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

Saturated fatty acids - Definition, structure & examples

Jan 2014  Home › Fats deficiency › Saturated fatty acids - Definition, structure and examples

Saturated fatty acids (SFA) are long-chain carboxylic acids characterized by the presence of carboxyl group. Their structure does not contain double bonds between the individual carbon atoms.

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SFA usually contain between 3 carbon atoms (propionic) and 36 carbon atoms (hexatriacontanoic). The chain of carbon atoms has the maximum possible hydrogen atoms for its structure. In nature, many saturated FAs occur and the difference between them is the number of carbon and hydrogen atoms present in their structures.

Definition of saturated fatty acid

Saturated fatty acid definition: "a fatty acid whose carbon chain cannot absorb any more hydrogen atoms; found chiefly in animal fats" definition by wordnetweb.princeton.edu/perl/webwn.
Definition of saturated FA: " A fatty acid, such as stearic acid, whose carbon chain contains no unsaturated linkages between carbon atoms and hence cannot incorporate any more hydrogen atoms" as defined by medical-dictionary.thefreedictionary.com.

Structure of saturated fatty acid

To understand the structure of SFA, we can take the example of butyric acid. Its chemical formula is CH3CH2CH2-COOH.

1.example of chemical structure of saturated FA
The structure shows that the carbon atoms contain no double bonds between them.

2.Example flat structure of SFA
In the diagram of flat structure, the positioning of the carbons and hydrogen atoms in relation to one another is clearly seen.

3.Example of ball-and-stick model of saturated FA.
The ball and stick model shows the positioning of atoms in a long-chain SFA is well defined.

Butric or butanoic acid is present in many milk products such as milk, butter and cheese. It is also produced by anaerobic fermentation in the intestines.

Examples of saturated fatty acids and structures

Lauric acid contains 12-carbon atoms and is a carbon-chain example of SFA. It is a medium chain FA with the molecular formula C₁₂H₂₄O₂. It is a white powdery solid. Coconut oil is a rich source. It is also present in smaller quantities in milk including breast milk. This FA has been found to increase the level of total cholesterol, particularly  high-density lipoprotein (HDL) cholesterol. Increase in HDL decreases the risk of atherosclerosis.

Myristic acid is an example of long chain SFA. It contains 14 carbon atoms in a chain. Its molecular formula is C₁₄H₂₈O₂. Its boiling point is 250.5 °C. It acts as an anchor in biomembranes. Esters of this SFA, known as myristate have high skin absorption rate and are used in cosmetic preparations and topical medicines. Nutmeg butter is a very rich source of this SFA. It is also present in coconut oil and animal lipids. It is also found in high concentrations in sperm whale oil.

Palmitic acid is an example of long chain SFA. It contains 16 carbons in a chain. It appears as white crystals. Its molecular formula is C₁₆H₃₂O₂ and its melting point is 62.9 °C. It is the most common SFA found in vegetable oils and animal lipids. Palm oil, palm kernel oil, and coconut oil are major plant sources of this SFA. It is also present in dairy products and meat. Its salt or ester is known as palmitate.

Stearic acid contains 36 hydrogen atoms and 18 carbon atoms in a long chain. Its an example of long chain SFA. It appears as white waxy solid with the melting point of 69.6 °C. Its molecular formula is C₁₈H₃₆O₂. It present in both animal and plant sources of SFA. Animal lipids, cocoa butter and shea butter contain abundant quantities of this SFA. Stearates are salts and esters of this SFA.
In general, intake of saturated fatty acids is directly linked to increase in cardiovascular risks.

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Saturated fatty acids - Definition, structure and examples