Phenylpropanoids
The reaction catalyzed by 3-deoxy-D-arabino-heptulosonate
7-phosphate synthase.
The reaction catalyzed by 3-dehydroquinate synthase.
The enzyme requires catalytic
amounts of NAD for activity.
The reaction catalyzed by 3-dehydroquinate dehydratase.
The reaction catalyzed by shikimate dedydrogenase.
The plant enzyme uses NADP,
whereas some microorganisms use pyrrolo quinoline quinone
as cofactor.
The reaction catalyzed by shikimate kinase.
The reaction catalyzed by 5-enolpyruvylshikimate
3-phosphate synthase.
The reaction catalyzed by chorismate synthase.
The enzyme requires catalytic amounts
of reduced flavin for activity.
The synthesis of
gallic acid,
hydroxycinnamat
es, stilbenes and
flavonoids by the
phenylpropanoid
pathway in plants
Coumarins
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Coumarin is the parent organic compound of a class of naturally
occurring phytochemicals found in many plant species. This oxygen
heterocycle is best known for its fragrance, described as a vanilla-like
odor or the aroma of freshly mowed hay. Identified in the 1820s,
coumarin has been synthesized in the laboratory since 1868 and used
to make perfumes and flavorings. It is also used to prepare other
chemicals -- in particular anticoagulants and rodent poison.
Coumarin is found in a variety of plants such as Tonka bean, lavender,
sweet clover grass, and licorice, but also occurs in food plants such as
strawberries, apricots, cherries, and cinnamon. It is thought to work by
serving as a pesticide for the plants that produce it.
Chemically, coumarin can occur either free or combined with the sugar
glucose to produce a coumarin glycoside. Medically, coumarin
glycosides have been shown to have blood-thinning, anti-fungicidal,
and anti-tumor activities. Dicumarol, a coumarin glycoside better known
as warfarin, is the most commonly used oral anticoagulant medication.
The biosynthesis of shikimate metabolites
Andrew R. Knaggs
Analytical Sciences Department, Discovery
Research,
GlaxoSmithKline Medicines Research Centre,
Gunnels Wood Road, Stevenage, Hertfordshire,
UK SG1 2NY
Received (in Cambridge, UK) 20th August 2002
First published as an Advance Article on the
web 29th November 2002
Covering 2000.
Previous review: Nat. Prod. Rep., 2001, 18, 334
Nat. Prod. Rep., 2003, 20, 119–136
FLAVONOID Biosynthesis
Anthocyanin biosynthetic pathway
Tannins: definition
The word tannin is very old and reflects a traditional technology.
"Tanning" (waterproofing and preserving) was the word used to
describe the process of transforming animal hides into leather by
using plant extracts from different plant parts of different plant
species.
• Plant parts containing tannins include bark, wood, fruit, fruitpods,
leaves, roots, and plant galls.
• Examples of plant species used to obtain tannins for tanning
purposes are wattle (Acacia sp.), oak (Quercus sp.), eucalyptus
(Eucalyptus sp.), birch (Betula sp.), willow (Salix caprea), pine
(Pinus sp.), quebracho (Scinopsis balansae) .
Tannins: definition
Tannins are phenolic compounds that precipitate proteins.
They are composed of a very diverse group of oligomers and polymers.
There is some confusion about the terminology used to identify or classify
a substance as a tannin, In fact,
• not only tannins bind and precipitate proteins (other phenolics such as
pyrogallol and resorcinol also have this property),
• not all polyphenols precipitate proteins or complex with polysaccharides
Tannins can complex with:
• Proteins
• Starch
• Cellulose
• Minerals .
Tannins: interaction with other
macromolecules
Tannins have a major impact on animal nutrition
because of their ability to form complexes with
numerous types of molecules, including, but not limited
to,
• Carbohydrates,
• Proteins,
• Polysaccharides,
• Bacterial cell membranes,
• Enzymes involved in protein and carbohydrates
digestion.
Characteristics of tannins
• oligomeric compounds with multiple structure
units with free phenolic groups,
• molecular weight ranging from 500
to >20,000,
• soluble in water, with exception of some high
molecular weight structures,
• ability to bind proteins and form insoluble or
soluble tannin-protein complexes.
TANNIN
• Condensed Tannin
– Epicatechin, Catechin
– Procyanidin, Anthocyanidin
– Leucoanthocyanidin (thru heat & acid)
• Hydrolyzable Tannin
– Gallotannin
– Egallitannin
Hydrolyzable tannins
• HTs are molecules with a polyol (generally D-glucose)
as a central core.
• The hydroxyl groups of these carbohydrates are
partially or totally esterified with phenolic groups like
gallic acid (-->gallotannins) or ellagic acid (-->
ellagitannins). HT are usually present in low
amounts in plants.
• Some authors define two additional classes of
hydrolyzable tannins: taragallotannins(gallic acid
and quinic acid as the core) and caffetannins
(caffeic acid and quinic acid)
Gallotannins
• The phenolic groups that esterify with the core are sometimes
constituted by dimers or higher oligomers of gallic acid (each
single monomer is called galloyl)
• Each HT molecule is usually composed of a core of D-glucose
and 6 to 9 galloyl groups
• In nature, there is abundance of mono and di-galloyl esters of
glucose (MW about 900). They are not considered to be tannins.
At least 3 hydroxyl groups of the glucose must be esterified to
exhibit a sufficiently strong binding capacity to be classified as a
tannin.
• The most famous source of gallotannins is tannic acid obtained
from the twig galls of Rhus semialata. It has a penta galloyl-Dglucose core and five more units of galloyl linked to one of the
galloyl of the core.
Ellagitannins
• The phenolic groups consist of
hexahydroxydiphenic acid, which spontaneously
dehydrates to the lactone form, ellagic acid.
• Molecular weight range: 2000-5000.
HT properties:
• hydrolyzed by mild acids or mild bases to yield
carbohydrate and phenolic acids
• Under the same conditions, proanthocyanidins
(condensed tannins) do not hydrolyze.
• HTs are also hydrolyzed by hot water or enzymes
(i.e. tannase).
Proanthocyanidins
(condensed tannins)
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PAs are more often called condensed tannins due to their condensed chemical
structure. However, HTs also undergo condensation reaction. The term,
condensed tannins, is therefore potentially confusing.
The term, proanthocyanidins, is derived from the acid catalyzed oxidation
reaction that produces red anthocyanidins upon heating PAs in acidic alcohol
solutions.
The most common anthocyanidins produced are cyanidin (flavan-3-ol, from
procyanidin) and delphinidin (from prodelphinidin)
PAs may contain from 2 to 50 or greater flavonoid units; PA polymers have
complex structures because the flavonoid units can differ for some substituents
and because of the variable sites for interflavan bonds.
Anthocyanidin pigments are responsible for the wide array of pink, scarlet, red,
mauve, violet, and blue colors in flowers, leaves, fruits, fruit juices, and wines.
They are also responsible for the astringent taste of fruit and wines.
PA carbon-carbon bonds are not cleaved by hydrolysis.
Depending on their chemical structure and degree of polymerization, PAs may
or may not be soluble in aqueous organic solvents.