Fuel Biochemistry of Triglycerides PDF

Lippincott’s illustrated reviews Chapter 16 – Page 181 Lecture 26 Biochemistry of Triglyceride 1  Specific Objectives By the end of this lecture students can be able to: Explain metabolic pathways of triglycerides. 2 3 Resynthesis of TAG and cholesteryl...

Lippincott’s illustrated reviews Chapter 16 – Page 181 Lecture 26 Biochemistry of Triglyceride 1  Specific Objectives By the end of this lecture students can be able to: Explain metabolic pathways of triglycerides. 2 3 Resynthesis of TAG and cholesteryl esters The mixture of lipids absorbed by the enterocytes migrates to the endoplasmic reticulum where biosynthesis of complex lipids takes place which synthesizes TAG 4 Secretion of lipids from enterocytes The newly resynthesized TAGs and cholesteryl esters are very hydrophobic, and aggregate in an aqueous environment and form chylomicrons. 5 Chylomicrons: The chylomicrons are synthesized in the mucosa (the lining) of the intestine. Chylomicrons is a small fat globule composed of protein and lipid which secreted in the blood and lymphatic vessels. 6  Fate of absorbed Fat The absorbed (exogenous) triglycerides are transported in blood as chylomicrons. The blood become milky after eating a meal rich in lipid due to increase level of chylomicrons. They are taken up by adipose tissue and liver. Liver synthesizes endogenous triglycerides. These are transported in the blood as VLDL (very low density lipoproteins) and are deposited in adipose tissue. Lipoprotein lipase breaks down the triglycerides in the lipoproteins to smaller fatty acids and monoglycerides that are transported into tissues and either burned for fuel or re-assembled into triglycerides for storage. 7 8  Triacylglycerols Triacylglycerols (nuteral fat) stored in adipose cells, and serve as the major energy reserve of the body. Structure of triacylglycerol (TAG): 9  Mono-, di-, and triacylglycerols consist of one, two, or three molecules of fatty acid esterified to a molecule of glycerol. Fatty acids are esterified through their carboxyl groups, resulting in a loss of negative charge and formation of “neutral fat.” 10 Different fates of TAG in the liver and adipose tissue In white adipose tissue, TAG is stored as fat droplets in the cytosol of the cells. It serves as “depot fat,” ready for mobilization when the body requires it for fuel. Little TAG is stored in the liver. 11  Instead, most is exported, packaged with other lipids and apoproteins to form lipoprotein particles called very-low density lipoproteins (VLDL). Nascent VLDL are secreted directly into the blood where they mature and function to deliver the endogenously derived lipids to the peripheral tissues. 12  Mobilization of stored fats TAGs provide concentrated stores of metabolic energy because they are highly reduced. The yield from the complete oxidation of fatty acids to CO2 and H2O is 9 kcal/g fat (as compared to 4 kcal/g protein or carbohydrate). 13  Hydrolysis of TAG The mobilization of stored fat requires the hydrolytic release of fatty acids and glycerol from their TAG form. This process is initiated by hormone-sensitive lipase (such as epinephrine or glucagon), which removes a fatty acid from carbon 1 and/or carbon 3 of the TAG. Additional lipases specific for diacylglycerol or monoacylglycerol remove the remaining fatty acid(s). 14 15 Fate of glycerol: The glycerol released during TAG degradation cannot be metabolized by adipocytes because they apparently lack glycerol kinase. Rather, glycerol is transported through the blood to the liver, where it can be phosphorylated. 16  The resulting glycerol phosphate can be used to form TAG in the liver, or can be participate in glycolysis or gluconeogenesis. 17 Fate of fatty acids: The free (unesterified) fatty acids move through the cell membrane of the adipocyte, and bind to plasma albumin. Fatty acids reach liver are undergo oxidation process to release energy. 18 Reference Book: Champe, P. C., Harvey, R. A. and Ferrier, D. R., 2005. Biochemistry “Lippincott’s Illustrated Reviews”, 5th or 6th Edition 19