biol 154: biological chemistry terpenes · pdf filephytol (the hydrophobic side chain of...
TRANSCRIPT
• The volatile component of fragrant materials in plant
belong to a class of compounds called terpenes.
• Terpenes/terpenoid compounds are widely distributed in
nature mostly in plants. They are polymers of isoprene
units joined head to tail.
• An isoprene unit contains 5-carbon atom put together in a
regular pattern, usually head-to-tail in terpenes up to 25
carbons
• The terpenes have been long associated with the term
Essential Oils comprising resins, steroids and rubber
• In fact, they are hydrocarbons that usually contain one or
more C=C double bonds, while the terpenoids are
oxygen-containing analogues of the terpenes
• They are thoroughly distributed in the plant kingdom,
especially in those plants that have abundant chlorophyll
2-Methyl-1,3-butadiene
• The double bond in the isoprene units are in the trans
geometric configuration. Isoprene units are produced from
acetyl CoA or acetic acid
• Terpenes can be cyclic or open chain molecules, carbon-
carbon saturated or unsaturated and may contain a
variety of functional groups such as aldehydes, alcohols,
hetons and esters
• Terpenes occur in rubber, carotenoids in carrots,
potatoes, yellow vegetables, vitamin A, E and K,
chlorophyll and urbiquinone.
• Terpenes containing 30 carbons or more are usually
formed by the fusion of two smaller terpene precursors
such that the head-to-tail "rule" appears to be violated
• In overall, terpenes hold potential interest practical
applications especially in the fragrance and flavour
industries, as well as in the pharmaceutical and chemical
industries.
Classification of Terpenes
• No of C No of Isoprene units Class of terpenes
• 5 1 Hemeterpene
• 10 2 monoterpenes
• 15 3 sesquiterpenes
• 20 4 diterpenes
• 25 5 sesterterpenes
• 30 6 triterpenes eg. Steroids
• 40 8 tetraterpenes eg. Carotenoids
• ˃40 78 polyterpenes eg. Rubber
Methods of extraction
1. Expression; squeezing the juice out of plant material
2. Steam distillation; suitable for the lower members
especially up to 20 carbon compounds.
3. Extraction with volatile solvent; vacuum distillation and
chromatographic method.
Isoprene Rule
• A compound is said to be a terpenoid if it can be broken
into 5-carbon units usually joined head to tail
• Example
Examples of Terpenes
• Monoterpenes are naturally occurring compounds, the
majority being unsaturated hydrocarbons (C10)
• But, some of their oxygenated derivatives such as
alcohols, ketones, and carboxylic acids are known as
monoterpenoids
• The branched-chain C10 hydrocarbons comprises of two
isoprene units and is widely distributed in nature with
more than 400 naturally occurring monoterpenes
identified
• Moreover, besides being linear derivatives (Geraniol, Citronellol), the monoterpenes can be cyclic molecules (Menthol – monocyclic; Camphor – bicyclic; Pinenes (α and β) – Pine genera) as well
• Thujone (a monoterpene) is the toxic agent found in Artemisia absinthium (wormwood) from which the liqueur, absinthe, is made
• Borneol and camphor are two common monoterpenes. Borneol, derived from pine oil, is used as a disinfectant and deodorant
• Camphor is used as a counterirritant, anaesthetic, expectorant, and antipruritic, among many other uses
Sesquiterpenes • Sesquiterpenes are biogenetically derived from farensyl
pyrophosphate and in structure may be linear, monocyclic
or bicyclic
• They constitute a very large group of secondary
metabolites, some having been shown to be stress
compounds formed as a result of disease or injury
• One group is the Sesquiterpene Lactones. Over 500
compounds of this group are known; they are particularly
characteristics of the Compositae but do occur
sporadically in other families
• Not only have they proved to be of interest from chemical
and chemotaxonomic viewpoints, but also possess many
antitumor, anti-leukaemic, cytotoxic and antimicrobial
activities
• They can be responsible for skin allergies in humans and
they can also act as insect feeding deterrents.
• Hemigossypol and related aldehydes together with the
dimeric gossypol are sesquiterpenes stress compounds
found in the subepidermal glands, immature flower buds
and seed kernels of the cotton plant (Gossypium spp.)
• Besides having insecticidal properties, gossypol is of
considerable pharmaceutical interest in that in humans, it
functions as a male anti-infertility agent
• In China, gossypol has been used to treat infertility in
males and studies have shown an increase in sperm
production
Diterpenes:
• The diterpenes, which contain 20 carbons (four C5 units), include
phytol (the hydrophobic side chain of chlorophyll), the gibberellin
hormones, the resin acids of conifer and legume species,
phytoalexins, and a host of pharmacologically important
metabolites, including taxol, an anticancer agent found at very low
concentrations (0.01% dry weight) in yew bark, and forskolin, a
compound used to treat glaucoma
• Some gibberellins have only 19 carbon atoms and are considered
norditerpenoids since they have lost 1 carbon through a metabolic
cleavage reaction
• Vitamin A, which combines with protein opsin to form the active
pigment of the retina
Triterpenes • These contain 30 carbon atoms, are generated by the
head-to head joining of two C15 chains, each of which constitutes three isoprene units joined head-to-tail
• This large class of molecules includes the brassinosteroids, the phytosterol membrane components, certain phytoalexins, various toxins and feeding deterrents, and components of surface waxes, such as oleanolic acid of grapes
• Squalene in shark liver oil, smaller quantities are in olive oil, rice bran oil, yeast, wheat oil. Squalene is used in biosynthesis of steroids
Tetraterpenes: • The most prevalent tetraterpenes (40 carbons, eight
isoprene units) are the carotenoid accessory pigments which perform essential functions in photosynthesis.
•Polyterpenes:
• These contain more than eight isoprene units, include the prenylated quinone electron carriers (plastoquinone and ubiquinone), long-chain polyprenols involved in sugar transfer reactions (e.g., dolichol), and enormously long polymers such as rubber (average molecular mass greater than 106 Da), often found in latex
• Natural rubber may be regarded as l, 4 adolition polymer, it is treated with elemental sulphur in a process called vulcanization
Biosynthesis of Terpenoid compounds
• The biosynthesis of all terpenoids can be divided into 4
steps
1. Synthesis of the fundamental precursor, Isopentenyl
pyrophosphate (IPP)
2. Repetitive addition of IPP to form a series of
prenylpyrophosphate homologs, which serve as the
immediate precursors of the different classes of
terpenoids.
3. Elevation of these allylic prenylpyrophosphates by
specific terpenoid synthases to yield terpenoid
skeletons.
4. Secondary modification of the skeletons (largely redox
reactions) to give rise to the functional properties and
great chemical diversity of this family of natural products
Compartmentation
• A more fundamental, and perhaps universal, feature of
the organization of terpenoid metabolism exists at the
subcellular level.
1. The sesquiterpenes (C15), triterpenes (C30), and
polyterpenes appear to be produced in the cytosolic and
endoplasmic reticulum (ER) compartments.
2. Isoprene, the monoterpenes (C10), diterpenes (C20),
tetraterpenes (C40), and certain prenylated quinones
originate largely, if not exclusively, in the plastids
• The evidence now indicates that the biosynthetic
pathways for the formation of the fundamental precursor
IPP differ markedly in these compartments, with the
classical acetate/mevalonate pathway being active in the
cytosol and ER and the glyceraldehyde
phosphate/pyruvate pathway operating in the plastids
The acetate/mevalonate pathway
1. Hydroxymethylglutaryl-coenzyme A (HMG-CoA) is the
precursor for terpenoid synthesis
• HMG-CoA is also an intermediate on the pathway for
synthesis of ketone bodies from acetyl-CoA. The enzymes
for ketone body production are located in the
mitochondrial matrix
• HMG-CoA destined for terpenoid synthesis is made
by equivalent, but different, enzymes in the cytosol. HMG-
CoA is formed by condensation of acetyl-CoA and
acetoacetyl-CoA, catalyzed by HMG-CoA Synthase
2. HMG-CoA reductase catalyzes production of mevalonate from HMG-CoA
• The carboxyl group of hydroxymethylglutarate that is in ester linkage to the thiol of coenzyme A is reduced first to an aldehyde and then to an alcohol
• NADPH serves as reductant in the 2-step reaction. Mevaldehyde is thought to be an active site intermediate, following the first reduction and release of CoA. HMG-CoA reductase is an integral protein of endoplasmic reticulum membranes
• The catalytic domain of this enzyme remains active following cleavage from the transmembrane portion of the enzyme
3. Mevalonate is phosphorylated by 2 sequential
phosphate transfers from ATP, yielding the
pyrophosphate derivative
Glyceraldehyde phosphate/pyruvate
pathway
• In plastids, pyruvate reacts with TPP to yiel a 2-C
fragment, hydroxylmethyl-TPP which condenses with
glyceraldehyde 3-phosphate
Synthesis of Terpenoids 1. Pyrophosphomevolanate decarboxylase catalyzes
ATP-dependent decarboxylation, with dehydration, to
yield isopentenyl pyrophosphate
• Isopentenyl pyrophosphate isomerase inter-converts
isopentenyl pyrophosphate
and dimethylallyl pyrophosphate
• The mechanism involves protonation followed by
deprotonation
2. Prenyl transferase catalyzes a series of head-to-tail
condensation reactions
• Dimethylallyl pyrophosphate reacts with isopentenyl
pyrophosphate to form geranyl pyrophosphate
• Condensation with another isopentenyl pyrophosphate
yields farnesyl pyrophosphate. Each condensation
reaction is thought to involve a
reactive carbocation formed as PPi is eliminated
• Prenyl transferase (Farnesyl Pyrophosphate Synthase)
has been crystallized with the substrate geranyl
pyrophosphate bound at the active site
2. Squalene synthase catalyzes head-to-head
condensation of 2 molecules of farnesyl pyrophosphate,
with reduction by NADPH, to yields qualene
• Tracer studies using 14C have confirmed that terpenoid compounds are all formed initially through acetate mevalonate pathway
• It has been shown that steroids are modified terponoid compound of cholesterol C27 may be regarded as C30 –C3. Carotenoids which are C40 may be regarded as terpenoid compounds
• Role of IPP
• To prevent esterification of the coo-funcctional group and makes the ester a good leaving group.
•
Carotenes • The term carotene (also carotin, from the Latin carota, or
carrot) is used for several related unsaturated
hydrocarbon substances having the formula C40Hx, which
are synthesized by plants but cannot be made by animals
• Carotene is an orange photosynthetic pigment important
for photosynthesis
• Carotenes are all coloured to the human eye. They are
responsible for the orange colour of the carrot, for which
this class of chemicals is named, and for the colours of
many other fruits and vegetables (for example, sweet
potatoes, chanterelle and orange cantaloupe melon)
• Carotenes are also responsible for the orange (but not all of the yellow) colours in dry foliage
• They also (in lower concentrations) impart the yellow coloration to milk-fat and butter
• Omnivorous animal species which are relatively poor converters of coloured dietary carotenoids to colourless retinoids have yellowed-coloured body fat, as a result of the carotenoid retention from the vegetable portion of their diet
• The typical yellow-coloured fat of humans and chickens is a result of fat storage of carotenes from their diets
• Carotenes contribute to photosynthesis by transmitting the light energy they absorb to chlorophyll
• They also protect plant tissues by helping to absorb the energy from singlet oxygen, an excited form of the oxygen molecule O2 which is formed during photosynthesis
• β-Carotene is composed of two retinyl groups, and is broken down in the mucosa of the human small intestine by β-carotene 15,15'-monooxygenase to retinal, a form of vitamin A
• β-Carotene can be stored in the liver and body fat and converted to retinal as needed, thus making it a form of vitamin A for humans and some other mammals
• The carotenes α-carotene and γ-carotene, due to their single retinyl group (β-ionone ring), also have some vitamin A activity (though less than β-carotene), as does the xanthophyll carotenoid β-cryptoxanthin.
• All other carotenoids, including lycopene, have no beta-ring and thus no vitamin A activity (although they may have antioxidant activity and thus biological activity in other ways)
• All carotenes absorb in the UV/VIS region; it means that
UV can be used to quantitate carotenes in natural
compounds
Retinol • Carotenoids are tetraterpenes, widely distributed in both
plants and animals. They are called lipodromic pigment meaning fat soluble
• They are coloured, yellow violet orange or red. In higher plants, carotenes are found in the leaves and chlorophyll
• They constitute the principal pigments of certain yellows, orange and red flowers
• Types of Carotenoids
• Carotenes - hydrocarbons soluble in pet ether.
• Xanthophylls - oxygenated derivatives of the hydrocarbons of
STEROIDS • Steroids are compounds when distilled in the presence of
selenium give a hydrocarbon called cyclopentano perhydro phenanthrene ring
Classification of Steroids
• Sterols
• Bile acids
• Sex hormones
• Adrenal cortex hormones
• Steroidal glycosides
• Hydrocarbons
Sterols • These are steroids with alcoholic OH functional group.
Sterols are widely distributed in nature
• They may occur as free compounds or as esters of some
fatty acids e.g. cholesterol – most abundant of sterols.
Cholesterol in the precursor of all steroids in animals.
Cholesterol is a white crystalline solid
• Features of sterols
1. It has fused rings labeled ABCD (present in all steroids).
2. It has 2 angular methyl groups at position 10 and 13; the methyl carbons are C19 and C18.
3. Side chain is bonded to carbon 17. There are 2 configurations, cis and trans
• A/B junction for all naturally occurring steroids could be cis or trans while B/C acid C/D are always trans
• In natural cholesterol, C19 is up (above the plane of the ring).
• Functions of cholesterol
1. Component of cell membranes
2. Synthesis of bile acids
3. Antheroclerosis
4. Synthesis of sex hormones
5. Vit D synthesis
Biosynthesis of cholesterol
• Cholesterol is synthesized from isoprene units origin from
isopentenyl pyrophosphate.
• Squalene oxidocyclase catalyzes a series of electron
shifts, initiated by protonation of the epoxide, resulting
in cyclization
• Structural studies of a related bacterial enzyme have
confirmed that the substrate binds at the active site in
a conformation that permits cyclization with only modest
changes in position as the reaction proceeds
• The product of the cyclization reaction is the
sterol lanosterol
• Conversion of lanosterol to cholesterol involves 19
reactions, catalyzed by enzymes associated with
endoplasmic reticulum membranes
• Additional modifications yield the various steroid
hormones or vitamin D
• Many of the reactions involved in converting lanosterol to cholesterol and other steroids are catalyzed by members of the cytochrome P450 enzyme superfamily
• The human genome encodes 57 members of the cytochrome P450 superfamily, with tissue-specific expression and intracellular localization highly regulated.
• Some P450 enzymes are localized in mitochondria. Others are associated with endoplasmic reticulum membranes
• Cytochrome P450 enzymes catalyze various oxidative reactions
• Many are mixed function oxidations (mono-oxygenations),
that require O2 as well as a reductant such as NADPH
• One oxygen atom is incorporated into a substrate and the
other oxygen atom is reduced to water
• An example is hydroxylation of a steroid, as in the
endoplasmic reticulum electron transfer pathway depicted
at right, NADPH transfers 2 electrons to cytochrome
P450 via a reductase that has FAD and FMN prosthetic
groups.
• P450 substrates include steroids, polyunsaturated fatty
acids, eicosanoids, retinoids, and various non-polar
xenobiotics (drugs and other foreign compounds)
• Some P450 enzymes have broad substrate specificity.
Mechanisms for detoxification of non-polar compounds
include reactions such as hydroxylations that increase
polarity, so that the products of these reactions can be
excreted by the kidneys
Chemistry of steroids 1. Steroids undergo reactions characteristic of the
functional groups presents
2. Reactions are greatly influenced by stereochemistry
and steric factors-the fused ring presents a rigid
structure.
3. There are 2 angular methyls on the B face. Therefore
many reagents preferentially attack at the relatively
unhindered and face especially when the reactions take
place near the angular methyls
Reactions
a) Hydrogenation occurs across the double bond. It is
possible for the Hs to attack at the α-face because the
angular methyl in on the B face
b) Esterification
OH
H2/Pt
OH
HH
RCOOH
OHOH
O
OHOH
Nu
Bile acids • These are isolated from the bile of higher animals. They
occur as salts or amides of either glycine or taurine
• This makes the molecule partly polar and partly non polar.
Bile acids are synthesized from cholesterol
• Bile acids resemble cholesterol, but have more polar
constituents
• Like the fatty acids, which have polar and non-polar
portions that allow them to act like detergents, the bile
acids also act like detergents
• They are stored in the bile in the gall bladder and act to
solubilize dietary lipids during digestion
• Glycocholate is a major bile salt
• Bile salts provide one way to break down cholesterol in
the body and, as noted in the previous paragraph,
blocking their normal reabsorption in the intestine requires
the body to break more cholesterol down for their
synthesis
• Their functions include
1. They are emulsifying agent in the gut, acids indigestion
of liquids
2. They provide suitable pH for action of lipases in the
small intestine
3. They are metabolites of cholesterol
4. They promote transport of liquids through aqueous
medium (blood and body fluids)
Vitamin D Synthesis • Vitamin D is derived from a cholesterol derivative, 7-
dehydrocholesterol, in a reaction that is initiated by ultraviolet light and which splits one of the rings
• The active form, calcitrol, is formed from Vitamin D3 by hydroxylation reactions in the liver and kidney
• Vitamin D deficiency causes a syndrome known as rickets, which is characterized by inadequate calcification of cartilage and bone
• Vitamin D deficiency in children was signficant in 17th century England. Today, many foods, including milk are fortified with vitamin D
HORMONES • Hormones are substances or chemical messengers produced
by endocrine glands that elicit specific effects in target tissues or organs
• They are required in minute quantities and are short-lived. E.g. insulin, glucagon, sex hormones, adrenocortical hormones, gastrointestinal hormones etc.
• An example of a plant hormone is ethylene (ethane). It regulates plant growth and fruit ripening and controls sex of flowers of certain plants
• 3-indole acetic acid in a naturally occurring hormone in plants that catalyzes fruit development, stem elongation, ethylene stimulation and synthesis, begins root formation in cuttings, decreases fruit loss and spoilage when the fruit drops from the tree
• Calcitriol (vitamin D hormone) is also included in this
group, although it has a modified steroid structure. The
most important steroid hormone in invertebrates is
ecdysone.
• Progesterone is a female sexual steroid belonging to the
progestin (gestagen) family. It is synthesized in the corpus
luteum of the ovaries
• The blood level of progesterone varies with the menstrual
cycle. The hormone prepares the uterus for a possible
pregnancy
• Following fertilization, the placenta also starts to
synthesize progesterone in order to maintain the pregnant
state.
• The development of the mammary glands is also
stimulated by progesterone.
• Estradiol is the most important of the estrogens. Like
progesterone, it is synthesized by the ovaries and, during
pregnancy, by the placenta as well
• Estradiol controls the menstrual cycle. It promotes
proliferation of the uterinemucosa, and is also responsible
for the development of the female secondary sexual
characteristics (breast, fat distribution, etc.).
• Testosterone is the most important of the male sexual
steroids (androgens)
• It is synthesized in the Leydig intersitial cells of the testes,
and controls the development and functioning of the male
gonads
• It also determines secondary sexual characteristics in
men (muscles, hair, etc.).
• Cortisol, the most important glucocorticoid, is
synthesized by the adrenal cortex
• It is involved in regulating protein and carbohydrate
metabolism by promoting protein degradation and the
conversion of amino acids into glucose
• As a result, the blood glucose level rises. Synthetic
glucocorticoids (e. g., dexamethasone) are used in drugs
due to their anti-inflammatory and immunosuppressant
effects.
•
• Aldosterone, a mineralocorticoid, is also synthesized in
the adrenal gland. In the kidneys, it promotes Na+
resorption by inducing Na+/K+ ATPase and Na+ channels.
• At the same time, it leads to increased K+ excretion. In
this way, aldosterone indirectly increases blood pressure.
• Calcitriol is a derivative of vitamin D. On exposure to
ultraviolet light, a precursor of the hormone can also arise
in the skin
• Calcitriol itself is synthesized in the kidneys. Calcitriol
promotes the resorption of calcium in the intestine and
increases the Ca2+ level in the blood.
Iodothyronines
• The thyroid hormone thyroxine (tetraiodothyronine, T4) and its active form triiodothyronine (T3) are derived from the amino acid tyrosine
• The iodine atoms at positions 3 and 5 of the two phenol rings are characteristic of them
• Post-translational synthesis of thyroxine takes place in the thyroid gland from tyrosine residues of the protein thyroglobulin, from which it is proteolytically cleaved before being released
• Iodothyronines are the only organic molecules in the animal organism that contain iodine. They increase the basal metabolic rate, partly by regulating mitochondrial ATP synthesis. In addition, they promote embryonic development.
PHEROMONES • Many animals and insects use chemicals to communicate
with others of the same species
• These chemicals are called pheromones. Pheromones
are substances secreted by insects or animals that trigger
a reaction in a member of the same species
• The reaction triggered may elicit
1. Immediate behavioural effect e.g. aggregation and
2. Primer effect, not immediate, physiological effect
• Types of Pheromones
1. Sex pheromones - this is species specific, it makes
sure they find a mate
2. Aggregation pheromones - helps to exert complete
attack on food or prey
3. Epideitic pheromones - helps to prevent
overcrowding; chemical mediated spacing of attack.
4. Alarm pheromones - helps to raise alarm of danger
to ants
• Sex Pheromones characteristics
1. Airborne
2. Contact
• They are perceived by antennae which have sense cells.
• Pheromones act on the sensory nerve cells at the mouth
parts or antennae
• Applications of pheromones
1. Monitoring population. Sex pheromone can be used to
lure and trap pests or insects. We can set up a trap and
put in it sex pheromone of a particular insect.
2. Pheromones can be used to cause disruption of mating
3. Pheromones can be combined with conventional
insecticides to control insect infestation of agricultural
fields.