Unsaturated fatty acid naturally occurring

Table 23.1 lists some of the most common fatty acids, and Table 23.2 gives the fatty acid composition of a number of common fats and oils. Notice that in the unsaturated fatty acids in Table 23.1 the double bonds are all cis. Many naturally occurring fatty acids contain two or three double bonds. The fats or oils that these come from are called polyunsaturated fats or oils. The first double bond of an unsaturated fatty acid commonly occurs between C9 and C10 the remaining double bonds tend to begin with Cl 2 and C15 (as in linoleic acid and linolenic acid). The double bonds, therefore, are not conjugated. Triple bonds rarely occur in fatty acids. 

In addition to the saturated fatty acids derived from acetate such as dodecanoic acid (lauric acid, Cn, Figure 11.16, n = 10), tetradecanoic acid (myristic acid, C, Figure 11.16, n = 12), hexadecanoic acid (palmitic acid, Qe, Figure 11.16, n = 14), and octadecanoic acid (stearic acid, Cis, Figure 11.16, n = 16) by a pathway such as that in Figure 11.16 (an abbreviated version of Schemes 11.37 and 11.38), many unsaturated fatty acids also occur naturally. 

Fatty acids occur mainly as esters in natural fats and oils but do occur in the unesterified form as free fatty acids, a transport form found in the plasma. Fatty acids that occur in natural fats are usually straight-chain derivatives containing an even number of carbon atoms. The chain may be saturated (containing no double bonds) or unsaturated (containing one or more double bonds). 

Most Naturally Occurring Unsaturated Fatty Acids Have c/s Double Bonds... 

Table 12.2 Some naturally occurring mono-unsaturated fatty acids...

First, most naturally occurring unsaturated fatty acids have double bonds in the cis isomeric configuration. .. 

Antioxidants These are naturally occurring compounds that have the ability to lower the levels of free radicals they include vitamins C and E, the carotenoids and the flavonoids. Vitamin E and the carotenoids are particularly important in preventing oxidation of the unsaturated fatty acids within the LDL particle and within membranes of cells. 

Most naturally occurring unsaturated fatty acids have cis double bonds. 

Trans fatty acids Trans fatty acids (Figure 27.13) are chemically classified as unsaturated fatty acids, but behave more like saturated fatty acids in the body, that is, they elevate serum LDL (but not HDL), and they increase the risk of CHD. Trans fatty acids do not occur naturally in plants and only occur in small amounts in animals. However, trans fatty acids are formed during the hydrogenation of liquid vegetable oils, for example, in the manufacture of margarine. 

Another alteration of unsaturated fatty acids is the formation of acetylenic groups (-OC-). This apparently occurs by dehydrogenation of -CH=CH- Examples of naturally occurring acetylenes are crepenynic acid (p. 381), alloxanthin (p. 1240), and the following remarkable hydrocarbon from the common cornflower Centaurea cyanusm ... 

All naturally occurring prostaglandins are derived through the cyclization of 20-carbon unsaturated fatty acids such as arachidonic acid, which in turn is synthesized from the essential fatty acid linoleic acid. The prostaglandins are named according to their ring substitutions and the number of additional side-chain double bands, as seen in prostaglandins E1( Fla, and F2a. 

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. 

Fatty Acids Fatty acids are long-chained carboxylic acids. Although their name includes the word fatty, fatty acids are not the same as fats. Fats are triglycerides, and fatty acids are one component of triglycerides. Naturally occurring fatty acids always have an even number of carbon atoms. Saturated fatty acids have carbon chains that contain only carbon-carbon single bonds. Unsaturated fatty acids have carbon chains that contain at least one carbon-carbon double bond. Monounsaturated fatty acids have one carbon-carbon double bond in the... 

But chrysanthemic acid derivatives are by far not the only examples of cyclopropane-containing structures in nature. In fact, the highly strained three-membered carbocycle is virtually ubiquitous. It occurs, for example, in every green plant in the form of 1-aminocyclopropanecarboxylic acid (ACC) 2, a direct precursor to the plant hormone ethylene [3]. In addition, the cyclopropane unit is found in a variety of other natural products, inter alia in terpenes and in various cyclopropanated fatty acids [4]. The biochemical precursors of the latter are unsaturated fatty acids, and in view of the existence of polyunsaturated fatty... 

Linoleic acid and a-linolenic acid are two naturally occurring unsaturated fatty acids that are components of fats. Explain how many cis-trans isomers exist for each of these fatty acids. Interestingly, only the all-cis isomers of each occur naturally. Draw the structure of the naturally occurring stereoisomer of linoleic and a-linolenic acid using skeletal structures. 

It is noteworthy that virtually all of the naturally occurring polyunsaturated fatty acids contain 18-22 carbon atoms. Linoleic acid is the predominant polyunsaturated fatty acid. It is of considerable interest that most animals cannot synthesize linoleic acid and must take it in the diet. If insufficient amounts of this acid are present in the diet of animals, severe symptoms, such as skin lesions, kidney damage, cataracts, increased permeability to water, and so on, can occur. Thus, the term essential fatty acids has been applied to these compounds. It is not certain, however, that dietary unsaturated fatty acids are needed by the human adult, but there is evidence of such a requirement by the human infant. 

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