HUFA and PUFA: Structures, Occurrence, Biochemistry And Their Health Benefits

Dharmendra Kumar Meena

Central Inland Fisheries Research Institute

Barrackpore 700120

Email: dkmeenafnb@gmail.com

Introduction

In 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-octadecadienoic) and a-linolenic (9-cis, 12-cis, 15-cis-octadecatrienoic) acids.

Both 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 cis-configuration 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

Linoleic 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..

γ-Linolenic acid ('GLA' or 6-cis,9-cis,12-cis-octadecatrienoic 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.

Arachidonic acid (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.

Arachidonic 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

a-Linolenic acid (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 or beta-oxidation. They are essential fatty acids.

Eicosapentaenoic acid ('EPA' or 20:5(n-3)) 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 and resolvins, 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 schizophrenia.

Docosahexaenoic acid ('DHA' or 22:6(n-3)) 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.

DHA 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

1 Linoleic and a-linolenic fatty acid

Linoleic and 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 Δ15 desaturases.

The 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.

In 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.

2. Bio synthesis of (n-6) Poly Unsaturated Fatty Acids

In 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)

The 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 20:3(n-6), 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).

The 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.

3. Bio synthesis of (n-3) Poly Unsaturated Fatty Acids

The a-linoleic acid, which is the primary precursor molecule for the (n-3) 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)

All the various intermediates may be found in tissues, especially those of fish, but eicosapentaenoic (20:5(n-3)), docosapentaenoic (22:5(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 22:5(n-3).

With 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.

In contrast to higher plants and mammals, the nematode Caenorhabditis elegans possesses all of the enzymes required for the synthesis of 20:4(n-6) and 20:5(n-3) fatty acids de novo, feats that can also be accomplished by the fungus, Mortierella alpina, and some mosses and red algae.

Essential fatty acids

As 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) to 20:4(n-6) 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.

Linoleate 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 eicosanoids, including prostaglandins (PG₁, PG₂ and PG₃ 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 possible here.

Nucleotide sequence size of fatty acid desaturase gene in various fishes (NCBI GENEBANK database)

Conclusion: 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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