Unsaturated fatty acids polyunsaturated

Palmitic acid and oleic acid are important constituents of most fats the triglycerides often contain one residue of each, with the third position, occupied by one of the other fatty acids. The unsaturated fatty acids have the cis configuration about the double bonds, causing the hydrocarbon chain to be bent. In consequence the chain cannot fit properly into a crystalline array of the saturated chains, which are most stable in the extended trans configuration, and the melting point of the fat is lowered. Fats with a high content of unsaturated fatty acids (polyunsaturated fats) are oils at room temperature. Vegetable oils are converted into solid fats... 

Type Saturated fatty acids Mono-unsaturated fatty acids Polyunsaturated fatty acids Oleic acid (o)-9) Smoke point (°C)... 

Linoleic and linolenic adds abound in certain vegetable oils, e.g. linseed oil. Mammals evidently are unable to synthesize them and therefore depend on them as essential food components (also called vitamin F see Chapt. XXII). Fish livers contain even more highly unsaturated fatty acids. Polyunsaturated fatty acids with 20-24 C atoms and 4-6 double bonds are synthesized in the mammalian organism from linolenic acid. They are components of several phosphatides (Klenk). 

Fatty acids can then be classified by their degree of unsaturation as well as their chain length. Thus, fatty acids are either saturated if they have no double bonds or unsaturated. Unsaturated fatty acids are further subdivided into monounsaturated diunsaturated and so on. Fatty acids with many double bonds are classed as polyunsaturated. Table 3 gives the approximate fatty acid compositions of several fats and oils, Table 4 lists the unsaturated fatty acids found in milk (note the very wide diversity). 

FIGURE 3-7 Pathways for the interconversion of brain fatty acids. Palmitic acid (16 0) is the main end product of brain fatty acid synthesis. It may then be elongated, desaturated, and/or P-oxidized to form different long chain fatty acids. The monoenes (18 1 A7, 18 1 A9, 24 1 A15) are the main unsaturated fatty acids formed de novo by A9 desaturation and chain elongation. As shown, the very long chain fatty acids are a-oxidized to form a-hydroxy and odd numbered fatty acids. The polyunsaturated fatty acids are formed mainly from exogenous dietary fatty acids, such as linoleic (18 2, n-6) and a-linoleic (18 2, n-3) acids by chain elongation and desaturation at A5 and A6, as shown. A A4 desaturase has also been proposed, but its existence has been questioned. Instead, it has been shown that unsaturation at the A4 position is effected by retroconversion i.e. A6 unsaturation in the endoplasmic reticulum, followed by one cycle of P-oxidation (-C2) in peroxisomes [11], This is illustrated in the biosynthesis of DHA (22 6, n-3) above. In severe essential fatty acid deficiency, the abnormal polyenes, such as 20 3, n-9 are also synthesized de novo to substitute for the normal polyunsaturated acids. 

Polyunsaturated fatty acids are characterized by a large number of C = C double bonds in their hydrocarbon chain. Stearic acid has no C = C double bonds and therefore is not unsaturated, let alone polyunsaturated. But eleostearic acid has three C = C double bonds and thus is polyunsaturated. Polyunsaturated fatty acids are recommended in dietary programs since saturated fats are linked to a high incidence of heart disease. Of the lipids listed in Table 28-2, safflower oil has the highest percentage of unsaturated fatty acids, predominately linoleic acid, an unsaturated fatty acid with two C=C bonds. 

As shown in Table 1, the acyl moiety of cardiolipin is comprised almost entirely of unsaturated fatty acids. Other membrane phospholipids such as phosphatidyl choline and phosphatidyl ethanolamine contain 1(M0 mol of saturatedfatty acids such as palmitic acid (Ci6 0) and stearic acid (Ci8 0) per 100 mol of total fatty acids. In particular,linoleic acid (Cl8 2) is the most abundant polyunsaturated fatty acid consisting of 80 mol%, linolenic acid (Cl8 3) 8 mol%, and oleic acid (Ci8 i) 6 mol%. Therefore, by using a commercially available cardiolipin purified from bovine heart mitochondria, we characterized auto-oxidation products by reverse phase HPTLC and reverse phase HPLC. 

Saturated fatty acids do not contain double bonds in the hydrocarbon chain. Unsaturated fatty acids contain from one to hve double bonds. Those with one double bond are known as monounsaturated, those with two as diunsatu-rated and those with more than two as polyunsaturated fatty acids. A brief summary of the roles of saturated and unsaturated fatty acids is given in Table 11.1. The proportion of these fatty acids in triacylglycerol in human adipose tissue is presented in Table 11.2. 

FIGURE 21-12 Routes of synthesis of other fatty acids. Palmitate is the precursor of stearate and ionger-chain saturated fatty acids, as well as the monounsaturated acids palmitoleate and oleate. Mammals cannot convert oleate to linoleate or a-linolenate (shaded pink), which are therefore required in the diet as essential fatty acids. Conversion of linoleate to other polyunsaturated fatty acids and eicosanoids is outlined. Unsaturated fatty acids are symbolized by indicating the number of carbons and the number and position of the double bonds, as in Table 10-1. 

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 ... 

Radical X , which initiates the reaction, is regenerated in a chain propagation sequence that, at the same time, produces an organic peroxide. The latter can be cleaved to form two additional radicals, which can also react with the unsaturated fatty acids to set up the autocatalytic process. Isomerization, chain cleavages, and radical coupling reactions also occur, especially with polyunsaturated fatty acids. For example, reactive unsaturated aldehydes can be formed (Eq. 21-14). 

One hundred and eighty-one molecular species of TGs have been identified 79 of them were saturated, 44 monounsaturated, and 58 polyunsaturated. The majority of the unsaturated TGs (61) contained only one unsaturated fatty acid, 41 contained two, and 5 had all three fatty acids unsaturated. Furthermore, ten TGs that contained linear or branched odd-carbon-number fatty acids have been identified. In Table 6, identified species are mentioned with retention times and peak numbers corresponding to the chromatogram in Fig. 43. 

The consensus is that fats containing unsaturated fatty acids are better for you than the corresponding saturated forms. Can this statement be explained by the ATP yield that results on complete oxidation (which in turn reflects the caloric content) Calculate the number of ATPs produced for the complete oxidation of arachidic (C2o o) and arachidonic (C2o 4) acids to assess any differences in energy value of saturated versus polyunsaturated fatty acids. 

Polyunsaturated fatty acids were epoxidized less regioselectively, generally <2 1 in favor of the double bond closer to the hydrophobic terminus. This result can be explained by a well known decrease of the rigidity of phospholipid bi layers on the addition of unsaturated fatty acids. However, when less than 20% cholesterol was added to the bilayer assembly the rigidity increased significantly and thus the regioselectivity. 

Typical examples are unsaturated fatty acids (Ferreri et al. 1999 Sprinz et al. 2000, 2001 Adhikari et al. 2001). The equilibrium constants for the oleic and linoleic systems are in the order of 10 dm3 mol1 and the reverse reaction in the order of 106 s"1 (Sprinz et al. 2000 and Sprinz, pers. comm.). In polyunsaturated fatty acids, such isomerizations could, in principle, also occur by an H-abstrac-tion/H-donation mechanism as discussed above. However, the rate of H-dona-tion of RSH to the pentadienylic radicals must be verylow (see above), and isomerization has been considered to occur only by the addition/elimination pathway (Sprinz et al. 2000). With the nucleobases, any thiyl addition can only be detected when the short-lived adduct is trapped by a fast reaction (Chap. 10.10). 

In nature, fish apparently acquire polyunsaturated lipids in one of two ways. The first of them conforms with the concept of Saigent and Henderson (1980), Watanabe (1982) and Henderson et al. (1985), that some species of fish do not need to synthesize long-chain polyenoic acids, since they occur in phytoplankton, which are eaten by zooplankton which in turn are food for fish. Takahashi et al. (1985) described the situation as unsaturated fatty acids being transferred from plant organisms to phytoplankton-eating fish to predatory fish. 

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