P-Coumaric acid


p-Coumaric acid is a hydroxycinnamic acid, an organic compound that is a hydroxy derivative of cinnamic acid. There are three isomers of coumaric acid—o-coumaric acid, m-coumaric acid, and p-coumaric acid—that differ by the position of the hydroxy substitution of the phenyl group. p-Coumaric acid is the most abundant isomer of the three in nature. p-Coumaric acid exists in two forms trans-p-coumaric acid and cis-p-coumaric acid''.
It is a crystalline solid that is slightly soluble in water, but very soluble in ethanol and diethyl ether.

Natural occurrences

p-Coumaric acid can be found in Gnetum cleistostachyum.

In food

p-Coumaric acid can be found in a wide variety of edible plants and fungi such as peanuts, navy beans, tomatoes, carrots, basil and garlic. It is found in wine and vinegar. It is also found in barley grain.
p-Coumaric acid from pollen is a constituent of honey.

Derivatives

can also be found in commercial breads containing flaxseed.
Diesters of p-coumaric acid can be found in carnauba wax.

Metabolism

Biosynthesis

It is biosynthesized from cinnamic acid by the action of the P450-dependent enzyme 4-cinnamic acid hydroxylase.
It is also produced from L-tyrosine by the action of tyrosine ammonia lyase.

Biochemistry

p-Coumaric acid is the precursor of 4-ethylphenol produced by the yeast Brettanomyces in wine. The yeast converts this to 4-vinylphenol via the enzyme cinnamate decarboxylase. 4-Vinylphenol is further reduced to 4-ethylphenol by the enzyme vinyl phenol reductase. Coumaric acid is sometimes added to microbiological media, enabling the positive identification of Brettanomyces by smell.
cis-p-Coumarate glucosyltransferase is an enzyme that uses uridine diphosphate glucose and cis-p-coumarate to produce 4′-O-β-D-glucosyl-cis-p-coumarate and uridine diphosphate. This enzyme belongs to the family of glycosyltransferases, specifically the hexosyltransferases.
Phloretic acid is found in the rumen of sheep fed with dried grass and is produced by hydrogenation of the 2-propenoic side chain of p-coumaric acid.
p-Coumaric acid serves as a photosensory chromophore when bound to a PYP domain.

As a cofactor of photoactive yellow proteins

p-Coumaric acid is the main enzyme cofactor of photoactive yellow proteins which are a homologous group of proteins found in many eubacteria. The importance of p-coumaric acid bound to PYP is due to its relative ease of forming stable crystals that diffract well for x-ray crystallography experiments, allowing biochemists and biophysicists to study the role of hydrogen bonding, molecular isomerization and photoactivity as biochemical phenomenon on a less complex scale than is present in other more complicated photosensitive proteins such as rhodopsin and cofactor retinal.

Photochemical transitions

It was originally believed that due to light emissions resembling that of retinal bound rhodopsin, the photosensor molecule bound to PYP should resemble the structure of retinal bound rhodopsin, the photosensor molecule bound to PYP should resemble the structure of retinal. Scientists were therefore amazed when the PYP Cys 69 was bound by a thiol ester linkage as the light sensitive prosthetic group p-coumaric acid. During the photoreactive mechanism:
  1. Light absorption yields the native protein to absorb a maximum wavelength of 446 nm, ε = 45500 M−1 cm−1.
  2. Within a nanosecond the absorbed maximum wavelength is shifted to 465 nm.
  3. Then on a sub-millisecond timescale is excited to a 355 nm state.
due to absorption of light causing photochemical transitions. To the right demonstrates the molecular structure of p-coumaric acid in the ligand binding site and hydrogen bonding interactions involved in the innate oxyanion hole. To the left is a crystallographic image of p-coumaric acid in the ligand binding site based on PyMOL rendering.
These observed phenomena are due to the trans–cis isomerization of the vinyl trans double bond in the p-coumaric acid. Scientists noted by observing the crystal structure of p-coumaric acid bound by PYP that the hydroxyl group connected to the C4 carbon of the phenyl ring appeared to be deprotonated – effectively a phenolate functional group. This was due to abnormally short hydrogen bonding lengths observed in the protein crystal structure.

Role of hydrogen bonding

s in proteins such as PYP take part in interrelated networks, where at the center of p-coumaric acid's phenolate O4 atom, there is an oxyanion hole that is crucial for photosensory function. Oxyanion holes exist in enzymes to stabilize transitions states of reaction intermediates, thus stabilizing the trans–cis isomerization of p-coumaric acid. During the transition state it is believed that the p-coumaric acid phenolate O4 takes part in a hydrogen bond network between Glu46, Tyr42 and Thr50 of PYP. These interactions are apart from the thiol ester linkage to Cys 69 keeping p-coumaric acid in the ligand binding cite. Upon transitioning to the cis-isomeric form of p-coumaric acid the favorable hydrogen bonds are no longer in close interaction.