Brief Information about Lipids

                                Lipids are a heterogeneous group of organic compounds characterized by their dissolvability in nonpolar solvents such as chloroform, ether, and benzene and by their poor solubility in water. Lipids might be polar or nonpolar. Polar lipids have constrained solubility in water since they are amphipathic, i.e., they possess hydrophilic and hydrophobic regions in the similar molecule. Major polar lipid group contain fatty acids, cholesterol, glycerophosphatides, and glycosphingolipids. Very short chain fatty acids and ketone bodies are immediately soluble in water. Nonpolar lipids serve principally as storage and transport forms of lipid and include triacylglycerols and cholesteryl esters. Lipids have numerous functions including the following: thermal insulation, energy storage (as triacylglycerol), metabolic fuels, membrane components, hormones, precursors of prostanoids and leukotrienes, emulsifying agents in the digestion and absorption of lipids, and surfactants in the alveolar membrane. 

                                Fatty acids that donot possess carbon-carbon double bonds are known as saturated and those with carbon-carbon double bonds as unsaturated. Fatty acids that possess an even number of carbon atoms and are acyclic, unbranched, nonhydroxylated, and monocarboxylic make up the largest group. The highly abundant saturated fatty acids in animals are palmitic and stearic acids. The melting point of fatty acids increases with increase in chain length, the even-numbered saturated fatty acids contain higher melting points than the odd-numbered. Among the even numbered, the presence of cis double bonds lowers the melting point significantly. Free fatty acids ionize at physiological pH (pK ~4.85) and exist only in minute quantities; in plasma, they typically are bound to albumin. 

                                They are usually present as esters or amides. Triacylglycerols packaged as chylomicrons in the intestinal epithelial cell are delivered to the blood circulation via the lymphatic system and are hydrolyzed to glycerol and fatty acids by endothelial lipoprotein lipase. Fatty acids are taken up by the cells of the tissue where the degradation happens, whereas glycerol is metabolized in the liver and kidney. Another means of triacylglycerol transport is very-low-density lipoprotein (VLDL), which is synthesized in the liver. Its triacylglycerol is also hydrolyzed by endothelial lipoprotein lipase.

Fatty acids:

A long hydrocarbon chain with carboxylic group is called as fatty acid. It can be of two types:

Saturated- without double bonds

Unsaturated- possess double bonds which are able to give cis or trans isomers.

Naturally occurring saturated fatty acids

Common Name	Systematic  Name	Molecular Formula	Structural Formula
Capric	n-Decanoic	C10H20O2	CH3[CH2]8COOH
Lauric	n-Dodecanoic	C12H24O2	CH3[CH2]10COOH
Myristic	n-Tetradecanoic	C14H28O2	CH3[CH2]12COOH
Palmitic	n-Hexadecanoic	C16H32O2	CH3[CH2]14COOH
Stearic	n-Octadecanoic	C18H36O2	CH3[CH2]16COOH
Arachidic	n-Eicosanoic	C20H40O2	CH3[CH2]18COOH
Behenic	n-Docosanoic	C22H44O2	CH3[CH2]20COOH
Lignoceric	n-Tetracosanoic	C24H48O2	CH3[CH2]22COOH
Cerotic	n-Hexacosanoic	C26H52O2	CH3[CH2]24COOH
Montanic	n-Octacosanoic	C28H56O2	CH3[CH2]26COOH

Lipid Classification:

Lipids might be classified based on their physical properties at room temperature, on polarity, or on their requirement for humans, but the preferable classification is depends on their structure.

Lipids can be classified into three major groups based on their structure:

·         Simple Lipids

·         Complex Lipids

·         Derived Lipids

Simple Lipids:

Simple Lipids are esters of fatty acids with glycerol or various other alcohols. Waxes and fats come under simple lipids. Esters of fatty acids with high molecular weight monohydric alcohol can be known as wax. The major simple lipids are triacylglycerols (fats). Triacylglycerols or triglycerides are esters of glycerol with three fatty acids.

Triacylglycerol

Complex Lipids:

Complex lipids contain both hydrophilic and hydrophobic residues. These can be classified into phospholipids and sphingolipids.

Phospholipids:

These are lipids with phosphate group in their molecules. Phospholipids contain two fatty acids, a phosphate group and a glycerol molecule. Phospholipids are major components of cell membranes.

Phosphatidic acid: 

It is important component in lipid bilayer. It comprises one molecule of glycerol, two fatty acid molecules and phosphoric acid. For the tricarbon compound glycerol, saturated fatty acid binds to Carbon 1, unsaturated fatty acid binds to Carbon2 and phosphate group binds to Carbon3. It acts as precursor for many lipid molecules biosynthesis. It influences membrane structure. It acts as signaling molecule. In Drosophila, it plays key role in phototransduction.

Phosphatidic acid

Phosphatidylethanolamine:

 It is also known as cephalin. In this, glycerol form ester bond with two fatty acids and phosphoric acid. Phosphoric acid is linked with ethanolamine. It is available abundantly in nervous tissues. It plays a key role in cytokinesis of cell division. It increases the rate of thrombin formation for blood clotting.

Phosphatidylethanolamine

Phosphatidylcholine:

It belongs to Lecithin group lipid substance. It can be found in egg yolks, soybeans etc. It comprises glycerophosphoric acid and choline with different fatty acids. Phosphotidylcholine is useful for treating ulcerative colitis via oral intake. It can be given as supplement to the patients with dementia. It is key component of pulmonary surfactant.

Phosphatidylcholine

Phosphatidylserine:

It is a glycerophospholipid. In this, serine is linked to glycerol with fatty acids via phosphodiester linkage. It plays an important role in apoptosis. It is present in cell membrane and plays a key role in cell signaling and blood clotting.

Phosphatidylinositol:

It contains mostly myo-Inositol molecule in its structure. Cell signaling, lipid signaling and membrane trafficking are major roles of phosphorylated forms of phosphotidylinositol.  

Phosphatidylinositol

Sphingolipids:

Lipids containing amino alcohol like sphingosine as backbone are called sphingolipids.

Sphingomyelin :

Sphingomyelin can be seen in myelin sheath of nerve cell. It is a type of sphingolipid with ceramide (sphingosine+fatty acid) and phosphocholine or phosphoethanolamine. Ester link is formed between head molecule and 1- OH group of a ceramide. It insulates electrical flux in nerve cell. It has a significant role in signal transduction. It is essential for apoptosis.

Sphingomyelin

Glycosphingolipids:

These are sphingolipids bound with carbohydrate moiety. These glycosphingolipids group include cerebrosides, gangliosides and globosides. 

Cerebrosides:

 In this, sugar residue, glucose or galactose is attached to ceramide through 1-OH moiety. Based on sugar molecule, these are of two types majorly, glucocerebrosides and galactocerebrosides. Galactocerebrosides can be seen in nerve cells and glucocerebrosides occur in rest of the cells.

Gangliosides:

Gangliosides comprises of ceramide and sialic acid linked to oligosaccharide chain. These are obtained from ganglion cells by Klenk, hence named as gangliosides. It helps in signal transduction. It plays an important role in immunology. It has a key role in carcinogenesis.

Globosides:

Globoside comprises of ceramide and more than one sugar residue as a side chain. Generally, sugars occur in the combination of N-acetylgalactosamine, D-glucose or D-galactose.

Derived Lipids:

These are components obtained by degradation of simple and compound lipids or precursors of them. They contain fatty acids and alcohols, sterols, hydrocarbons and Vit A, D, E, and K (fat soluble vitamins).

In the next post, I will discuss about derived lipids in detail.