Principles of Biochemistry

6.3.2.2 Unsaturated Fatty acids

This group of fatty acids are characterized by having one (monounsaturated) or more double bonds (polyunsaturated). In most of the unsaturated fatty acids, there is a double bond (∆⁹) between carbon atom 9 and 10 .

e.g. Structure of oleic acid C18:1, ∆^{9 10 9}

CH₃-CH₂-CH₂-CH₂-CH₂-CH₂-CH₂-CH₂-CH=CH-CH₂-CH₂-

CH₂-CH₂-CH₂-CH₂-CH₂-COOH

Oleic acid (n⁹)

The additional double bonds are usually present between the ∆9 double bond and the methyl terminal end of the chain.

In fatty acids containing two or more double bonds, the double bonds are separated by a methylene group:–CH=CH–CH₂–CH=CH–The double bonds of nearly all the naturally occurring unsaturated fatty acids are with the cis geometrical configuration, which produces a rigid bend in the aliphatic chain. Unsaturated fatty acids may be further subdivided as follows:

a) Monounsaturated (monoethenoid, monoenoic) acids: A monounsaturated fatty acid is a fatty acid with a carbon chain in which one carbon–carbon double bond is present.

e.g. Oleic acid: It is fatty acid with18 carbon atoms and a single double bond. The structure is represented as follows

Oleic acid C18:1; ∆ ⁹

18 10 9 1

CH₃ (CH₂)₇CH=CH (CH₂)₇ COOH

or

C18:1 ω9 or 18:1 n-9

ω1 2 3 4 5 6 7 8 9 10

CH₃-CH₂-CH₂-CH₂-CH₂-CH₂-CH₂-CH₂-CH₂=CH - (CH₂)₇COOH

Structure of Oleic acid

b) Polyunsaturated (polyethenoid, polyenoic) acids (PUFA): The polyunsaturated fatty acid is a fatty acid with a carbon chain in which two or more carbon-carbon double bonds are present. They are classified as follows according to the number of double bonds present.

i) Dienoic acid: fatty acid with 2 double bonds

e.g. Linoleic acid ω6, C18:2 or 18:2 n- 9,12

ω1 2 3 4 5 6 7 8 9 10 18

CH₃-CH₂-CH₂-CH₂-CH₂-CH=CH-CH₂-CH = CH-(CH₂)₇COOH

n18 12 9 1

ii) Trienoic acid: fatty acid with 3 double bond

e.g. Linolenic acid

ω3, C18:3 or n-3, 6, 9

 ω1 2 3 4 5 6 7 8 9 10 18

CH₃-CH₂-CH=CH-CH₂- CH= CHCH₂CH = CH (CH₂)₇COOH

n18 15 12 9 1

iii) Tetraenoic acid

e.g. Arachidonic acid : ω6, C 20:4 or

ω⁶ 6 9, 12, 15

CH₃CH₂CH₂CH₂CH₂CH=CHCH₂CH=CHCH₂CH=CHCH₂CH=CH(CH₂)₃ COOH

n 20 14 11 8 5 

iv) Pentaenoic acid

e.g. Eicosa pentaenoic acid (EPA) ω3

ω³

CH₃CH₂CH=CHCH₂CH=CHCH₂CH=CHCH₂CH=CHCH₂CH=CH(CH₂)₃ COOH

20 17 14 11 8 5

v) Hexoenoic acid

e.g Docosahexa enoic acid (DHA) ω3

n22 19 17 14 11 8 5

CH₃CH₂CH=CHCH₂CH=CHCH₂CH=CHCH₂CH=CHCH₂CH=CHCH₂CH=CH(CH₂)₃ COOH

ω³