Phenylpropanoids metabolism

The metabolic pathway of phenylpropanoids involves a number of enzymes.

Biosynthesis

Shikimate pathway

The shikimate pathway is a seven step metabolic route used by bacteria, fungi, and plants for the biosythesis of aromatic amino acids (phenylalanine, tyrosine, and tryptophan).

Cinnamic and p-coumaric acids biosynthesis

In plants, the biosynthesis of all phenylpropanoids begins with the amino acids phenylalanine and tyrosine.

Phenylalanine ammonia-lyase (PAL, a.k.a phenylalanine/tyrosine ammonia-lyase) is an enzyme responsible for the transformation of L-phenylalanine or tyrosine into trans-cinnamic acid or p-coumaric acid respectively and ammonia.

Trans-cinnamate 4-monooxygenase (cinnamate 4-hydroxylase) is the enzyme responsible for the transformation of trans-cinnamate into 4-hydroxycinnamate (p-coumaric acid). 4-Coumarate-CoA ligase is the enzyme responsible for the transformation of 4-coumarate (p-coumaric acid) into 4-coumaroyl-CoA.^[1]

Other hydroxycinnamic acids biosynthesis

• Cinnamyl-alcohol dehydrogenase (CAD), an enzyme responsible for the transformation of cinnamyl alcohol into cinnamaldehyde
• Sinapine esterase, an enzyme responsible for the transformation of sinapoylcholine into sinapate (sinapic acid) and choline
• Trans-cinnamate 2-monooxygenase, an enzyme responsible for the transformation of trans-cinnamate (cinnamic acid) into 2-hydroxycinnamate
• Caffeate O-methyltransferase, an enzyme responsible for the transformation of caffeic acid into ferulic acid
• Caffeoyl-CoA O-methyltransferase, an enzyme responsible for the transformation of caffeoyl-CoA into feruloyl-CoA
• 5-O-(4-coumaroyl)-D-quinate 3'-monooxygenase, an enzyme responsible for the transformation of trans-5-O-(4-coumaroyl)-D-quinate into trans-5-O-caffeoyl-D-quinate
• Sinapoylglucose—choline O-sinapoyltransferase, an enzyme responsible for the transformation of 1-O-sinapoyl-beta-D-glucose into sinapoylcholine (sinapine)
• Sinapoylglucose—malate O-sinapoyltransferase, an enzyme responsible for the transformation of 1-O-sinapoyl-beta-D-glucose into sinapoyl-(S)-malate

Conjugation enzymes

• 2-coumarate O-beta-glucosyltransferase, an enzyme responsible for the transformation of trans-2-hydroxycinnamate into trans-beta-D-glucosyl-2-hydroxycinnamate
• Cinnamoyl-CoA reductase, an enzyme responsible for the production of cinnamoyl-CoA from cinnamaldehyde
• Hydroxycinnamate 4-beta-glucosyltransferase, an enzyme responsible for the transformation of p-coumaric acid into 4-O-beta-D-glucosyl-4-hydroxycinnamate
• Shikimate O-hydroxycinnamoyltransferase, an enzyme responsible for the transformation of 4-coumaroyl-CoA into 4-coumaroylshikimate
• Quinate O-hydroxycinnamoyltransferase, an enzyme responsible for the transformation of feruloyl-CoA into O-feruloylquinate
• Sinapate 1-glucosyltransferase, an enzyme responsible for the transformation of sinapate (sinapic acid) into 1-sinapoyl-D-glucose
• Coniferyl-alcohol glucosyltransferase, an enzyme responsible for the transformation of coniferyl alcohol into coniferin

Glucosidases

• Coniferin beta-glucosidase, an enzyme responsible for the transformation of coniferin into coniferol

Stilbenoids biosynthesis

• Pinosylvin synthase, an enzyme responsible for the formation of pinosylvin from cinnamoyl-CoA
• Trihydroxystilbene synthase, an enzyme responsible for the production of resveratrol from 4-coumaroyl-CoA

An alternative bacterial ketosynthase-directed stibenoids biosynthesis pathway exists in Photorhabdus bacterial symbionts of Heterorhabditis nematodes, producing 3,5-dihydroxy-4-isopropyl-trans-stilbene for antibiotic purposes.^[2]

Coumarins biosynthesis

• Scopoletin glucosyltransferase, an enzyme responsible for the transformation of scopoletin into scopolin

Chalcones biosynthesis

4-Coumaroyl-CoA can be combined with malonyl-CoA to yield the true backbone of flavonoids, a group of compounds called chalconoids, which contain two phenyl rings. Naringenin-chalcone synthase is an enzyme that catalyzes the chemical reaction 3 malonyl-CoA + 4-coumaroyl-CoA → 4 CoA + naringenin chalcone + 3 CO₂.

Flavonoids biosynthesis

Conjugate ring-closure of chalcones results in the familiar form of flavonoids, the three-ringed structure of a flavone.

Biodegradation

Hydroxycinnamic acids degradation

• Caffeate 3,4-dioxygenase is an enzyme that uses 3,4-dihydroxy-trans-cinnamate (caffeic acid) and oxygen to produce 3-(2-carboxyethenyl)-cis,cis-muconate.

References