LIPIDS IN HUMAN BEINGS

LIPIDS

 

The term lipid describes a group of biological compounds which are not soluble in water but are usually soluble in many organic solvents like ether chloroform and benzene etc. Thus, unlike the other major groups of biological molecules like proteins and carbohydrates and nucleic acids, lipids are categorized by a physical property rather than by structural features. Lipids play a very  important role in the life of an organism. The lipids are divided in to sub groups depending on their functional characteristics.

They are, Fatty acids, Fats and oils Soaps and detergents, waxes, Phospholipids, terpenes, steroids, lipid soluble vitamins and bio synthetic pathways. Each lipid has a different structure. All the lipids have a large number of Carbon hydrogen bonds which  make them a highly energy rich class of compounds.

 

Classification of lipids

Lipids are classified into two major groups depending upon their chemical composition.

A. Simple lipids:

Simple lipids can be segregated into structural types, which are fatty acids, waxes, triglycerides and sterols. A fatty acid is a long-chain monocarboxylic acid, and a wax is the ester of a long-chain alcohol and a fatty acid. A triglyceride is the ester of a glycerol that contains three fatty acid molecules. Sterols are a special class of alcohols, containing a fused four-ring structure, or steroid nucleus. Sterols may combine with a fatty acid to form sterol esters.

 

B. Complex lipids:

Among the complex lipids, important structural types are phosphoglycerides, phosphosphingolipids, and glycolipids. The parent phosphoglyceride, phosphatidic acid  is similar in structure to a triglyceride except that the 3-hydroxyl group of the glycerol component is esterified to phosphoric acid rather than to fatty acids.

Further esterification of the phosphoric acid of phosphatidic acid with a variety of small, hydroxyl-containing molecules leads to a series of derived phosphoglycerides, including phosphatidyl choline commonly known as lecithin, phosphatidyl ethanolamine and phosphatidyl serine.

The phosphosphingolipids are derived from sphingosine, a long-chain dialcohol with an amino group. The formation of an amide with an FA at one point along this chain yields ceramide. Esterification of a ceramide derivative with phosphorylcholine yields sphingomyelin, which is the major phospho-sphingolipid. If ceramide is instead linked to a simple sugar, a cerebroside glycolipid is formed. Further addition of several amino sugars yields the lipids called gangliosides.

 

Structure of different types of lipids

 

Every Lipid has a different structure. But they all have a large number of carbon hydrogen bonds in them. The length of the chain which is generally linear is less than the chain length of proteins. The molecular mass of lipids is in between amino acids and proteins.

Lipids are classified into two groups depending upon their chemical structure: fatty acid based lipids and iso-prene based lipids.

 

Fatty acid based lipids:

 

Fatty acids are saturated or unsaturated monocarboxylic acids. Unique arrangement of fatty acids provide the molecule with a non polar hydrophobic end and a polar hydrophilic end. Fattyacids called unsaturated when there is a double between two carbon atoms and saturated when there is no double bond between carbon atoms.

 

Iso-prene based lipid:

 

Iso-prene is a branched five carbon chain. Iso-prene is aliquid and flammable unsaturated hydrocarbon. Iso-prene has a broad spectrum of naturally occuring isoprene lipids is obtained from the linking of isoprene monomers such as estrogen, testosterone and cholestrol.

There are some following structures of different types of lipids:

 

Structure and Function of lipids

 

Triacylglycerol

 

These are the composed of glycerol (1,2,3-trihydroxypropane) and  three fatty acids to form a triester. Complete hydrolysis of triacylglycerols yields three fatty acids and a glycerol molecule. Most of the naturally occurring oil and fats consist of triacylglycerol.

Fatty acids

These are the defining constituresponsible for the distinctive physical and metabolic properties. They are also important in non-esterified form. In the body they are released from triacylglycerols during fasting to provide a source of energy. Linoleic and linolenic acids are essential fatty acids because  they cannot be synthesised by animals and must come from plants through diet. They are precursors of arachidonic, eicosapentaenoic and docosahexaenoic acids that 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 mostly esterified first to triacylglycerols or structural lipids in tissues.

Steroids

Members of the steroid family are present in plants, yeast, protozoa, and higher forms of life. Steroids has a variety of biological functions, from participation in cell membrane structure to regulation of physiological events. Naturally occurring steroids and their synthetic forms are used mostly in medical practice. Each steroid contains three fused cyclohexane (six-carbon) rings and a fourth cyclopentane ring. Naturally occurring steroids have an oxygen-containing group at carbon-3. Shorthand formulas for steroids indicate the presence of double bonds, as well as the structure and position of oxygen-containing or other organic groups.

 

Waxes

In their most common form, wax esters consist of fatty acids esterified to long-chain alcohols with similar chain-lengths. The latter tend to be saturated or have one double bond only. Such compounds are found in animal, plant and microbial tissues and they have a variety of functions, such as acting as energy stores, waterproofing and lubrication.

In some tissues, such as skin, avian preen glands or plant leaf surfaces, the wax components can be much more complicated in their structures and compositions. They can contain aliphatic diols, free alcohols, hydrocarbons (e.g. squalene), aldehydes and ketones. 

Phospholipids

There are two classes of phospholipids. The first is the glycerophospholipid  sphingolipids.

Glycerophospholipids

They are themselves divided into two groups phosphatides, a molecule composed of glycerol substituted with two fatty acid esters. They are the main component of cell membrane. They are the reservoir for intracellular messenger. Anchors some proteins for cell membrane. They act as co-factor of enzymes and biological detergents.

 

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 a most abundant sphingolipid found in animal tissues. Sphingomyelin is an important building block of membranes. Complex sphingolipids are located mainly in the plasma membrane of mammalian cells where they have a structural function, although they also act as adhesion sites for proteins from the extracellular tissue. therefore, they have analogous intracellular functions in all cellular compartments, including the nucleus. It is now evident that sphingolipids and their metabolites have important roles in signal transduction. In recent years, it has become apparent that sphingolipids are involved in the patho-physiology of  many of the more common human diseases including diabetes, many different cancers, microbial infections, Alzheimer's disease and other neurological syndromes, and diseases of the cardiovascular and respiratory systems. Sphingolipids and their metabolism are therefore likely to prove of ever increasing interest to scientists.

Sphingolipid

Thus, as a whole lipids are an essential part of the diet. Some lipids are most essential because they cannot form in our body but needs them to form important molecules and structures. Other lipids have been identified as increasing the risk of heart disease and other serious conditions, including obesity, diabetes and atherosclerosis.