Figure 1: Fatty acids, one of the most complex categories of lipids, are typically a linear chain of carbon atoms in shape, which are classified based on the number of carbon atoms, the number of double bonds, and the position of the first double bond. The linear chain has methyl terminus on one side, and a carboxylic acid terminus on the other side. Fatty acids without double bonds are known as saturated, those with one double bond as monounsaturated, while the presence of more the one double bond confers the status of polyunsaturated. Docosahexaenoic acid (DHA) is defined as a C22:6, n3 because it is composed of 22 carbon atoms (C22), 6 double bonds are present in the carbon chain, and n3 stands for the position of the first double bond starting from the methyl terminal. Fatty acid synthesis is committed by elongases, specific enzymes that elongate the chain by insertion of a double carbon at time (fatty acids with odd carbon chains are not present in mammalian); and desaturases which insert the double bond at a specific position starting from the methyl terminus, i.e. classified as n3, n6 and n9 desaturases. Mammalian species do not possess enzymes to insert double bonds at n-3 or n-6 position and linoleic acid and α-linolenic acid are provided only by dietary sources. Given the importance in metabolism and several physiological processes, these fatty acids are known as essential fatty acids. The common name is given instead of the scientific name for easier recognition.
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