Apigenin is biosynthetically derived from the general
phenylpropanoid pathway and the flavone synthesis pathway. The phenylpropanoid pathway starts from the aromatic amino acids L-phenylalanine or
L-tyrosine, both products of the
Shikimate pathway. When starting from L-phenylalanine, first the amino acid is non-oxidatively deaminated by
phenylalanine ammonia lyase to make cinnamate, followed by oxidation at the
para position by
cinnamate 4-hydroxylase to produce
p-coumarate. As L-tyrosine is already oxidized at the
para position, it skips this oxidation and is simply deaminated by
tyrosine ammonia lyase to arrive at
p-coumarate. To complete the general phenylpropanoid pathway,
4-coumarate CoA ligase substitutes coenzyme A at the carboxy group of
p-coumarate. Entering the flavone synthesis pathway, the type III
polyketide synthase enzyme
chalcone synthase uses consecutive condensations of three equivalents of
malonyl-CoA followed by aromatization to convert
p-coumaroyl-CoA to
chalcone.
Chalcone isomerase then isomerizes the product to close the pyrone ring to make
naringenin. Finally, a flavanone synthase enzyme oxidizes naringenin to apigenin. Two types of flavone synthase (FNS) have been described; FNS I, a soluble enzyme that uses 2-oxogluturate, Fe2+, and ascorbate as cofactors and FNS II, a membrane bound, NADPH dependent cytochrome p450 monooxygenase. == Glycosides ==