The Shikimate Pathway
It’s berry good for you!
Important products of the
shikimate pathway
• Animals do not operate shikimate
pathway, only plants & microbes do
• Essential aromatic amino acids
(phenylalanine, tyrosine & tryptophan)
must be obtained through our diet!
• Vitamins E and K have precursors from
shikimate pathway
Important products of the
shikimate pathway
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Hydroxybenzoic acids (e.g. gallic acid)
Coumarins
Phenylpropanoids (e.g. cinnamic acid derivatives)
Lignans
Stilbenes
Flavonoids
Catechins
Tannins
Isoflavonoids
Phylloquinones
Formation of shikimic acid, which can be converted to an aromatic ring by
reduction and dehydration, provides a precursor for many phenolics
Conversion of shikimic acid
to aromatic amino acids:
precursors to C6C3 unit
In most plants & microbes,
the phenylpyruvic acids are
precursors to L-Phe and Tyr.
Animals do not operate the
shikimate pathway and
must get Phe and Tyr from
their diet.
In plants, L-Phe and
L-Tyr are often deaminated by
the enzyme PAL
(phenyl ammonium
lyase) to form the C6C3
cinnamic starter unit
Cinnamic and p-coumaric
(p-hydroxycinnamic) acids
and their derivatives are
very common in plants
They may be functionalized on
aromatic ring with additional OH
or OMe groups, or the carboxylic
acid group may be reduced to
aldehyde or alcohol
They may be found esterified
with sugars, quinic acid, or
terpenes. They also serve as
precursors for biosynthesis of
flavonoids and stilbenes
Many essential oils of spices are derived from the cinnamates
Coumarins are derived from o- or p-hydroxycinnamic (coumaric) acid
Formation of a
blood-thinning dimer
consumption of warfarin-treated bait,
rats die from internal haemorrhage.
Other coumarin derivatives employed
as rodenticides include coumachlor and
coumatetralyl (Figure 4.32). In an
increasing number of cases, rodents are
becoming resistant to warfarin, an ability
which has been traced to elevated
production of vitamin K by their
intestinal microflora.
Origins of hydroxycinnamic acid derivatives and lignans
1)
2)
3)
HSCoA
NADPH
NADPH
HCAs may be
functionalized with
sugars, quinic acid
amines, etc.
Assembly of
lignans occurs
by phenolic
oxidative
coupling
J. Agric. Food Chem (2004) 52:5843-48
Flavonoids, friends and relatives
C6C3 from tyrosine
+ C6 from acetate
The shikimate and acetate pathways
combine to produce some key 2o
metabolites. 4-hydroxycinnamoyl CoA is
the precursor to hydroxycinnamic acids,
stilbenes & flavonoids
Pathway to stilbenes involves
a different folding pattern than
the path leading to chalcones
and and flavonoids
Flavanones are precursors to the
other flavonoids such as flavonols
Biosynthetic routes to flavonoid skeletons
ALL are antioxidants; some are more effective
than others based on structure
Catechins (flavan-3-ols)
found in tea, chocolate, berries
--Antiproliferative against
epidermal carcinomas
--Iron chelation
--Antiinflammatory
--Hypolipidemic
Flavonols (citrus, berries, onions)
--Antiproliferative (breast, colon cancer)
--Reduces platelet aggregation
Anthocyanins – found in dark red foods
--Reduce oxidative damage to neurons,
vascular cells, antiinflammatory
Polyphenols
• Term encompasses all flavonoids, tannins,
stilbenoids, and phenolic acids
• All have varying levels of antioxidant and
antiinflammatory activities as well as effects on
specific biological pathways/diseases
• Bioavailability of polyphenolics is a complex issue
and depends on structure, metabolic transformation
in small intestine, colon and liver
• Berry polyphenols study (Koli, et al, 2010) reports
on a clinical trial of 72 adults consuming berry
products 2x daily (837mg/day total, including 62 mg
phenolic acids) .
Results
• Plasma quercetin levels increased by 51-84% in
treatment group; plasma phenolic acids (p-coumaric,
caffeic, vanillic, protocatechuic) increased by 21-40%
• Flavonoid metabolites (hydroxyphenylacetic acids,
homovanillic acid) increased 7-31% in plasma
• Urinary excretion of quercetin, p-coumaric acid and 3hydroxyphenyl acetic acid increased, but not the other
phenolic acids.
• The high concentrations of some metabolites suggest
that their biological effects should be studied
separately
• Anthocyanins and PACs were not analyzed
nor were tissue concentrations analyzed.
Tannins
Proanthocyanidins (poly-flavan-3-ols)
or “condensed tannins”
are oligomers of catechin or epicatechin
units formed by phenolic radical coupling
Ellagic acid
Formed by radical coupling of
two gallic acids and lactone
formation
A “B-type” proanthocyanidin, like
those found in cacao. More on
A-type PACs later…
Ellagitannins contain a central
ellagic acid unit esterified with
sugars or more gallic acid units
Tannins
Pentagalloylglucose – a hydrolyzable
tannin in which glucose is esterified with
gallic acid
Theaflavins & thearubigens:
Found in fermented tea are
dimeric catechin structures in which
oxidative processes catalyzed by
enzyme polyphenol oxidase have
led to the formation of a seven
membered tropolone ring.
Cinchonains
“Flavonolignan”
Cinchonains were recently identified in leaves of
bilberry, the European blueberry
(Vaccinium myrtillus)
A study in Journal of Ethnopharmacology
(Qa’dan, et al, 2009) reported that cinchonain 1b
enhances the release of insulin from pancreatic
Islet cells – may play a role in treating type-2
diabetes
Radical coupling of
caffeic acid with
epicatechin, followed by
lactonization
Leaves of other Vaccinium species (V. vitis-idaea
and V. angustifolium) are being investigated
for their mechanisms of anti-diabetic activity
based on their use to treat diabetes in Cree
traditional medicine in Northern Quebec.
Green tea catechins (Camellia sinensis)
Galloylated catechins (flavan-3-ols)
from tea are some of the most wellstudied polyphenolics
Content/200 mL cup
EGCG = epigallocatechin gallate (142 mg)
EGC = epigallocatechin (65 mg)
ECG = epicatechin gallate (28 mg)
GCG = gallocatechin gallate (17 mg)
caffeine (76 mg)
Fresh tea leaves are steamed or
dried at elevated temps green tea
Black tea is fermented and tends to
keep longer. Catechins are oxidized to
theaflavins and thearubigens.
Biological activities of green tea/
EGCG
• Reduces risk of esophageal, skin, stomach
& other cancers
• induces apoptosis in tumor cells
• lowers total cholesterol, improves HDL/LDL
ratio
• strong antioxidant, mild antiinflammatory
• brain and bone protection
• accelerates weight loss?
• Rat study: protection of eye from oxidative
damage (J. Agric. Food Chem. 2010,
58:1523-1534)
Bioavailability of tea catechins to
the eye (Chu, et al, 2010)
• 11 groups of rats (n=6) were fed
green tea extract at dose of 550 mg/kg
• Rats were sacrificed @ various timepoints
• Tissues homogenized, extracted & analyzed by HPLC (no
details given!)
• Pharmacokinetics of biodistribution varied between the
catechins vs. catechin gallates and between the tissues
• Catechins were present in much higher quantity in most
tissues than catechin gallates
• Maximum conc. reached in 1 h for plasma, 3 – 5 h for humor,
5 – 10 h for retina, lens & cornea
• Marker of oxidative stress (8-iso-PGF2a) decreased in
humors, cornea & lens
• Distribution routes may differ between compounds
Isoflavonoids: radical rearrangement of flavonoid skeleton
produces isoflavonoids, found widely in legumes, soy products
Coumestrol, a coumestan
found in clovers
Phytoestrogenic activity
estradiol
• “Phytoestrogen” refers to non-steroidal plant materials having estrogenic properties.
• The planar isoflavonoids and coumestans mimic the shape and polarity of the steroid
hormone estradiol and are able to bind to estrogen receptors
• They stimulate estrogenic response in some tissues, have opposite effect in others
• Known to cause reproductive problems in grazing animals
• As part of the diet they can influence overall estrogenic activity in the body
• Isoflavonoid-rich foods/herbs may counter some of the side-effects of menopause
• There is mounting evidence that phytoestrogens provide other beneficial effects:
helping prevent heart attacks/CVD, protect against osteoporosis, lower the risk of breast
and uterine cancer, etc.
Antioxidants and more…Phylloquinones and tocopherols are part
phenolic, part isoprenoid
Coenzyme Q10:
Redox carrier for
electrons in human
mitochondrial ETS
Vitamin K1
Sources: plants, primarily green veggies
Role: blood clotting – needed for carboxylation of Glutamate
residues in prothrombin
Inhibited by warfarins (coumadin)
Vitamin E
Sources: cereals, seed oils
eggs, soybean, corn oil, barley
--Free radical scavenger
--Protects lipids in LDL and cell
membranes from oxidation
--decreases coronary artery
lesions, but effect on CVD
mortality still not proven
Biosynthesis of Vitamin E:
Shikimate pathway-derived
4-hydroxyphenyl pyruvic acid is
alkylated with isoprenoid chain
from mevalonate pathway
Rings are methylated by SAM
Cyclization of phenol with chain
forms chroman ring
Tocopherols differ in pattern of
methylation on the ring
Oxidation of phenolic ring to
quinone forms plastoquinones,
ubiquinones.
Less common in food: phenolics made exclusively through acetate pathway
Starter unit
acetyl-CoA +
Formation
of non-shikimate,
polyketide
phenolics
via the acetate
pathway from
a poly-b-keto
ester precursor
Formation of anthrones and anthraquinones
(polyketide phenolics)
Don’t be fooled! Compare
these structures to that of
the flavonoids – polyketide
vs. shikimate-derived
St. John’s Wort
Hypericum perforatum
Anthrones and their derivatives are found in
numerous herbs/botanicals including
Senna, Cascara and Aloe.
They act as laxatives/purgatives
a constituent of St. John’s Wort