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LIPID METABOLISM Part 2

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LIPID METABOLISM Part 2 LIPID METABOLISM part 2 Dr. Ahmed Ali Hussein PhD of Biochemistry 2018-2019 Biosynthesis of triglyceride in the intestinal mucosal cells: The biosynthesis of triglycerides occurs in the intestinal mucosal cells using glycerol, monoglyceride (MG) or α -glycerophosphate through several steps as follows: 1. Activation of the free fatty acids: For a fatty acid to react it is first activated. This can occur by Thiokinase using CoA & energy in the form of ATP as follows: 2. Formation of α-Glycerophosphate: α -Glycerophosphate can be formed from either of two sources: a. Metabolism of triglycerides: as mentioned one of the products is glycerol. Glycerol can be phosphorylated to α -Glycerophosphate as follows: In liver, kidney, mammary gland and mucosal intestinal cells. In this tissue the glycerokinase enzyme is active. b. From glucose by glycolysis as follows: CH2 OH CH2 OH Glycolysis dehydrogenase Glucose C O CH OH OH NADH + H OH CH2 O P O NAD CH2 O P O OH OH Dihydroxy acetone α-Glycerophsphate Phosphate In muscle and adipose tissue glycerokinase lack in this tissue so glycerol-3-p (glycerophsphate) formed from glycolysis. 3- Formation of T.G. : Acyl CoA transferase : enzyme responsible for the transfer of Acyl group to glycerol-3-p . The T.G. synthesized in the tissue for stored the energy, all the energy is in F.A. Oxidation of fatty acids :- (β – oxidation ) The process at which the fatty acid oxidation in the matrix of mitochondria to acetyl CoA , and liberate a high energy in form of FADH2 and NADH . This FADH2 and NADH converted to ATP in the respiratory chain . β- oxidation this name derived from the oxidation of β- carbon atom of F.A. with removal 2 carbon atom each time as Acetyl CoA . This acetyl CoA enter krebs cycle and generate further ATP Summary of the energy yield from the oxidation of Palmitic acid (16 carbons) . Energetic of fatty acid oxidation: The no. of cycles to reach the end products is equal to the No. of carbon atoms of the fatty acid divided by 2 minus 1 e.g. palmitic acid (16 carbon atom). No. of Cycles is =16/2 = 8 8 - 1 = 7 No. of high energy phosphate bonds is 5 in each term, so 7 X 5 = 35 ATP In this way 8 Acetyl CoA is formed, so 8 X 12 = 96 ATP & so +35 From β- Oxidation +96 From TCA cycles -2 In step no. 1 (formation of palmitoyl Co A) _____ 129 ATP net energy Fatty acid synthesis ( Lipogenesis ) F.A. synthesized in the cytosol of many tissues include : Liver , kidney , mammary gland , adipose tissues . Acetyl CoA is the precursor of FA synthesis which provide from glucose . FA synthesis require the NADPH as Coenzyme . which provide from HMPs (hexose monophosphate shunt). Both metabolic processes found in the cytosol of the cell . Lipogenesis increased in diet rich in carbohydrates and decreased when energy intake is decreased . Differences between F.A. oxidation (β-oxidation) and F.A. synthesis(lipogenesis): FA oxidation FA synthesis 1 Named lipolysis Named lipogenesis 2 Take place in matrix of Take place in cytosol mitochondria 3 Utilize the FAD and NAD as Require the NADPH as Coenzyme . Coenzyme and liberate high energy 4 HMPs doesn’t important for HMPs is very important for FA FA oxidation synthesis . Cholesterol The main sterol in human and animals , it found in all tissue and blood . Source of cholesterol :- Exogenous (dietary cholesterol ) Endogenous (synthesized cholesterol ) Dietary cholesterol :- Cholesterol in diet present as free cholesterol and esterified cholesterol . In intestine lumen esterified cholesterol is rapidly hydrolyzed to free cholesterol by cholesterol esterase . Free cholesterol then absorbed to mucosal intestine cells together with dietary free cholesterol and other lipids . About 30 % - 60 % of dietary cholesterol is absorbed daily . In mucosal intestine cells most of free cholesterol is re- esterified by the action of ACAT (Acyl cholesterol Acyl transferase) enzyme . ACAT Free cholesterol esterified cholesterol Intestinal cholesterol bind with protein and form chylomicron , which synthesis in mucosal intestinal cells. The main lipid in chylomicron is T.G. and then the cholesterol . Chylomicron transport to the extrahepatic tissues (mainly muscles and adipose tissues). Synthesized cholesterol :- Cholesterol synthesized mainly in the liver . Acetyl CoA is the precursor of cholesterol synthesis . Synthesized cholesterol transported from the liver to extrahepatic tissue by VLDL . VLDL is synthesized in the liver . The main lipids in VLDL are T.G. and cholesterol . Function of cholesterol in tissues :- Synthesis of bile acids . Synthesis of Vit. D . Synthesis of steroid hormone . Incorporated to cell membrane . Steps of cholesterol biosynthesis :- Formation of mevalonic acid (6 Carbon atoms) . Addition of 6 units of isoprenoid to from squalene . Formation of lanosterol . Conversion of lanosterol into cholesterol (27 carbon atoms). Esterification of cholesterol :- After synthesis, cholesterol is esterified Esterification of cholesterol means that the cholesterol bind with FA (at the OH position). By ester bond and form cholesterol ester . Cholesterol esterified in tissues and blood . Intracellular esterification :- ACAT :- enzyme responsible for the esterification of cholesterol in tissues . (Acyl Cholesterol Acyl Transferase ) ACAT require energy (ATP) and CoA . Intravascular esterification :- LCAT :- enzyme responsible for the esterification of cholesterol in blood . LCAT does not require ATP and CoA . The FA is transfer from 2nd carbon atom of lecithine (phospholipid in cell membrane) Lecithine + Free cholesterol LCAT cholesterol ester + lysolecithine Important of esterification :- 1- Increase the ability of lipoproteins to transport the lipid in blood . 2- Protect the cell from free radicals which occur by free cholesterol . Ketogenesis • Ketogenesis is a metabolic pathway that produces ketone bodies, which provide an alternative form of energy for the body. The process supplies the needed energy of certain organs, especially the brain cardiac, skeletal muscles. • Ketone bodies are water soluble molecules produced by the liver from fatty acids during low food intake or fasting. They are also formed when the body experienced starvation, carbohydrate restrictive diet, and prolonged intense exercises. • but insufficient ketogenesis can cause hypoglycemia and excessive production of ketone bodies leads to a dangerous state known as ketoacidosis • These are soluble in aquous solution, so in plasma these are transported as such and do not required any lipoproteins. • Ketogenesis produces acetone, acetoacetate, and beta-hydroxybutyrate molecules by breaking down fatty acids. • The three ketone bodies, each synthesized from acetyl-CoA molecules, are: • Acetoacetate, which can be converted by the liver into β-hydroxybutyrate, or spontaneously turn into acetone • Acetone, which is generated through the decarboxylation of acetoacetate, either spontaneously or through the enzyme acetoacetate decarboxylase. • β-hydroxybutyrate is generated through the action of the enzyme D-β- hydroxybutyrate dehydrogenase on acetoacetate . Reaction 1: Condensation In ketogenesis, • two molecules of acetyl CoA combine to form acetoacetyl CoA and HS — CoA. • this condensation is in the opposite direction of the last step of β oxidation. Reaction 2: Hydrolysis The hydrolysis of acetoacetyl CoA • forms acetoacetate, a ketone body, and HS — CoA. Acetoacetate can enter the citric acid cycle by reforming acetyl CoA for energy production or break down into other ketone bodies. Reaction 3: Hydrogenation Acetoacetate is reduced by 2H from NADH + H+ to β-hydroxybutyrate, which is considered a ketone body even though it does not contain a keto group. Reaction 4: Decarboxylation Acetoacetate can also undergo decarboxylation to yield acetone, a ketone body, and CO2. • The presence of ketone bodies in the blood is termed ketosis and the presence of ketone bodies in the urine is called ketonuria . Regulation • Ketogenesis may or may not occur, depending on levels of available carbohydrates in the cell or body. This is closely related to the paths of acetyl-CoA: • When the body has ample carbohydrates available as energy source, glucose is completely oxidized to CO2; acetyl-CoA is formed as an intermediate in this process, first entering the citric acid cycle followed by complete conversion of its chemical energy to ATP in oxidative phosphorylation. • When the body has excess carbohydrates available, some glucose is fully metabolized, and some of it is stored in the form of glycogen or, upon citrate excess, as fatty acids. (CoA is also recycled here.) • When the body has no free carbohydrates available, fat must be broken down into acetyl-CoA in order to get energy. Acetyl-CoA is not being recycled through the citric acid cycle because the citric acid cycle intermediates (mainly oxaloacetate) have been depleted to feed the gluconeogenesis pathway, and the resulting accumulation of acetyl-CoA activates ketogenesis. • The brain cannot receive fatty acids, which cannot pass through the blood-brain barrier. The liver, in order to keep supplying the brain with glucose, must convert amino acids, glycerol, pyruvate, and lactate into glucose. This process is called gluconeogenesis, and also produces the two ketone bodies acetoacetate and beta- hydroxybutyrate. It releases these ketone bodies, along with glucose, into the blood stream to feed the brain. By this point, the muscles and other organs have mainly switched to fatty acids for energy, conserving the glucose for the brain Pathology • Both acetoacetate and beta-hydroxybutyrate are acidic, and, if levels of these ketone bodies are too high, liberate H+ ions,the pH of the blood drops, resulting in ketoacidosis. Ketoacidosis is known to occur in untreated type I diabetes (diabetic ketoacidosis) and in alcoholics after prolonged binge-drinking without intake of sufficient carbohydrates (alcoholic ketoacidosis). KB concentration when increased in body indicates: Excessive production acetyl CoA (increased Lipolysis) or Depressed utilization of acetyl CoA by TCA-Cycle. Main causes are uncontrolled Diabetes, starvation, prolonged fasting, excessive diarrhea/ vomiting. .
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