Understanding the classification of Lipids


(In this topic you can find the answer to Question L-02

Lipids are a diverse group of compounds that can be extracted from biological material using non-polar solvents. They are not soluble in water and other polar solvents. (Some lipids are amphipatic, though, since a part of the molecule is hydrophobic, while the other part is hydrophilic).

 

As you can see, the definition of lipids is not a structural concept, but a functional one. It explains that lipids are very different from the structural point of view.

Different kind of lipids

Different kind of lipids

    Anyway, since some of them show structural similarities, these are used to form the different subclasses or these compounds. 
  
According to specific structural similarities, lipids are classified in the following subgroups: 
 
 

 

 

 

 

      Fatty acids

      All of them are carboxylic acids with a hydrocarbon chain usually lineal. Natural fatty acids generally have an even number of carbon. They can be saturated or unsaturated, essentials or no essential, cis or trans (you can find the different criteria for the classification of fatty acids in this post)

 

Acyl glicerols or Neutral Fats

      They are esters of fatty acids with glycerol. Depending on the number of fatty acids that are esterified to the glycerol, they can be sub classified as monoacylglycerols, diacylglycerols and triacylglycerols.

 

Waxes

      From the structural point of view, they are esters of fatty acids with an alcohol different from glycerol. From the biomedical point of view, the only important waxes are the cholesterol esters (but nobody called them “waxes”!).

 

Phosphoglycerides 

      The structural backbone of this group of lipids is the phosphatidate: esters of diacylglycerols with phosphoric acid. This group includes lecithins, cephalines, phosphatidyl inositols and other important compounds. (In this link you can find a description of the different sub classes of phosphoglycerides and their functions)

 

Sphingolipids

      Lipids belonging to this group have in common the alcohol sphingosin in their structure. Some sphingolipids, like sphingomyelin, are linked to phosphatated groups (sphingomyelin and phosphoglycerides are called phospholipids).  Other sphingolipids do not have phosphate, but they are linked to carbohydrates, forming the glycosphingolipids. The intracellular accumulation of sphingolipids because of genetic deficit of enzymes related to their metabolism, produce diseases called sphingolipidoses. These conditions include Tay-Sachs disease, Niemann- Pick disease, Fabry’s Disease, Gaucher’s disease, etc. depending on the missing enzyme and/or the kind of sphingolipid that is accumulated.

 

Isoprenoid Lipids:

      These lipids have in common that they are formed by isoprene units (methyl butadiene). In some cases, the presence of isoprene is obvious in the structure, like in the terpens (Vitamin A, Vitamin E, Coenzyme Q). In other cases, the isoprene units are evident in the process of synthesis, like in the biosynthesis of steroids, the other important group of isoprenoid lipids.

 

Eicosanoid Lipids:

      These lipids are derivative of Arachidonic acid, an unsaturated fatty acid with 20 carbons (Eico is the Greek prefix for twenty, in the same way that deca means ten and penta indicates five). This group includes prostaglandins, leukotriens and thromboxans.