and PUFA: Structures, Occurrence, Biochemistry And Their Health
Dharmendra Kumar Meena
Central Inland Fisheries Research Institute
higher plants, the number of double bonds in fatty acids only rarely
exceeds three, but in algae and animals there can be up to six. Two
principal families of polyunsaturated fatty acids occur in nature
that are derived biosynthetically from linoleic
(9-cis, 12-cis, 15-cis-octadecatrienoic)
of the parent fatty acids can be synthesised in plants, but not in
animal tissues, and they are therefore essential dietary components
(see below). Polyunsaturated fatty acids can be found in most lipid
classes, but they are especially important as constituents of the
phospholipids, where they appear to confer distinctive properties to
the membranes, in particular by decreasing their rigidity. The
exception is the sphingolipids, where they are rarely detected in
other than trace amounts. The lipids of all higher organisms contain
appreciable quantities of polyunsaturated fatty acids ('PUFA') with
methylene-interrupted, i.e. with two or more double bonds of the
separated by a single methylene group. The term 'homo-allylic' is
occasionally used to describe this molecular feature.
(n-6) family of poly
unsaturated fatty acids
acid is a ubiquitous
component of plant lipids, and of all the seed oils of commercial
importance. For example, corn, sunflower and soybean oils usually
contain over 50% of linoleate, and safflower oil contains up to 75%.
Although all the linoleate in animal tissues must be acquired from
the diet, it is usually the most abundant di- or polyenoic fatty acid
in mammals (and in most lipid classes) typically at levels of 15 to
25%, although it can amount to as much as 75% of the total fatty
acids of heart cardiolipin. It is also a significant component of
fish oils, although fatty acids of the (n-3)
family tend to predominate in this instance..
acid ('GLA' or
acid or 18:3(n-6))
is usually a minor component of animal tissues in quantitative terms
(< 1%), as it is rapidly converted to higher metabolites. It is
found in a few seed oils, and those of evening primrose, borage and
blackcurrant have some commercial importance. Evening primrose oil
contains about 10% GLA, and is widely used both as a nutraceutical
and a medical product.
(5-cis, 8-cis, 11-cis, 14-cis-eicosatetraenoic
acid or 20:4(n-6))
is the most important metabolite of linoleic acid in animal tissues,
both in quantitative and biological terms. It is often the most
abundant polyunsaturated component of the phospholipids, and can
comprise as much as 40% of the fatty acids of phosphatidylinositol.
As such, it has an obvious role in regulating the physical properties
of membranes, but the free acid is also involved in the mechanism by
which apoptosis is regulated.
acid is frequently found as a constituent of mosses, liverworts and
ferns, but there appears to be only one definitive report of its
occurrence in a higher plant (Agathis
robusta). The fungus Mortierella
alpina is a commercial source or
arachidonate via a fermentation process.
(n-3) family of poly
unsaturated fatty acids
(9-cis, 12-cis, 15-cis-octadecatrienoic
acid or 18:3(n-3))
is a major component of the leaves and especially of the
photosynthetic apparatus of algae and higher plants, where most of it
is synthesised. It can amount to 65% of the total fatty acids of
linseed oil, where its relatively susceptibility to oxidation has
practical commercial value in paints and related products. In
contrast, soybean and rapeseed oils have up to 7% of linolenate, and
this reduces the value of these oils for cooking purposes.
a-Linolenic acid is the biosynthetic precursor of jasmonates in
plants, which appear to have functions that parallel those of the
eicosanoids in animals. In animal tissue lipids, a-linolenic acid
tends to be a minor component (<1%), the exception being grazing
non-ruminants such as the horse or goose, where it can amount to 10%
of the adipose tissue lipids. As with linoleate, the remaining
members of the (n-3)
family of fatty acids are synthesised from a-linolenate in animal
and plant tissues by a sequence of elongation and desaturation
reactions as described below, while shorter-chain components may also
be produced by alpha
They are essential fatty acids.
acid ('EPA' or
is one of the most important fatty acids of the (n-3)
family. It occurs widely in algae and in fish oils, which are major
commercial sources, but there are few definitive reports of its
occurrence in higher plants. It is an important constituent of the
phospholipids in animal tissues, especially in brain, and it is the
precursor of the PG3
series of prostaglandins
which have anti-inflammatory effects (see the appropriate web page
for further discussion). There is currently great interest in the
role of this acid in treating neurological disorders such as
acid ('DHA' or
is usually the end point of a-linolenic acid metabolism in animal
tissues. It is a major component of fish oils, especially from tuna
eyeballs, and of animal phospholipids, those of brain synapses and
retina containing particularly high proportions. Indeed, there is
evidence that increased levels of this fatty acid are correlated with
improved cognitive and behavioural function in the development of the
human infant. While DHA is found in high concentrations in many
species of algae, especially those of marine origin, it is not
present in higher plants.
has been shown to be the precursor of docosanoids,
termed 'resolvins' or 'protectins', which are analogous to the
eicosanoids and have potent anti-inflammatory and immuno-regulatory
actions. The concentration of DHA in tissues has been correlated with
a number of human disease states, and it is essential to many
neurological functions. As a phospholipid constituent, it has
profound effects on the properties of membranes, modulating their
structure and function. In such an environment, DHA is believed to be
more compact than more saturated chains with an average length of
8.2Å at 41°C compared to 14.2Å for oleic chains. This is the
result of adoption of a conformation with pronounced twists of the
chain, which reduce the distance between the ends.
Bio synthesis of fatty acids
Linoleic and a-linolenic fatty acid
a-linolenic acids are synthesised in plant tissues from oleic acid
by the introduction of double bonds between the existing double bond
and the terminal methyl group by the sequential action of Δ12 and
main substrate for the Δ12 desaturase is 1-acyl,
2-oleoyl-phosphatidylcholine in the endoplasmic reticulum of the cell
(although other lipids may also be substrates in chloroplasts). The
newly formed linoleate is then transferred by a variety of mechanisms
to other lipids. Phosphatidylcholine can also be the substrate for
further desaturation, but in leaf tissue in a number of plant species
it appears that most a-linolenate is formed by desaturation of
linoleic acid linked to monogalactosyldiacylglycerols. Those plants
that produce significant amounts of 16:3(n-3)
add further complications to the problem, and it is evident that much
remains to be learned of the overall process.
fact, two distinct desaturases have been characterized that can
insert the Δ12 double bond, i.e. a plastidial enzyme (FAD6), which
uses the terminal methyl group as a reference point and is an ω6
desaturase as it introduces the double bond six carbons from the
terminal carbon, and secondly an extra-plastidial oleate Δ12
desaturase (FAD2) that is selective for C-12, 13 oxidation
independently of chain length. The latter is related closely to an
enzyme in the seeds of castor oil (Ricinus
communis) that converts oleate to
(R)-12-hydroxystearate. Indeed, whether the product is a hydroxyl
group or a double bond may depend on the nature of only four amino
acid residues. Less is know of the desaturase (FAD3) that converts
linoleate to a-linolenate, but it is argued that it should be
considered as an ω3 rather than as a Δ15 enzyme. It also has much
in common with hydroxylase enzymes. Infrequently in plants, a double
bond is inserted between an existing double bond and the carboxyl
group as in the biosynthesis of γ-linolenic acid in evening primrose
and borage seed oils.
In this instance, the double in position 6 is inserted after those in
positions 9 and 12.
synthesis of (n-6) Poly Unsaturated Fatty Acids
animal tissues, additional double bonds can only be inserted between
an existing double bond and the carboxyl group. The linoleic acid,
which is the primary precursor molecule for the (n-6)
family of fatty acids, must come from the diet. Biosynthesis of
polyunsaturated fatty acids requires a sequence of chain elongation
and desaturation steps, as illustrated below, and the various enzymes
require the acyl-Coenzyme A esters as substrates not intact lipids
(unlike plants). The liver is the main organ involved in the process.
(Bio synthetic path way of
(n-6) Poly Unsaturated Fatty Acids)
first step is believed to be rate limiting and involves desaturation
with the introduction of a double bond in position 6 to form
γ-linolenic acid. Chain elongation by a two-carbon unit gives
which is converted to arachidonic acid by a Δ5 desaturase. This is
the main end-product of the process. However, two further
chain-elongation steps yield 24:4(n-6),
which can be further desaturated by a Δ6 desaturase to 24:5(n-6).
All the enzymes to this stage are located in the endoplasmic
reticulum of the cell, but the last fatty acid must be transferred to
the peroxisomes for retro-conversion (β-oxidation) to 22:5(n-6).
marine parasitic protozoon Perkinus
marinus (and at least three other
unrelated unicellular organisms) synthesises arachidonic acid by an
alternative pathway in which elongation of linoleic to 11,
14-eicosadienoic acid is followed by sequential desaturation by Δ8
and Δ5 desaturases.
synthesis of (n-3) Poly Unsaturated Fatty Acids
a-linoleic acid, which is the primary precursor molecule for the
family of fatty acids in animal tissues, must come from the diet. The
main pathway to the formation of docosahexaenoic acid (22:6(n-3))
requires a sequence of chain elongation and desaturation steps (Δ5
and Δ6 desaturases), as illustrated below, with acyl-Coenzyme A
esters as substrates. Thus, a-linoleic acid is sequentially
elongated and desaturated, with double bonds being inserted between
existing double bonds and the carboxyl group, as far as 24:6(n-3).
The final steps of what has been termed the 'Sprecher' pathway
involve retro-conversion, i.e. removal of the first two carbon atoms
by a process of β-oxidation, and take place in the peroxisomes of
the cell (as in the case of the (n-6)
family of fatty acids).
(Bio synthetic pathway of (n-3)
Poly Unsaturated Fatty Acids)
the various intermediates may be found in tissues, especially those
of fish, but eicosapentaenoic (20:5(n-3)),
and docosahexaenoic (22:6(n-3))
acids tend to be by far the most abundant. In human tissues, the
rates of conversion of a-linoleic acid to longer-chain metabolites
is very low, suggesting that a high proportion of the latter must
come from the diet (meat, eggs and fish) in normal circumstances.
Δ4, Δ5 and Δ8 Desaturase has been found in
certain micro-algae of marine origin (e.g. Pavlova
salina), suggesting that a more
direct route may exist in this instance, i.e. via desaturation of
acetyl-CoA as the primary precursor, the synthesis of 22:6(n-3)
by the route described above involves approximately 30 distinct
enzymes and 70 reactions. However, an entirely different and much
simpler pathway catalysed by a distinct polyketide synthase has been
found in marine microbes. The conventional view of polyketides is of
secondary metabolites consisting of multiple building blocks of
ketide groups (—CH2—CO—),
which are synthesised by a polyketide synthase. This is an enzyme
system similar to the fatty acid synthase in bacteria in that it uses
acyl carrier protein as a covalent attachment for chain synthesis and
proceeds in iterative cycles. However, the double bonds are
introduced during the process of fatty acid synthesis in contrast to
the elongation-desaturation pathway. Much remains to be learned of
this process in relation to DHA synthesis, but it is believed that as
the chain elongates the ketones groups are reduced to hydroxyls, and
this is followed by dehydration reactions to introduce the double
bonds. Thus, aerobic desaturation is not required for introducing
double bonds into the existing acyl chain, and it is sometimes termed
an 'aerobic' pathway, although it can occur under aerobic conditions.
contrast to higher plants and mammals, the nematode Caenorhabditis
elegans possesses all of the
enzymes required for the synthesis of 20:4(n-6)
fatty acids de novo,
feats that can also be accomplished by the fungus, Mortierella
alpina, and some mosses and red
Essential fatty acids
linoleic and linolenic acids cannot be synthesised in animal tissues
and must be obtained from the diet, i.e. ultimately from plants.
There is an absolute requirement for these 'essential
fatty acids' for growth,
reproduction and good health. Young animals deprived of these fatty
acids in the diet rapidly display the effects, including diminished
growth, liver and kidney damage, and dermatitis; these eventually
result in death. A key biochemical parameter is the 'triene-tetraene'
ratio, i.e. the ratio of 20:3(n-9)
fatty acids in plasma; levels greater than 0.4 reflect essential
fatty acid deficiency. It takes longer for the effects to become
apparent in older animals, which may have substantial stores of
essential fatty acids in their body fats, but symptoms will appear
eventually. The effects of essential fatty acid deficiency have been
seen in human infants, on adults on parenteral nutrition or with
certain genetic disorders. The absolute requirements are dependent on
a number of factors, including species and sex, but are usually
considered to be 1-2% for linoleate, and somewhat less for
linolenate. In contrast, the requirement for a-linolenate in fish is
higher than for linoleate. For some years it was believed that cats
lacked a Δ6 desaturase and had an absolute requirement for
arachidonic acid especially in their diet, i.e. they were obligate
carnivores, but this now appears not to be the case.
and linolenate may in fact be less important than their longer-chain
metabolites in animal biology. The functions of arachidonic,
eicosapentaenoic and docosahexaenoic acids that make them essential
are only partly understood. They are signalling molecules and are
involved in the regulation of gene expression. They are precursors of
including prostaglandins (PG1,
series), thromboxanes, leukotrienes, and lipoxins, which have a
variety of important biological properties. In addition, these
polyunsaturated fatty acids confer distinctive attributes on the
complex lipids that may be required for their function in membranes.
Although the actual
requirement for polyunsaturated fatty acids is relatively low,
general nutritional advice for the human diet until relatively
recently was that they should comprise a substantial part of the
daily intake. Now it is recognized that the propensity of such fatty
acids for oxidation can lead to potentially harmful levels of
hydroperoxides in tissues. Higher relative proportions of monoenes
are now recommended. Detailed discussion of this topic is not
sequence size of fatty acid desaturase gene in various fishes (NCBI
Sequence Size (bp)
Zebra fish Δ6/Δ5
Common Carp Δ6
Rainbow Truo tΔ6
Nile Tilapia Δ6
Atlantic Salmon Δ5
Gilthead Seabream Δ6
Now a days cardiovascular disease problem are becoming prominent
issue not only in older age group but it has seen in very young
generations also. Highly Unsaturated and Poly Unsaturated Fatty Acids
are very important biomolecule which can cope up with this problem
but problem is that no much studies been done on this aspect.
Different family of fatty acids has its own properties functional,
synthetic pathways so it is must to explore these all areas to take
their whole profits in human health. In this article these all
briefly described and it will ignite a deep interest among the
research to explore these possibilities.
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acid as a bioactive molecule. J.
Clin Invest., 107,
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Holman, R.T. (Editor). Progress
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of docosahexaenoic acid (DHA, 22:6-4,7,10,13,16,19): two distinct
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role of omega-3 long-chain polyunsaturated fatty acids in health and
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