Lipid Chemistry Lecture (5) - Biochemistry I PDF

Biochemistry I Lipid Chemistry Lecture (5) Dr/ Mohamed M. Salama Lecturer of Biochemistry and Molecular Biology Course Code: CPB 402 & PB 402  By the end of this lecture, the students should be able to know: OBJECTIVE  Compound lip...

Biochemistry I Lipid Chemistry Lecture (5) Dr/ Mohamed M. Salama Lecturer of Biochemistry and Molecular Biology Course Code: CPB 402 & PB 402  By the end of this lecture, the students should be able to know: OBJECTIVE  Compound lipids A. Phospholipids  Glycero-phospholipids  Sphingo-phospholipids B. Glycolipids  Cerebrosides  Sulpholipids  Gangliosides C. Lipoproteins Phospholipids  A. Phospholipids: These contain in addition to fatty acids and alcohol, a phosphate radical. They also have nitrogen containing bases. The alcohol of phospholipids is either: Glycerol (glycero-phospholipids) or Sphingosine (sphingo-phospholipids).  1- Phosphatidic acid (Diacylglycerol phosphate) Consists of glycerol which is esterified with: Saturated F.A. at position (1). Unsaturated F.A. at position (2). Phosphoric acid at position (3). Function: It has no function. It is produced as intermediate in the synthesis of triacylglycerols and phospholipids.  2- Cardiolipin (diphosphatidylglycerol) Consists of 2 molecules of phosphatidic acid linked together by glycerol. Function: Cardiolipin is the major lipid in mitochondrial membrane.  3- Lecithin (Phosphatidyl choline) It consists of phosphatidic acid, the phosphate of which is esterified with nitrogenous base (choline). Functions: 1. It enters in the structure of cell membrane. It is the most abundant phospholipids in cell membrane. 2. Lecithin acts as lipotropic factor i.e. prevent accumulation of fat in liver. 3. It acts as body store of choline. Choline is important for: It is important in nerve transmission (acetylcholine). It acts as methyl donor in transmethylation reaction. 4. Lecithin forms cholesterol esters. 5. Dipalmityl lecithin acts as a surfactant in the lung. Dipalmityl lecithin (i.e. lecithin which contains 2 palmitic acid residues) acts as a surfactant in the lung. Dipalmityl lecithin continuously secreted by the lung cells in the alveolar wall, forming a monolayer over the watery surface of the alveolus and so lowers the surface tension. This helps expiration and inspiration. In premature babies: Lungs do not secrete enough surfactant. This leads to lung collapse and death from respiratory failure (respiratory distress syndrome). Treatment of this case needs putting the premature babies in incubator and administration of surfactant locally in the lung.  4- Cephalin (Phosphatidyl ethanolamine) It consists of phosphatidic acid, the phosphate of which is esterified with nitrogenous base (ethanolamine). Function: It is one of activating factors of coagulation mechanism.  5- Lysophospholipids (Lysophosphatides) If the fatty acid in position 2 of lecithin or cephalin was removed, we get lysolecithin and lysocephalin respectively.  6- Phosphatidyl serine It is a cephalin like phospholipids but contains amino acid serine rather than ethanolamine.  7-Lipositol (Phosphatidyl inositol) It consists of phosphatidic acid, the phosphate of which is esterified with inositol. Function: It is present in cell membrane.  8- Plasmalogens -They are similar to lecithin and cephalin but fatty acid in position (1) is replaced by unsaturated alcohol attached in ether linkage instead of ester linkage of F.A. Function: It is present in high conc. in brain and nerve tissues.  9- Sphingomyelin -Differ from all other types of phospholipids in that they contain amino alcohol sphingosine instead of glycerol. - A fatty acid usually saturated or monounsaturated is attached in amide linkage to the amino group of sphingosine forming ceramide.  9- Sphingomyelin A phosphate is esterified to the one position of sphingosine and choline is esterified to the phosphate. Function: Sphingomyelins are found in large quantities in brain and nerve tissues. Glycolipids  B. Glycolipids These are compound lipids containing carbohydrate radical. They also contain sphingosine and therefore they can be classified with sphingomyelin as "sphingolipids".  Cerebrosides Glycolipids include Sulpholipids Gangliosides  1- Cerebrosides They are called simple glycolipids. These consist of sphingosine, FA and sugar (usually galactose or rarely glucose). The F.A connected to the amino group of sphingosine in amide linkage and the sugar is connected to the primary alcohol group of sphingosine in glycosidic linkage.  According To The Type Of Fatty Acid, They Can Be Classified Into: 1) Kerasin: the fatty acid is lignoceric acid (C24, saturated). 2) Nervon: the fatty acid is nervonic acid (C24, unsaturated, ω9). 3) Oxynervon: the fatty acid is oxynervonic acid (hydroxy nervonic acid). 4) Cerebron: the fatty acid is cerebronic acid (C24, hydroxy saturated).  Function of cerebrosides: - Cerebrosides are present in many tissues especially in the brain and myelin of nerve fibers. - They act as insulators of nerve impulse (Nerve impulse conduction).  2- Sulpholipids (sulphatides) - These consist of cerebrosides in which the galactose is esterified at C3 with sulphate group. - They are found in the brain and kidneys.  3- Gangliosides They are called complex glycolipids. Upon hydrolysis they give: 1. Ceramide (sphingosine and fatty acid). 2. Hexoses (glucose and galactose). 3. Hexosamines: Sialic acid (N-acetyl neuraminic acid) (NANA). N-acetyl galactosamine. Function of gangliosides: - They act as receptors at cell membrane. - They are present at high concentration in brain. Lipoproteins  C- Lipoproteins Def.: These are compound lipids formed of lipids conjugated with protein. Functions: 1. They are present in cell membrane, mitochondria and plasma (plasma lipoproteins). 2. Plasma lipoproteins convert water insoluble lipids into water soluble complexes. This facilitates transport of lipids between blood and different tissues.  12 % Blood from VLDL Liver and intestine