Polyunsaturated fatty acids - Structure

Jan 2014   Home › Deficiency diseases of nutrients › Structure of polyunsaturated fatty acids

Polyunsaturated fatty acids (PUFAs) are carboxylic acids with long aliphatic chains. They have more than one carbon double bond (C=C) in their chemical structure.

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In nature, PUFAs exist attached to triglycerides or phospholipids.
In some conditions they exist in "free' form and are known as "free fatty acids". Human body has the capacity to synthesize many of these polyunsaturated FAs.

Some of the polyunsaturated FAs such as α-linolenic (omega-3, ALA) and linoleic (omega-6, LA) are essential fatty acids (EFAs).
Though they are essentially required for many biological functions, they cannot be synthesized by human body.

They have to be ingested regularly through foods to maintain health.
These polyunsaturated EFAs are also precursors for the synthesis of other important long-chain desaturated PUFAs like
eicosapentaenoic acid (EPA),
docosahexaenoic acid (DHA) and
arachidonic acid (ARA).

In some situations EPA, DHA or ARA become conditionally essential.  This happens in in case of dietary deficiency, diseases and developmental conditions affecting absorption, assimilation and utilization or impairment of the production of the converting enzymes. In such conditions they have to be included in the diet for carrying on normal body functions.

The nutritional sources of PUFAs include both plant and animal sources. Fish oil, krill oil, sardines, cod liver oil, algal oil, olive oil and walnuts are good sources of the important PUFAs.

Structure of polyunsaturated fatty acids

The structures of these PUFAs have a typical carboxyl (-COOH) end and a methyl (CH3) end. The methyl end is considered as the end of the hydrocarbon chain and the carbon at this end is known as omega (ω) carbon'. The nomenclature, omega-3, omega-6, omega-6 etc., denotes the position of the first double bond in the carbon chain from the 'omega carbon'.

To understand the nomenclature in respect to positioning of double bonds in carbon chain, we can consider the chemical structure of ALA.

The chemical structure of polyunsaturated alpha-linolenic acid showing physiological numbering (red) and chemical numbering (blue) conventions.

In the above illustration the ω carbon end is on the right and the first c=c bond is at the third carbon from that end. Hence this PUFA is omega-3.

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Structure of polyunsaturated fatty acids