanthocyanidins, and anthocyani-dins.

Modification by hydroxylation of the A- and B-ring, methylation of hydroxyl groups as well as glycosylation and acylation reactions result in the immense diversity of flavonoids found in nature (10). The detailed biosynthesis steps of various flavonoids have been extensively described (12). Recent advances on flavonoid biosynthesis were restricted to specific steps and filled some of the remaining gaps in the metabolic pathway. Yet the key reaction to the anthocyanins as well as the formation of epicatechin, the proanthocyanidins, and some minor flavonoid-related compounds, such as aurones and dihydrochalcones, still remains a matter of debate (12).

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