Lec18_05_26_04 - Chapter 18 Metabolism Movement of Lipids...

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Chapter 18 Metabolism & Movement of Lipids Cells use lipids for storing energy, building membranes, signaling within and between cells, sensing the environment, covalently modifying proteins, forming specialized permeability barriers, and protecting cells from highly reactive chemicals. Fatty acids are oxidized in mitochondrion to release energy for cellular functions and stored as triglycerides. Fatty acids also precursor for phospholipids, structural backbones of cell membranes. Cholesterol is precursor for steroid hormones and other biologically active lipids that function in cell-cell signaling. Also derived from cholesterol biosynthesis are fat soluble vitamins including Vitamin A (impt in retinal pigments), Calcium metabolism by Vitamin D, protection against oxidants Vitamin E and cofactor for Vitamin K in formation of blood clots. Chapter focuses on movement of lipids especially membrane components between organelles and between cells (Fig 18-1) . Conclude chapter by focusing on atherosclerosis and cardiovascular disease the number one killer in Western world. 18.1 Phospholipids and sphingolipids: synthesis and intracellular movement. Cells require production of additional lipid membrane in order to divide. The important components are sphingolipids, phospholipids, and sterols (i.e. cholesterol) Tab 18-1. The regulation of fatty acid synthesis plays a key role in the regulation ofmembrane synthesis as a whole. A fatty acid consists of a long hydrocarbon chain with a carboxyl group at one end ( Fig 18-2). Saturated fatty acids have only single bonds, whereas unsaturated fatty acids have one or more double bonds in hydrocarbon chain. The major fatty acids in phospholipids contain 14, 16, 18, or 20 carbon atoms and include bother saturated and unsaturated chains. Saturated fatty acids containing 14 or 16 carbon atoms are made from acetyl CoA by acetyl-CoA carboxylase and fatty acid synthase. Enzymes in animals are found in cytosol. Desaturase enzymes are located in the ER and introduce double bonds at specific positions in some fatty acids, which creates a kink in the hydrocarbon chain that interrupts intermolecular packing. Membranes and triglycerides droplets whose components are high in unsaturated fatty acids (i.e. corn oil, olive oil) tend to be more fluid at RT than those with lots of saturated fatty acids (animal fats). In addition human cannot synthesize certain essential polyunsaturated fatty acids including linoleic acid and linolenic acid that must be consumed in diet. Fatty acids can be derived from enzymatic hydrolysis of triglycerides (primary stored form of fatty acids) which consist of three fatty acyl chains esterified to glycerol (Fig 18-2). Unesterified fatty acids move within cells bound to small cytosolic proteins. Fatty acid binding proteins (FABPs) belong to a group of small cytosolic proteins that facilitate the intracellular movement of many lipids. They consist of a hydrophobic binding pocket lined by β -sheets ( Fig 18-3) . FABPs are important in conversion of fatty acids to energy in muscle and adipocytes that store fatty acids.
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