Compound Lipids Biochemistry PDF

Biochemistry Compound (complex) lipids Dr. Abdisamad Hashi 1 Phospholipids Phospholipids are compound lipids containing phosphoric acid, in addition to fatty acids, nitrogenous base and alcohol. There are two classes of Phospholipids: - 1. Glycerophospholipids – th...

Biochemistry Compound (complex) lipids Dr. Abdisamad Hashi 1 Phospholipids Phospholipids are compound lipids containing phosphoric acid, in addition to fatty acids, nitrogenous base and alcohol. There are two classes of Phospholipids: - 1. Glycerophospholipids – that contain glycerol as the alcohol. 2. Sphingophospholipids – that contain sphingosine as the alcohol. 2 Glycerophospholipids  Glycerophospholipids / phosphoglycerides – are the major lipids that occur in biological membranes.  They consist of glycerol-3-phosphate esterified at its C1 and C2 with fatty acids.  Usually, C1 contain a saturated fatty acid while C2 contains unsaturated fatty acid. 3  Types of Glycerophospholipids 1. Phosphatidic acid, 2. Lecithin, 3. Cephalins, 4. Phosphatidylinositol, 5. Phosphatidylserine and 6. Cardiolipin. 4 Phosphatidic acid  This is the simplest phospholipid. It does not occur in good concentration in the tissues.  Basically, phosphatidic acid is an intermediate in the synthesis of triacylglycerol and phospholipids. 5 Lecithin (phosphatidylcholine)  These are the most abundant group of phospholipids in the cell membranes.  Chemically, lecithin (Greek: lecithos – egg yolk) is a phosphatidic acid with choline as the base.  Phosphatidylcholines represent the storage form of body’s choline for nerve transmission and transmethylation.  Lecithin acts as lipotropic factor I.e. prevent accumulation of fat in liver (fatty liver). 6 Lecithin (phosphatidylcholine) Lecithin forms cholesterol esters: Cholesterol esters is transported to the liver and excreted with bile. This prevents atherosclerosis. Lecithin prevents gall stones: lecithin in bile solubilizes cholesterol and prevent cholesterol stones in gall bladder 7 Dipalmitoyl lecithin  Dipalmitoyl lecithin is a lecithin which contains 2 palmitic acid residues. It is an important phosphatidylcholine found in lungs.  It is a surface active agent and prevents the adherence of inner surface of the lungs due to surface tension.  Respiratory distress syndrome in infants is a disorder characterized by the absence of dipalmitoyl lecithin.  In premature babies, lungs do not secrete enough surfactant. This leads to lung 8 collapse and death from respiratory Cephalins (phosphatidylethanolamine)  Ethanolamine is the nitrogenous base present in cephalins. Thus, lecithin and cephalin differ with regard to the base.  Cephalins participate in blood clotting. 9 Phosphatidylinositol  Thisis an important component of cell membranes.  The action of certain hormones (e.g. oxytocin, vasopressin) is mediated through PI. 10 Phosphatidylserine The amino acid serine is present in this group of Glycerophospholipids. Phosphatidylthreonine is also found in certain tissues 11 Cardiolipin  It is so named as it was first isolated from heart muscle.  Structurally, cardiolipin consists of two molecules of phosphatidic acid held by an additional glycerol through phosphate groups.  It is an important component of inner mitochondrial membrane.  Cardiolipin is the only phosphoglyceride that possesses antigenic properties.  It stimulates antibody formation i.e. antigenic. 12 Cardiolipin 13 Sphingomyelines Sphingosine is an amino alcohol present in sphingomyelines. They do not contain glycerol at all. Sphingosine is attached by amide linkage to a fatty acid to produce ceramide. Sphingomyelines are important constituent of myelin and found in good quantity in brain and nervous system. 14 Niemann Pick's disease  It is accumulation of large amounts of sphingomyelin in liver due to deficiency of sphingomyelinase enzyme.  It leads to mental retardation and death in early life. 15 Ceramide  Ceramide acts as a second messenger (signaling molecule) by regulating programmed cell death (apoptosis), cell cycle and cell differentiation. A ceramide containing a 30-carbon fatty acid is a major component of skin.  It regulates skin’s water permeability. 16 Glycolipids  Glycolipids are important constituent of cell membrane and nervous tissues (particularly the brain).  Cerebrosides are the simplest form of Glycolipids.  They consist of ceramide (sphingosine attached to a fatty acid) and one or more sugars.  Sulpholipids (sulphatides) are cerebrosides containing sulphate group (attached to sugar).  Galactocerebroside & glucocerebroside are the most important glycolipids. 17 Structure of galactosylceramide 18 Cerebrosides (simple glycolipids)  Upon hydrolysis ,they give Sphingosine, Fatty acid and Sugar.  According to the type of fatty acid, they may be classified into: - Kerasin (lignoceric acid), Narvon (nervonic acid), Oxynervon (oxynervonic acid) and Cerebron (cerebronic acid).  Functions of cerebrosides: - 1. Cerebrosides are present in many tissues especially in the brain and myelin of nerve fibres. 2. They act as insulators of nerve impulse. 19 Gaucher’s disease  Accumulation of cerebrosides (sphingollpids) in phagocytes due to deficiency of glucocerebrosldase enzyme.  Manifestations: mental retardation, hepatomegally and bone disorders. 20 Gangliosides  These are found in ganglions and are the most complex form of glycosphingolipids.  Upon hydrolysis they give Ceramide (sphingosine and fatty acid), Hexoses (glucose and galactose) and Hexosamines: Sialic acid (N- acetylneuraminic acid) and N- acetylgalactosamine).  The most important gangliosides present in the brain are GM1, GM2, GD, and GT. 21 Functions of gangliosides 1. They act as receptors at cell membrane. 2. They are present in high concentration in brain.  Degradation: By hexosaminidase enzyme.  The ganglioside, GM2 that accumulates in Tay-Sachs disease.  Accumulation of gangliosides in brain and intestine due to deficiency of hexosaminidase enzyme.  Manifestations: mental retardation, hepatomegally, blindness and death in early life. 22