Figure 1 Fatty-acid structure and nomenclature. (A) Chemical formula and carbon atom numbering system for a 16-carbon saturated fatty acid (16:0). (B) Schematic representation of 16:0. (C) A monounsaturated fatty acid, 18:1 n-9, showing the double bond nine carbon atoms from the methyl end (carbon 18). (D) The essential n-6 fatty acid 18:2n-6, where the first double bond is found six carbon atoms from the methyl end. The two double bonds are separated by a methylene (-CH2-) group. (E) The essential n-3 fatty acid 18:3n-3, where the first double bond is found three carbon atoms from the methyl end. (F) Phytanic acid, a dietary ,3-methyl-branched-chain fatty acid (3,7,11,15-tetramethyl 16:0). The methyl group on carbon 3 prevents this fatty acid from degradation by ^-oxidation. (G) Pristanic acid (2,6,10,14-tetramethyl 15:0) is the product of phytanic acid a-oxidation, in which a single carbon (carbon 1) is lost. The methyl group on carbon 2 does not preclude subsequent degradation by ^-oxidation.
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