Polyunsaturated fatty acids structure

The prostaglandins (qv) constitute another class of fatty acids with aUcycHc structures. These are of great biological importance and are formed by i vivo oxidation of 20-carbon polyunsaturated fatty acids, particularly arachidonic acid [27400-91-5]. Several prostaglandins, eg, PGE [745-65-3] have different degrees of unsaturation and oxidation when compared to the parent compound, prostanoic acid [25151 -18-9]. 

The composition of some common fets end oils. Fatty acids provide the R. R, and R" hydrocarbon residues in the general structure of a fat or oil shown in the text. Fats and oils all contain a mixture of saturated, monounsaturated. and polyunsaturated fatty acids. 

Dietary polyunsaturated fatty acids (PUFAs), especially the n-3 series that are found in marine fish oils, modulate a variety of normal and disease processes, and consequently affect human health. PUFAs are classified based on the position of double bonds in their lipid structure and include the n-3 and n-6 series. Dietary n-3 PUFAs include a-linolenic acid, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) whereas the most common n-6 PUFAs are linoleic acid, y-linolenic acid, and arachidonic acid (AA). AA is the primary precursor of eicosanoids, which includes the prostaglandins, leukotrienes, and thromboxanes. Collectively, these AA-derived mediators can exert profound effects on immune and inflammatory processes. Mammals can neither synthesize n-3 and n-6 PUFAs nor convert one variety to the other as they do not possess the appropriate enzymes. PUFAs are required for membrane formation and function... 

I. Petitpas, T. Grime, A. A. Bhattacharya, S. Curry, Crystal Structures of Human Serum Albumin Complexed with Monounsaturated and Polyunsaturated Fatty Acids , J. Mol. Biol. 2001, 314, 955-960. 

This structure shows a triglyceride with three identical saturated fatty acids. Tripalmitin, in which all fatty acids are palmitic acid (n = 14), provides one example of a fat. Triolein is an oil containing only oleic acid moieties esterified to glycerol. In contrast to these two examples, it is by no means necessary that the three fatty acid groups be derived from only one fatty acid. For example, we might have a triglyceride that contains one saturated fatty acid, say palmitic acid, one monounsaturated fatty acid, say oleic acid, and one polyunsaturated fatty acid, perhaps arachidonic acid. 

Consequently any of the above factors or conditions could result in failure to produce sufficient amounts of these polyunsaturated fatty acids, which could result in modification of the type of fatty acids present in phospholipids in membranes, and hence the structure of the membranes. 

Figure 11.26 The structures of the prostaglandin E series produced from three polyunsaturated fatty acids containing 20 carbon atoms but a different number of double bonds. The number of double bonds in the three different acids produces prostaglandins of the E series with a different number of double bonds outside the cyclopentane ring. It is this number which influences the function of the prostaglandin and similarly the function of prostacyclins and thromboxanes (see text). Note, PGEi has one double bond, PGE2 has two double bonds and PGE3 has three double bonds outside the cyclopentane ring.

Approximately 60% of the dry weight of the brain is fat, a considerable proportion of which is polyunsaturated fatty acids that are present in plasma membranes. It would not be surprising if replacement of the unsaturated acids by the saturated fatly acids in membrane structure due to a dietary deficiency of polyunsaturated fatty acids played some part in development of mental illness. Indeed, it has been found that supplementation of a normal diet with polyunsaturated fatly acids can improve some mental disorder (see chapter 11). 

Cholesterol The pathway for synthesis of cholesterol is described in Appendix 11.9. Cholesterol is important in the structure of membranes since it can occupy the space that is available between the polyunsaturated fatty acids in the phospholipid (Chapter 4). In this position, cholesterol restricts movement of the fatty acids that are components of the phosphoglycerides and hence reduces membrane fluidity. Cholesterol can be synthesised de novo in proliferating cells but it can also be derived from uptake of LDL by the cells, which will depend on the presence of receptors for the relevant apoUpoproteins on the membranes of these cells (Appendix 11.3). 

Marine fishes are rich sources of structurally diverse bioactive compounds including polyunsaturated fatty acids, polysaccharides, minerals, vitamins, antioxidants, enzymes, and bioactive peptides (Kim et ah, 2008). Marine fish-derived ACE inhibitory peptides have been purified from enzymatic digestion of various fish materials from Alaska pollack (Nakajima et ah, 2009), bonito (Fujita et ah, 2000 Hideaki et ah, 1993 Yokoyama et ah,... 

This cascade however may be propagated throughout the cell unless terminated by a protective mechanism (see below) or a chemical reaction such as disproportionation, which gives rise to a non-radical product. Polyunsaturated fatty acids, found particularly in membranes, are especially susceptible to free radical attack. The effects of lipid peroxidation are many and various. Clearly, the structural integrity of membrane lipids will be adversely affected. In the lipid radical produced, the sites of unsaturation may change, thereby altering the fluidity of the membrane (see chap. 3). Lipid radicals may interact with other lipids and... 

Jasmonic acid is an 18-carbon pentacyclic polyunsaturated fatty acid derived from lino-lenic acid, plays a role in plants similar to arachidonic acid,58 and has a structure similar to... 

Oxidation of unsaturated fatty acids The oxidation of unsatu rated fatty acids provides less energy than that of saturated fatly I acids because they are less highly reduced and, therefore, tom I reducing equivalents can be produced from these structured Oxidation of monounsaturated fatty acids, such as 18 1(9) (deb acid) requires one additional enzyme, 3,2-enoyl CoA isomeras which converts the 3-cis derivative obtained after three rounds (3-oxidation to the 2-trans derivative that can serve as a substra for the hydratase. Oxidation of polyunsaturated fatty acids, sue ... 

Full fat milk contains about 3% fat and triacylglycerols account for about 95% of the lipid fraction. Other components of the lipid fraction are diacylglycerols, cholesterol, phospholipids, and free fatty acids. The lipid structures contain many fatty acids from all major classes, that is, saturated, monounsaturated, and polyunsaturated fatty acids (Haug et al., 2007). More than 60% of the fatty acids in cow s milk and consequently in dairy products are saturated, including shorter and medium... 

Fatty acids (which are also known as lipids ) consist of a carboxyl function with an aliphatic chain. The aliphatic chain can be either saturated (i.e., no double bonds), monounsaturated (i.e., one double bond), or polyunsaturated (i.e., multiple double bonds). See Figure 110 for examples of fatty acid structures. 

There were marked differences in the fatty acid patterns of the lipids of gills and kidneys, those from seawater fish possessing more w-3 in proportion to w-6 fatty acids and a higher proportion of total polyunsaturated fatty acids. They suggested that the role of polyunsaturates in membrane permeability and plasticity might account for the observation, and pointed out that the w-3 structure allows a greater degree of unsaturation than do the <0-6 or w-9 series. 

Lipids are of special concern in comparing plastic metabolism between the two groups of fish. Triacyl-glycerols, cholesterol ethers and non-esterified fatty acids, which are the direct sources of energy, have already been discussed in the previous section. We now turn to phospholipids and cholesterol, which are essential to the structure of cell membranes, and to polyunsaturated fatty acids, which determine to a large extent the functional activity of these membranes. 

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