VPB 112: General Veterinary Biochemistry (1+1)

NOMENCLATURE OF FATTY ACID

• In the naming of fatty acids, saturated acids end in - anoic acid e.g. octanoic acid and the unsaturated fatty acids with double bonds end in – enoic acids, e.g. octadecenoic acid (oleic acid).
• Carbon atoms are numbered from the carboxyl carbon (carbon No.1). The carbon atom adjacent to the carboxyl carbon (No. 2) is also known as the α - carbon. Carbon atom No.3 is the β - carbon and the distal methyl carbon is known as the ω - carbon or n- carbon atom.
• The fatty acids are generally designated as follows (Δ, delta system of numbering). For example, oleic acid is written as 18: 1,Δ⁹. The number 18 indicates the number of carbon atoms, 1 indicates the number of double bond and the superscript 9 indicates the position of the double bonds i.e. the double bond is between carbon atoms 9 and 10 of the fatty acid. Some times the Δ is omitted and oleic acid is indicated as (18: 1;9. )
• Alternatively, the position of a double bond can be denoted by counting from the distal end, with the carbon atom, (the methyl carbon) as number (ω-system). For example ω -3 fatty acid, linolenic acid (has a double bond between ω-3 and ω-4 C atom), ω-6 fatty acid, linoleic and arachidonic (has a double bond between ω-6 and ω-7 C atom) and the ω 9 fatty acid, oleic acid (has a double bond between the C atoms ω-9 and ω-10 of the fatty acid.
• The pKa of fatty acid carboxyl group is about 4.8. Hence, fatty acids are ionized at physiologic pH and so it is appropriate to refer to them according to their carboxylate forms i.e. palmitate or hexadecanoate.
• The properties of fatty acids and the compounds that contain them are dependent on the chain length and the degree of unsaturation.
• Non- polar hydrocarbon chain of fatty acids accounts for the poor solubility of fatty acid in water. The carboxyl group is polar (ionized at neutral pH) and accounts for the slight solubility of fatty acids in water.
• Unsaturated fatty acids have lower melting points than saturated fatty acids of the same length. For eg. the melting point of stearic acid is 69.6^o C, whereas that of oleic acid is 13.4^o C. The melting point of polyunsaturated fatty acids of the C18 series is even lower. The melting point of arachidonic acid is - 49.5^o C.
• The chain length also affects the melting point. The melting temperature of palmitic acid (C16) is 62.8^o C. The melting point of stearic acid (C18) is 69.68^o C.
• Fatty acids with short chain length and unsaturation enhance the fluidity of membranes.