Daidzein (7-hydroxy-3-(4-hydroxyphenyl)-4H-chromen-4-one) is a naturally occurring compound found exclusively in soybeans and other legumes, and structurally belongs to a class of compounds known as isoflavones. Daidzein and other isoflavones are produced in plants through the phenylpropanoid pathway of secondary metabolism and are used as signal carriers, and defense responses to pathogenic attacks. Upon consumption of isoflavone-rich foods, daidzein has poor bioavailability and low water solubility.
Total isoflavones in soybeans are—in general—37 percent daidzein, 57 percent genistein and 6 percent glycitein, according to USDA data. Soy germ contains 41.7 percent daidzein.
The biosynthesis of daidzein begins with L-phenylalanine and undergoes a general phenylpropanoid pathway where the shikimate derived aromatic ring is shifted to the adjacent carbon of the heterocycle. The process begins with phenylalanine ligase (PAL) cleaving the amino group from L-Phe forming the unsaturated carboxylic acid, cinnamic acid. Cinnamic acid is then hydroxylated by membrane protein cinnamate-4-hydroxylase (C4H) to form p-coumaric acid. P-coumaric acid then acts as the starter unit which gets loaded with coenzyme A by 4-coumaroyl:CoA-ligase (4CL). The starter unit (A) then undergoes three iterations of malonyl-CoA resulting in (B), which enzymes chalcone synthase (CHS) and chalcone reductase (CHR) modify to obtain trihydroxychalcone. CHR is NADPH dependent. Chalcone isomerase (CHI) then isomerizes trihydroxychalcone to liquiritigenin, the precursor to daidzein.
A radical mechanism has been proposed in order to obtain daidzein from liquiritigenin, where an iron-containing enzyme, as well as NADPH and oxygen cofactors are used by a 2-hydroxyisoflavone synthase to oxidize liquiritigenin to a radical intermediate (C). A 1,2 aryl migration follows to form (D), which is subsequently oxidized to (E). Lastly, dehydration of the hydroxy group on C2 occurs through a 2-hydroxyisoflavanone dehydratase (specifically GmHID1) to give daidzein.
|
|