How Lipids are Classified?

Lipids are classified on many counts. They are classified as simple or complex based on the presence or absence of fatty acid molecules in the backbone structure.

They are also classified as storage or structural based on their functions. However, the classification presented hereunder, is based on the backbone structure.

Lipids (SI. Nos. 2-4 in the outline classification) bears one or more fatty acid molecules in the structure or is derived from fatty acids. Therefore, it is imperative to discuss the structure and properties of fatty acids.

Fatty acids:

Fatty acids are carboxylic acids (4 – 36 carbon atoms) with aliphatic hydrocarbon chains. However, the more commonly occurring fatty acids have even number of carbon atoms (12 – 24) in an unbanked chain.

Some fatty acids have saturated hydrocarbon chains bearing no double bond (palmitic and stearic acid); while others have one or more double bonds (oleic acid). These are known as unsaturated fatty acids. Presence of double bonds decreases the melting point of the fatty acid.

Eicosanoids:

These are derived from a 20 carbon poly-unsaturated fatty acid and comprise of prostaglandins, thromboxanes and leukotrienes. Prostaglandins were originally isolated from the seminal plasma. However, it is now found to be present in all mammalian tissues. These act as local hormones carrying messages to nearby cells.

Thromboxanes are produced by platelets (thrombocytes). These help in the formation of blood clots and reduce the flow of blood to the clot. Leukotrienes, produced by leucocytes induce the contraction of muscles present in the air passage to the lung.

Storage lipids:

These lipids are stored in tissues and form an alternate source of energy to glucose during emergency situations.

Acryl glycerol’s (Triglycerides): These are fatty acid esters of glycerol. When all the three hydroxyl groups of the glycerol molecule are esterified by one each of fatty acid molecules, it results in a triacylglycerol or triglyceride or neutral fat.

This is the most common type of storage lipid in plant and animal organisms. Triglycerides are non-polar and hydrophobic molecules, essentially insoluble in water. The melting point of a triglyceride is affected by the melting point of the fatty acid molecule present in the structure.

Unsaturated fatty acids with double bonds in the hydrocarbon chain decrease the melting point of a triglyceride. Triglycerides extracted from the source in plants have a mixture of unsaturated fatty acids and hence are liquids at room temperature, while those from animals have saturated fatty acids and are usually semisolids at room temperature. The cooking oils used in our kitchens are extracted from the plants and are called vegetable oils.

These oils are liquids at body temperature and hence, reduce the incidence of cardiac diseases. Human fat is made up of fat cells storing neutral fat and inter-cellular substance, together constituting the adipose tissue.

There is a thick layer of sub-cutaneous neutral fat called blubber in the blue whale. Animals of Polar Regions have a thick layer of sub-cutaneous fat tissue made up of neutral fat. It insulates the body from the freezing cold.

Waxes: Biological waxes are long chain (14-36 carbon atoms) saturated or unsaturated fatty acids with long chain (16 – 30 carbon atoms) alcohols.

Their melting points (60° – 100° C) are generally higher than triglycerides. The skin glands of birds and other vertebrates secrete wax, which keeps the skin water proof. Certain plants also secrete wax to prevent an excessive aerial loss of water. Biological waxes have a variety of application in pharmaceutical and cosmetic industries.

Lanolin (from lamb wool), bees wax, carnauba wax (from a Brazilian tree) and wax extracted from spermaceti oil of sperm whale are widely used in the manufacture of lotions, ointments and polishes.

Structural lipids:

Phospholipids: Phospholipids (also known as phosphoglycerides) constitute the largest class of lipids in all biological membranes. Structurally, the third hydroxy! Group of the glycerol molecule is esterifies to phosphoric acid.

The other two hydroxyl groups are esterifies to on each of fatty acid molecules. This results in a parent molecule of all phospholipids, the phosphatidi acid.

A highly polar or a charged group is attached to the phosphoric acid by a phosphodieste bond resulting in a phospholipids’ molecule (It is named in accordance with the attache polar or the charged head group. For example, phosphatidyl choline and phosphatidyl ethanolamine have choline and ethanolamine, respectively as the polar head group.

Phosphatidyl serine, phosphatidyl glycerol, phosphatidyl inositol biphosphate and cardiolipin are the other examples. All phospholipid molecules are amphipathic, i.e. they have two distinct ends, one water loving or polar (hydrophilic) head and two water hating or non-polar (hydrophobic) tails.

It is due to this property that there are two layers of phospholipid molecules in all biological membranes, which spread as sheets.

The polar heads orient towards the outer and cytoplasmic side, while the non-polar tails are present in the inner side forming the hydrophobic core of the membrane. This property of biological membranes does not allow polar (water soluble) molecules and ions to pass through them with relative ease.

Some phospholipids from unicellular organisms and vertebrate heart tissue contain ether linkages instead of ester linkages. In unicellular organisms, one molecule of fatty acid is linked to the C_] of glycerol by an ether linkage forming a compound known as plasmalogen. Platelet activating factor is another such phospholipids in vertebrates. It is released from basophils, which stimulates platelet aggregation and release of serotonin.

Sphingolipids: These are the second largest class of membrane lipids. Like phospholipids, these are amphipathic having a polar head and two non-polar tails. Unlike phospholipids, they do not have glycerol in their backbone. All sphingolipids are derived from a compound called sphingosine or 4-sphingenine. A molecule of fatty acid is attached to the amino group in an amide linkage at the C₂ resulting in ceramide. Ceramide is the parent compound for all sphingolipids. A polar head group is attached to the first carbon atom by a phosphodiester orglycosidic bond. This results in an amphipathic molecule. There are three groups of sphingolipids: sphingomyelin, neutral glycospihngolipids and gangliosides

(a) Sphingomyelins: In sphingomyelins, the polar head group is phosphocholine or phosphoethanolamine. It is present in the plasma membrane of all animal cells and myelin, an insulating membranous sheath of some axons.

(b) Glycosphingolipids: These are present on the outer face of plasma membranes. The polar head groups are one or more monosaccharides attached to the hydroxyl group at C, of ceramide. A cerebroside results, when a single monosaccharide unit is the head group. Galactose is the head group in the plasma membrane of neural tissue, while glucose in the non-neural tissue. Glycosphingolipids act as cell surface recognition molecules (antigens), especially as blood group substances.

2.1.4.2 (c) Gangliosides: Gangliosides are complex sphingolipids with oligosaccharides as head groups having, /V-acetyl neuraminic acid (sialic acid) at the termini. Sialic acid confers a negative charge to the sphingolipid at pH 7.0.

Steroids:

Steroids are oxidized derivatives of sterols. Structurally all sterols are made up of a cyclopentanophenanthrene (steroid) r jt leus consisting of three phenanthrene rings (A, B¹ and C), to which is attached a cyclopentenyi ring, D. It is a 19 carbon structure.

All steroid hormones are derived from the sterol, cholesterol, a 27 carbon compound. An 8 carbon side chain is attached to the steroid nucleus to result in a cholesterol molecule. Cholic acids (glycocholic acid and taurocholic acid), metabolic products of cholesterol are formed in the liver. These are released to the gall bladder, where they form bile salts. Vitamin D a fat soluble vitamin is derived from the sterol, ergosterol.

Isoprenoids (terpenoids):

Isoprenoids are multiples of a 5 carbon structure, isoprene. Two isoprene units condense head to tail or tail to tail to form a monoterpene. Functional isoprenoids may be mono-, di- or tri-teipenes. Terpenes include ubiquinone (a component of the respiratory chain) and dolichol, which takes part in glycoprotein synthesis. Plant derived isoprenoids include menthol, camphor, rubber, b-carotene (provitamin A), fat soluble vitamins like vitamin A, E and K and plant oils like geraniol and limonene.

Plasma lipoproteins:

Cholesterol and other lipids are essentially insoluble in water. They are carried from the place of origin to the end user tissue as plasma lipoproteins. A plasma lipoprotein is a macromolecular complex of a carrier protein called apolipoprotein and the transported lipid. There are four classes of human plasma lipoproteins: chylomicrons, very low density lipoproteins (VLDL), low density lipoproteins (LDL) and high density lipoproteins (HDL).