Initially it was believed that lipids are oily materials with two main purposes - to serve as a source of energy and as the building blocks of membranes. In 1929, George and Mildred Burr however dispelled this myth and showed that linoleic acid was an essential dietary constituent and played a vital role in many processes in the body.
Bergström, Samuelsson and others in 1964 found that essential fatty acid arachidonate was the precursor of the prostaglandins with their effects on inflammation and other disease states and lipids gained a new importance among biochemists.
Types of lipids include:
- Storage lipids
- Structural lipids
- Other lipids
These are the defining constituents of lipids and are in large part responsible for the distinctive physical and metabolic properties. They are also important in non-esterified form.
In the body these are released from triacylglycerols during fasting to provide a source of energy.
Linoleic and linolenic acids are essential fatty acids, in that they cannot be synthesised by animals and must come from plants via the diet. They are precursors of arachidonic, eicosapentaenoic and docosahexaenoic acids, which are vital components of all membrane lipids.
Fatty acids in diet are short and medium chain length are not usually esterified. Once within the body they are oxidized rapidly in tissues as a source of ‘fuel’.
Longer chain fatty acids are usually esterified first to triacylglycerols or structural lipids in tissues.
These form the primary storage form of long chain fatty acids for energy and structure formation of cells. These are composed of glycerol (1,2,3-trihydroxypropane) and 3 fatty acids to form a triester. Triglycerides are found in blood tests. Complete hydrolysis of triacylglycerols yields three fatty acids and a glycerol molecule. Polyunsaturated fatty acids are important as constituents of the phospholipids and form the membranes of the cells. Most of the natural fats and oils of commerce consist of triacylglycerols
Tri-, Di- and Monoacylglycerols
1,2-Diacylglycerols are formed as intermediates in the biosynthesis of triacylglycerols. These also function as second messengers in many cellular processes. Monoacylglycerols are produced when triacylglycerols are digested in the intestines of animals.
Cholesterol is a ubiquitous component of all animal tissues. Most of it is present in the membranes. It occurs in the free form and esterified to long chain fatty acids (cholesterol esters) in animal tissues, including the plasma lipoproteins. Cholesterols are precursor of bile acids, vitamin D and steroidal hormones.
Complex Lipids in Membranes
Cellular membranes control the transport of materials, including signalling molecules and can change in form to enable budding, fission and fusion. The cell membranes have a water loving or hydrophilic constituent and a hydrophobic or water repelling constituent making them amphiphilic.
There are two classes of phospholipids. The first are the glycerophospholipids, which are themselves subdivided into two groups. Phosphatides, is molecules composed of glycerol substituted with two fatty acid esters. Three alcohols that form phosphatides are choline, ethanolamine, and serine.
The second are sphingolipids. Sphingolipids have a long-chain or sphingoid base, such as sphingosine, to which a fatty acid is linked by an amide bond. Sphingomyelin is by far the most abundant sphingolipid in animal tissues. Sphingomyelin is an important building block of membranes
These are molecules wherein fatty acids are linked directly to a sugar backbone. These form part of the cell membrane bilayer as well. In the saccharolipids, a monosaccharide substitutes for the glycerol backbone present in glycerolipids and glycerophospholipids.
Proteolipids and Lipoproteins
These are proteins that are covalently bound to fatty acids or other lipid moieties, such as isoprenoids, cholesterol and glycosylphosphatidylinositol. These include HDL (high density lipoprotein), LDL (low density lipoprotein), VLDL (very low density lipoprotein) etc. according to their molecular size.
These are made by polymerization of acetyl and propionyl subunits using enzymes. These form large number of secondary metabolites and natural products from animal, plant, bacterial, fungal sources. Antimicrobials or antibiotics like erythromycins, tetracyclines and anticancer agents like epothilones are polyketides.