Medical brain notes 25

  Lipid Metabolism Introduction Lipids are organic compounds of biological nature that includes fats, oils and waxes. They are insoluble in water but soluble in nonpolar solvents such as ether, chloroform and benzene. Lipids are utilizable by living organisms. In the normal mammal at least 10 to 20 percent of the body weight is lipid. They form important dietary constituent on account of their high calorific value and fat soluble vitamins (vitamins A, D, E and K) along with the essential fatty acids. Lipids are distributed in all organs, particularly in adipose tissues in which lipids represent more than 90 percent of the cytoplasm of a cell. Biological functions of Lipids Lipids are stored in a relatively water - free state in the tissues in contrast to carbohydrates which are heavily hydrated to perform a wide variety of functions.   1.             Body lipids are reservoir of potential chemical energy. Lipids can be sto...

  Biological functions of Lipids Lipids are stored in a relatively water - free state in the tissues in contrast to carbohydrates which are heavily hydrated to perform a wide variety of functions.   1.             Body lipids are reservoir of potential chemical energy. Lipids can be stored in the body in almost unlimited amount in contrast to carbohydrates. Furthermore, lipids have a high calorific value (9.3 calories per gram) which is twice as great as carbohydrate. Large amount of energy is stored as lipid than as carbohydrates. 2.             Lipids which forms the major constituent of biomembranes are responsible for membrane integrity and regulation of membrane permeability. 3.             The subcutaneous lipids serve as insulating materials against atmospheric heat and cold and protect internal organs. 4. ...

  Fatty acids The fatty acids are the basic units of lipid molecules. Fatty acids are derivatives of aliphatic hydrocarbon chain that contains a carboxylic acid group. Over 200 fatty acids have been isolated from various lipids. They differ among themselves in hydrocarbon chain length, number and position of double bonds as well as in the nature of substituents such as oxy-, keto-, epoxy groups and cyclic structure. Depending on the absence, or presence of double bonds, they are classified into saturated and unsaturated fatty acids. Saturated fatty acids, do not contain double bonds. The hydrocarbon chain may contain 12 to 18 carbon atoms. eg. palmitic and stearic acids CH 3  (CH 2 ) 14  COOH     -  Palmitic acid (C-16) CH 3  (CH 2 ) 16  COOH     -  Stearic acid (C-18) Unsaturated fatty acids are classified into different types depending on the number of double bonds present in the hydrocarbon chain. These fatty ...

  Biosynthesis of fatty acids 1.             Biosynthesis of fatty acids occurs in all organisms and in mammals it occurs mainly in adipose tissue, mammary glands, and liver. 2.             Fatty acid synthesis takes place in the cytosol in two steps. ·                Formation of medium chain fatty acid of chain length 16 carbon atoms. ·                Lengthening of this carbon chain in microsomes for larger fatty acids. 3.             Acetyl CoA serves as a source of carbon atoms for saturated as well as unsaturated fatty acids. Acetyl CoA can be formed from excessive dietary glucose and glucogenic amino acids (amino acids which can be converted to glucose). Carbohydrates and ami...

  Oxidation of Fatty Acids The digestion of fats starts in the small intestine. Fats are emulsified by the bile salts and hydrolysed by the pancreatic lipases to form free fatty acids. These free fatty acids combine with glycerol (produced by the glycolytic process) to form triglycerides. They combine with proteins to form lipoproteins and enter into circulation to perform various biolological functions such as oxidation, storage and formation of new lipids. Thus the various fatty acids may exist in the free form as well as in the esterified form (Triglyceride) in blood. Fatty acids are the immediate source for oxidation of fats in various tissues viz. liver, adipose tissue, muscles, heart, kidney, brain, lungs and testes.   1.   β -Oxidation Fatty acids are oxidised to CO 2  and water with the liberation of large amount of energy. Oxidation is brought about in the mitochondria because all the enzymes required for oxidation are present in the mitochondria. Oxida...

  Cholesterol Biosynthesis Cholesterol is an animal sterol which occurs either free or as fatty esters. As it was first isolated from human gallstones deposited in the bile duct, it is named as cholesterol (Greek word chole means – bile, sterol). Cholesterol is composed of 1,2-cyclopentanoperhydrophenanthrene ring system. Although cholesterol is an essential compound for life for the synthesis of hormones, bile acids and vitamin D, it is not necessary to supply it in the diet because it can be synthesised in the cell from acetyl CoA Carbohydrates, amino acids, fatty acids and glycerol which gets converted to Acetyl CoA can also serve as a source for cholesterol synthesis. The liver plays a decisive role in the cholesterol metabolism which accounts for 90% of the overall endogenic cholesterol and its esters. A man weighing 70 kg contains about 140 grams of cholesterol. The cholesterol concentration of blood in human is between 150 to 250 mg per 100 ml, being distributed equally betw...

  Biosynthesis of cholesterol Important intermediates of cholesterol Biosynthesis and enzymes involved. 1. Formation of acetyl CoA A molecule of acetic acid combines with coenzyme A (CoA) to produce Acetyl CoA in the presence of an enzyme Acetyl CoA synthetase. 2. Formation of acetoacetyl CoA Two molecules of acetyl-CoA condense to form an acetoacetyl-CoA molecule, catalyzed by the enzyme “thiolase”. 3. Formation of HMG CoA The acetoacetyl-CoA further undergoes condensation with one more molecule of acetyl-CoA to form HMG-CoA (3-Hydroxy 3-Methyl Glutaryl-CoA). The enzyme which mediates this reaction is called HMG-CoA synthetase. 4. Formation of mevalonate The HMG-CoA is reduced to form mevalonate by NADPH + H+ dependent reductase (HMG-CoA reductase). This is the rate limiting enzyme in the pathway of cholesterol biosynthesis. 5. Mevalonate thus formed is then converted to squalene through various steps. 6. Squalene, with the formation of various intermediates finally give rise to t...