There is growing recognition that many phenolic secondary metabolites present in foodstuffs may possibly exert beneficial effects on human health. This may to some degree be mediated via antioxidant actions, but a range of more specific pharmacological effects have also been proposed. Given this background, there may be favourable consequences for the general health of Western populations as a result of optimising the phenolic content of the diet. This paper reviews what is known of the function of phenolics both in the plant and in man. It also describes current understanding of the biosynthesis of phenolics in plants, with emphasis on where potential controlling steps may exist. Finally, advances in identification and isolation of the genes coding for phenolic biosynthetic enzymes or regulatory proteins are also summarised. Taken together, this information provides a basis for attempts to modify and optimise the phenolic content of food crops, using either conventional plant breeding along with manipulation of agronomic practices, or else the more targeted approaches of modern molecular biology.



© 2000 Society of Chemical Industry

Phenols in the plant and in man. The potential for possible nutritional enhancement of the diet by modifying the phenols content or profile

Isoflavones have drawn much attention because of their benefits to human health. These compounds, which are produced almost exclusively in legumes, have natural roles in plant defense and root nodulation. Isoflavone synthase catalyzes the first committed step of isoflavone biosynthesis, a branch of the phenylpropanoid pathway. To identify the gene encoding this enzyme, we used a yeast expression assay to screen soybean ESTs encoding cytochrome P450 proteins. We identified two soybean genes encoding isoflavone synthase, and used them to isolate homologous genes from other leguminous species including red clover, white clover, hairy vetch, mung bean, alfalfa, lentil, snow pea, and lupine, as well as from the nonleguminous sugarbeet. We expressed soybean isoflavone synthase in Arabidopsis thaliana, which led to production of the isoflavone genistein in this nonlegume plant. Identification of the isoflavone synthase gene should allow manipulation of the phenylpropanoid pathway for agronomic and nutritional purposes.

Identification and expression of isoflavone synthase, the key enzyme for biosynthesis of isoflavones in legumes.

Complex reactions catalyzed by cytochrome P450 enzymes.

A rapid and substantial progress to a broad understanding of flavonoid biosynthesis marked the previous reviews of this series of books (Hahlbrock and Grisebach, 1975; Ebel and Hahlbrock, 1982; Heller and Forkmann, 1988), and several other reviews have also appeared (Harborne, 1988; Zaprometov, 1989; Stafford, 1990). Advances achieved in the past few years are restricted to specific steps, filling some of the remaining gaps in this metabolic pathway. Among these reactions are the formation of 3-deoxyanthocyanins and 5-deoxyflavonoids, and the synthesis of the structurally derived pterocarpans, which represent the most prominent group of flavonoid phytoalexins. Among the flavonoid modifications, enzymatic formation of flavonoid sulfate esters and aromatic acylation of anthocyanins with hydroxycinnamic acid glucose esters as donors can be mentioned. Yet the key reaction to one of the main flavonoid classes, 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.

Biosynthesis of flavonoids

Chalcone O-methyltransferase (ChOMT) and isoflavone O-methyltransferase (IOMT) are S-adenosyl-l-methionine (SAM) dependent plant natural product methyltransferases involved in secondary metabolism in Medicago sativa (alfalfa). Here we report the crystal structure of ChOMT in complex with the product S-adenosyl-l-homocysteine and the substrate isoliquiritigenin (4,2′,4′-trihydroxychalcone) refined to 1.8 A as well as the crystal structure of IOMT in complex with the products S-adenosyl-l-homocysteine and isoformononetin (4′-hydroxy-7-methoxyisoflavone) refined to 1.4 A. These two OMTs constitute the first plant methyltransferases to be structurally characterized and reveal a novel oligomerization domain and the molecular determinants for substrate selection. As such, this work provides a structural basis for understanding the substrate specificity of the diverse family of plant OMTs and facilitates the engineering of novel activities in this extensive class of natural product biosynthetic enzymes.

Structures of two natural product methyltransferases reveal the basis for substrate specificity in plant O-methyltransferases.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/0014-5793%2890%2980410-K

Reaction mechamism of oxidative rearrangement of flavanone in isoflavone biosynthesis

P-450-Dependent oxidative rearrangement in isoflavone biosynthesis: Reconstitution of P-450 and NADPH:P-450 reductase

The reaction mechanism of oxidative rearrangement in the conversion of liquiritigenin, a flavanone, into 2,7,4′-trihydroxyisoflavanone was studied in elicitor-challenged Pueraria lobata cell cultures. The involvement of cytochrome P-450 in the reaction, hydroxylation associated with 1,2-aryl migration, was proved previously by the inhibition experiments with carbon monoxide and P-450 inhibitors. In order to obtain rigorous evidence proving that the enzyme is a P-450, a reconstitution experiment was performed with solubilized cytochrome P-450 and NADPH:cytochrome P-450 reductase fractions. During these studies we noticed that various biosynthetic reactions can be interpreted as P-450-mediated reactions associated with migration or bond cleavage. Ring contraction of 7-hydroxy-kaurenoic acid in gibberellin biosynthesis, the formation of a furan ring in furanocoumarin biosynthesis and several rearrangement reactions in steroid metabolism are discussed as examples of P-450 reactions associated with migration or bond cleavage.

Muhammed Faisal Hashim

Papers

Reaction mechamism of oxidative rearrangement of flavanone in isoflavone biosynthesis

P-450-Dependent oxidative rearrangement in isoflavone biosynthesis: Reconstitution of P-450 and NADPH:P-450 reductase

P‐450‐Dependent Oxidative Rearrangement in Isoflavone Biosynthesis: Reconstitution of P‐450 and NADPH:P‐450 Reductase.