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UA BIOC 460 - Fatty acid oxidation

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Bioc 460 - Dr. Miesfeld Fall 2008 1 of 9 pages Figure 2. Figure 1. Lecture 34 - Fatty acid oxidation Key Concepts - Overview of lipid metabolism - Reactions of fatty acid oxidation - Energy yield from fatty acid oxidation - Formation of ketone bodies Overview of lipid metabolism Carbohydrate metabolism is but one component of energy production and storage. In fact, a much larger percentage of the total energy reserves in animals is lipids in the form of fat deposits consisting of energy-rich fatty acids. As shown in figure 1, there are three basic sources of fatty acids in animals that can be used for energy conversion processes, 1) fatty acids present in triacylglycerols (figure 2) obtained from the diet, 2) fatty acids stored as triacylglycerols in adipose tissue that are released by hydrolysis following hormone stimulation (glucagon or epinephrine signaling), and 3) fatty acids synthesized in the liver from excess carbohydrates and exported as triacylglycerols. Fatty acids in dietary triacylglycerols are transported from the intestines to the rest of the body by large lipoprotein particles called chylomicrons. Hormone signaling releases fatty acids from adipose tissue that bind to an abundant transport protein in serum called albumin. Lastly, fatty acids synthesized in the liver are carried through the body as triacylglycerols by very low density lipoprotein (VLDL) particles. Palmitate is a C16 saturated fatty acid that can be carried through the body as a protein-fatty acid complex. Fat is stored in fat cells (adipocytes). Obesity, especially childhood obesity, can be due to both more fat storage per cell, and to a larger number of adipocytes (figure 2). In contrast, in normal healthy adults, the onset of old age and reduced metabolic rates leads to weight gain resulting primarily from storing more fat per cell (although adults can also add more fat cells if they become obese).Bioc 460 - Dr. Miesfeld Fall 2008 2 of 9 pages Olestra Pathway Questions for Fatty Acid Metabolism 1. What purpose does fatty acid metabolism serve in animals? • Fatty acid oxidation in mitochondria is responsible for providing energy to cells when glucose levels are low. Triacylglycerols stored in adipose tissue of most humans can supply energy to the body for ~3 months during starvation. • Fatty acid synthesis reactions in the cytosol of liver and adipose cells convert excess acetyl CoA that builds up in the mitochondrial matrix when glucose levels are high into fatty acids that can be stored or exported as triacylglycerols. 2. What are the net reactions of fatty acid degradation and synthesis for the C18 fatty acid palmitate? Fatty acid oxidation: Palmitate + 7 NAD+ + 7 FAD + 8 CoA + 7 H2O + ATP --> 8 acetyl CoA + 7 NADH + 7 FADH2 + AMP + 2 Pi + 7 H+ Fatty acid synthesis: 8 Acetyl CoA + 7 ATP + 14 NADPH + 14 H+ --> palmitate + 8 CoA + 7 ADP + 7 Pi + 14 NADP+ + 6 H2O 3. What are the key enzymes in fatty acid metabolism? Fatty acyl CoA synthetase – enzyme catalyzing the "priming" reaction in fatty acid metabolism which converts free fatty acids in the cytosol into fatty acyl-CoA using the energy available from ATP and PPi hydrolysis. When the energy charge in the cell is low, the fatty acyl-CoA is used for fatty acid oxidation inside the mitochondria, however, when the energy charge is high, the fatty acyl-CoA is used to synthesize triacylglycerols or membrane lipids. Carnitine acyltransferase I - catalyzes the commitment step in fatty acid oxidation which links fatty acyl-CoA molecules to the hydroxyl group of carnitine. The activity of carnitine acyltransferase I is inhibited by malonyl CoA, the product of the acetyl-CoA carboxylase reaction, which signals that glucose levels are high and fatty acid synthesis is favored. Acetyl CoA carboxylase - catalyzes the commitment step in fatty acid synthesis using a biotin-mediated reaction mechanism that carboxylates acetyl CoA to form the C3 compound malonyl CoA. The activity of acetyl CoA carboxylase is regulated by both reversible phosphorylation (the active conformation is dephosphorylated) and allosteric mechanisms (citrate binding stimulates activity, palmitoyl-CoA inhibits activity). Fatty acid synthase - this large multi-functional enzyme is responsible for catalyzing a series of reactions that sequentially adds C2 units to a growing fatty acid chain covalently attached to the enzyme complex. The mechanism involves the linking malonyl-CoA to an acyl carrier protein, followed by a decarboxylation and condensation reaction that extends the hydrocarbon chain. 4. What are examples of fatty acid metabolism in real life? A variety of foods are prominently advertised as "non-fat," even though they can contain a high calorie count coming from carbohydrates. Eating too much of these high calorie non-fat foods (e.g., non-fat bagels) activates the fatty acid synthesis pathway resulting in the conversion of acetyl CoA to fatty acids which areBioc 460 - Dr. Miesfeld Fall 2008 3 of 9 pages Figure 3. Figure 4. stored as triacylglycerols. Olestra is a fat substitute composed of a sucrose molecule with several fatty acids attached. Transport and storage of fatty acids and triacylglycerols Much of the triacylglycerol stored in adipose tissue originates from dietary lipids. Fats that enter the small intestine from the stomach are insoluble and must be emulsified by bile acids such as glycocholate which are secreted by the bile duct and function as detergents to promote the formation of micelles. Lipases are water soluble enzymes in the small intestine that hydrolyze the acyl ester bonds in triacylglycerols to liberate free fatty acids which then pass through the membrane on the lumenal side of intestinal epithelial cells (figure 3). Pancreatic lipase cleaves the ester bond at the C-1 and C-3 carbons to release two free fatty acids and monoacylglyclerol, whereas, other intestinal lipases cleave at the C-2 carbon to generate glycerol and fatty acid. The absorption and transport of dietary triacylglycerols can be broken down into five steps, 1) emulsification of triacylglycerols by bile acids, 2) hydrolysis of fatty acids by intestinal lipases, 3) resynthesis of triacylglycerols inside intestinal epithelial cells, 4) packaging of triacylglycerols into large lipoprotein particles called chylomicrons, and 5) export of the chylomicrons to the lymphatic system. Chylomicrons transport the triacylglycerols to adipose


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UA BIOC 460 - Fatty acid oxidation

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