Fatty acids (FAs) are a class of lipids consisting of carbon, hydrogen, and oxygen, arranged as a linear carbon chain skeleton of variable length, generally with an even number of atoms, with a carboxyl group at one end.
Fatty acids from 2 to 30 carbons or more occur, but the most common and important ones contain between 12 and 22 carbon atoms and are found in many different animal and plant fats.
They are rarely free in nature and are the main components of:
- triacylglycerols (or triglycerides);
- monoacylglycerols (the last two families of compounds are often added to processed foods);
- phospholipids of cell membranes;
- sterol esters.
Far from only being a convenient unit for energy storage, they are also essential for metabolic and structural activities. In this regard fatty acids, unlike proteins and nucleic acids, have the singular ability to be incorporated into tissues intact, thereby altering tissue acyl compositions.
Although most of them have unbranched structure, there are many that have a branched chain. Some of these, like phytanic acid, occur frequently but in small amounts in animal fats, waxes, and marine oils. They are rare in plant lipids, while being major components of the lipids of gram-positive bacteria.
For a list of the most common saturated, monounsaturated and polyunsaturated fatty acids, see the page List of fatty acids.
Depending on their degree of saturation/unsaturation in the carbon chain, they can be divided into three classes:
- saturated fatty acids (SFAs), if no double bond is present;
- monounsaturated fatty acids (MUFAs), if only one double bond is present;
- polyunsaturated fatty acids (PUFAs), if two or more double bonds are present.
Moreover on the basis of the absence/presence of double/triple bonds they can be grouped into two broad classes:
- saturated FAs, if there are no double bonds in the carbon chain;
- unsaturated FAs, if there are one or more double bonds in the carbon chain.
On the basis of the ability or not to synthesize them de novo from endogenous precursors by animals, and whose deficiency can be reversed by dietary addition, they can be classified as:
- essential fatty acids
- not essential
Finally, depending only on chain length they can be functionally divided into:
- short chain FAs (SCFAs): up to 6 carbon atoms;
- medium chain FAs (MCFAs): from 8 to 12 carbon atoms;
- long chain FAs (LCFAs): from 14 to 18 carbon atoms;
- very long chain FAs (VLCFAs): from 20 carbon atoms onwards.
Short chain saturated fatty acids, thanks to their high water solubility, short chain length and small molecular size, are rapidly absorbed in the intestine and are transported, bound to albumin, to the liver by the portal vein, not requiring chylomicrons formation.
The pathway of absorption and transport of MCFAs is similar.
LCFAs and VLCFAs have a low or absent water solubility and hence, after intestinal absorption, are secreted into lymphatic system as component of chylomicrons, and by the thoracic duct are transferred into the circulatory system at the subclavian vein level (venous blood).
Other classes of fatty acids
- Oxygenated fatty acids
They contain hydroxyl, keto, and epoxy groups; an example is ricinoleic acid, the major fatty acid in castor oil.
- Cyclic fatty acids
They contain a cyclic unit with three, five, like prostaglandins, or even six carbon atoms.
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