Phosphoglycerides

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Phosphoglycerides

The second largest class of complex lipids consists of the phosphoglycerides also called as glycerophosphates. They are the major components of biological membranes. They control the permeability of plant and animal cell. They consist of Sn glycerol-3-phosphate. The fatty acids are esterified at C-1 and C2 position, and phosphate at C-3. They must contain one phosphate as the name suggests. As these compounds contain a polar head in addition to their non-polar hydrocarbon chain, they are called amphipathic or polar lipids. The parent compound is phosphatidic acid

The most abundant phosphoglyceride in plants and animals is phosphatidylethanolamine and phosphatidylcholine (lecithin). Lecithin’s contains palmitic, stearic, oleic, linoleic and arachidonic acid as the most abundant fatty acids. They are methylating agents and are thought to have the active role in nerve impulse. Cardiolipins are components of membranes of heart cells.

These are the phospholipids in which there is ether linkage instead of ester linkage at C-1 of glycerol. It contains alfa, beta-unsaturated alkenyl hydrocarbon (cis bond) joined by ether linkage. About half of the heart phospholipids are plasmalogens. Their exact function is not known, perhaps it confers resistance to lipases.

Phosphoglycerides can be hydrolyzed

Phosphoglycerides can be hydrolyzed into their components by enzymes called phospholipases as shown below. They are specific for specific sites. Lysolecithin can be obtained by the action of lecithinase, which removes fatty acids from C-2 position and is present in snake venom and the sting of bees. The lecithinase leads to hemolysis of red blood cell, as the lecithin is the major component of membranes.

Sphingolipids

Sphingomyelins

The most common sphingolipids are ceramides having either a phosphocholine or phosphoenol moiety so that they can also be classified as sphingophospholipids. The membranous myelin sheath that surrounds and electricity insulate many nervous cell axons is particularly rich in sphingomyelin

Cerebrosides

The simplest sphingolipids are ceramides with head groups that consist of single sugar residue. Galactocerebroside is most prevalent in the neuronal cell membrane of the brain and has a beta-D-galactose head group. The glucocerebrosides contain glucose as the moiety.

Gangliosides

They are the most complex glycosphingolipids. They are the ceramides attached with oligosaccharides that include at least one sialic acid residue. The gangliosides are primary components of cell surface membrane and constitute a significant fraction (6%) of brain lipids. They have considerable physiological functions. Their carbohydrate head group, which extends beyond the surface of cell membrane, act as a specific receptor for pituitary glycoprotein hormones that regulate a number of important physiological functions. They are also a specific determinant of cell-cell recognition.

Glycolipids

They contain one or more sugar residues attached to either diacylglycerol or a sphingosine derivative and do not contain phosphate. For example, glucoglycerides are the glycosides of diacylglycerol.

Waxes

Waxes are simple lipids containing one molecule of fatty acid esterified with one molecule of a high molecular weight monohydroxy alcohol or sterol. They differ from fats and oils in that the glycerol is replaced with high molecular weight alcohol or sterol.

They are saponifiable and less prone to atmospheric oxidation.

Beeswax, carnauba wax (from carnauba plant) and spermaceti (from sperm whale) are examples of true wax. Beeswax contains palmitic acid esterified with hexacosanol or triacontanol. Carnauba wax is the hardest known wax which consists of fatty acid esterified with tetratriacontanol.

Waxes are found as protective coating on the skin, fur, and feathers of animals and birds and on the leaves and fruits of higher plants, and exoskeleton of many insects.

Steroids

Steroids which are mostly of eukaryotic origin, are derivative so cyclopentanoperhydrophenanthrene, are a compound that consists of four fused rings. All steroid originate from triterpene squalene. Various steroids vary in respect of side chain attached at C-17.

Cholesterol

This is the most abundant steroid in animals, distributed in the plasma membrane. It is found in less extent in cell organelle membranes as compared to the plasma membrane. It is further classified as sterol as it has C3-OH group and its branched aliphatic side chain of 8 to 10 carbon atoms at C17. In plasma, it is esterified to fatty acid as a cholesteryl ester.

The solubility of cholesterol is low in the water, at 25°C, i.e., 0.2mg/100ml. In plasma, the concentration is 150-200 mg/100 ml in normal adult human beings. This high solubility is due to its association with proteins forming LDL (Low-density lipoproteins) and VLDL (Very Low-Density Lipoproteins). It has been observed that 30% cholesterol is existing in free form and rest as fatty acyl derivative. This molecule is of great interest to the scientist world over as it leads to high risk of atherosclerosis (thickening of arteries) and hence heart attack. About thirteen Nobel prizes have been awarded for the work of cholesterol. Plants contain very little cholesterol but synthesize other sterols. Yeast and fungi also synthesize sterols, which differ from cholesterol in their aliphatic side chain and number of double bonds. Prokaryotes contain little, if any, sterol.

Sterol hormones

In mammals, cholesterol is a precursor of steroidal hormones, substances that regulate a great variety of physiological functions. The important steroidal hormones are a corticosteroid, testosterone, estradiol, and progesterone.

The important bile salt derived from these sterols in the animal system are cholic acid, chenodeoxycholic acid etc. They are the important constituents of the bile juice and help in the digestion of lipids in the intestine.

Prostaglandins

The very name is prostaglandins was coined as they were recognized first in prostate glands. They are C20 unsaturated hydrocarboxylic acids with a cyclopentane ring in the molecule. The parent compound is prostanoic acid.

Arachidonic acid is a precursor of prostaglandins. The structure of arachidonic acid is as shown below. It is synthesized from the linoleic acid (essential fatty acid).

They are three major classes of primary prostaglandins i.e. A, E (ether soluble), F (phosphate buffer soluble). One of the example is PGE1

The lipid bilayer is formed when the molecules have a cross-section of head and tail similar. This is normally with the phospholipids, sphingolipids, glycolipids, and gangliosides. Proteins are embedded inside these lipid layers like ice (proteins) floating in water (Sea of lipid). The superficial proteins are called extrinsic and those crossing through are called intrinsic.

They are natural mediation of inflammation. To stop inflammation corticosteroid that inhibits their biosynthesis are used as an anti-inflammatory. Aspirin also inhibits oxygenase that forms prostaglandins from arachidonic acid.

Terpenes

They are the class of lipids which are non-saponifiable in nature. They are made multiple units of five carbon hydrocarbon, isoprene (2-methyl-1, 3butadiene).

They have arranged either head to tail or tail to tail containing two, three, four six or eight units forming linear or cyclic compounds.

  • Monoterpenes geraniol, limonene, and menthol are the major components of geranium, lemon oil, clove oil and mint oil.
  • The diterpene phytol, a linear terpenoid is a component of photosynthetic pigment chlorophyll.
  • The beta-carotene (tetraterpene containing eight isoprene units) is a precursor of Vit-A
  • Another class includes fat-soluble vitamin-E and Vit-K
  • Ubiquinone or coenzyme-Q acts as the hydrogen carrier in oxidative phosphorylation in mitochondria.

Membranes

The thin impermeable barrier around the cytoplasm or cell organelles is termed as membranes. They chiefly constitute amphipathic lipids and proteins. Among the amphipathic lipids, glycerophospholipids, sphingolipids, and sterol are the chief constituents. When they are mixed with water, they aggregate to be away from water. They align in such a manner that their hydrophobic region aggregate and polar region interacting with water. Micelles are relatively small, a spherical structure involving a few dozen to few hundred molecules that have cone shape as shown. The free fatty acids and lysophospholipids usually the micelles.

 

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