General Aspects Structures and Nomenclature

A fatty acid molecule consists of a hydrocarbon chain with an acidic carboxyl group at one end and a terminal methyl group at the other. In the case of a saturated fatty acid, all the carbon atoms in the chain are linked by single bonds, whereas an unsaturated fatty acid is defined by the presence of one or more double bonds in the chain. Most polyunsaturated fatty acids of animal tissues belong to either the omega-6 (o6) or omega-3 (o3) series (also referred to as n-6 and n-3, respectively). These terms indicate the positioning of the double bonds in the chain. Thus, for an o6 fatty acid, the double bond nearest to the methyl end is located between carbon atoms 6 and 7, counting from the methyl terminus. Similarly, the double bond nearest to the methyl end of an o3 fatty acid forms the link between carbon atoms 3 and 4. Fatty acids are symbolized by a shorthand nomenclature. For example, linoleic acid is abbreviated to 18:2o6, indicating a chain length of 18 carbon atoms with 2 double bonds, the first double bond being located between carbons 6 and 7 from the methyl end. Other polyunsaturated fatty acids with important functions in animals are a-linolenic (18:3o3), arachidonic (20:4o6), eicosapentaenoic (20:5o3), and docosahexaenoic (22:6o3) acids. In the diets and tissues of animals, fatty acids are mainly present in the esterified form, as triacylglycerols, phospholipids, or cholesteryl esters, with only traces occurring in the free (unesterified) form.

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