Fatty acids: structure and classification

Chemical structure of fatty acids

Fatty Acids
Fig. 1 – Olive Oil

Fatty acids (FA) consist 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);
  • diacylglycerols;
  • 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 FA, 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.

Classification of fatty acids

Depending on their degree of saturation/unsaturation in the carbon chain, they can be divided into three classes:

  • saturated fatty acids (SFA), if no double bond is present;
  • monounsaturated fatty acids (MUFA), if only one double bond is present;
  • polyunsaturated fatty acids (PUFA), 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 FA, if there are no double bonds in the carbon chain;
  • unsaturated FA, 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:

Finally, depending only on chain length they can be functionally divided into:

  • short chain FA (SCFA): up to 6 carbon atoms;
  • medium chain FA (MCFA): from 8 to 12 carbon atoms;
  • long chain FA (LCFA): from 14 to 18 carbon atoms;
  • very long chain FA (VLCFA): from 20 carbon atoms onwards.

SCFA, 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 MCFA is similar.
LCFA and VLCFA 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.
References

Akoh C.C. and Min D.B. “Food lipids: chemistry, nutrition, and biotechnology” 3th ed. 2008

Arienti G. Le basi molecolari della nutrizione. Seconda edizione. Piccin, 2003

Bender D.A. “Benders’ dictionary of nutrition and food technology”. 2006, 8th Edition. Woodhead Publishing. Oxford

Burr G. and Burr M. A new deficiency disease produced by the rigid exclusion of fat from the diet. J Biol Chem 1929;82:345-67 [Full Text]

Chow Ching K. “Fatty acids in foods and their health implication” 3th ed. 2008

Cozzani I. e Dainese E. “Biochimica degli alimenti e della nutrizione”. Piccin Editore, 2006

Mahan L.K., Escott-Stump S.: “Krause’s foods, nutrition, and diet therapy” 10th ed. 2000

Rosenthal M.D., Glew R.H. Mediacal biochemistry. Human metabolism in health and disease. John Wiley & Sons, Inc. 2009

Shils M.E., Olson J.A., Shike M., Ross A.C.: “Modern nutrition in health and disease” 9th ed. 1999

Stipanuk M.H.. Biochemical and physiological aspects of human nutrition. W.B. Saunders Company-An imprint of Elsevier Science, 2000

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