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Chapter 23 23 1 Fatty acids are important factors in storing energy for two main reasons One is because the carbon in fatty acids is almost entirely reduced meaning when it is oxidized it releases more energy than carbon in other compounds that is not as reduced Two because fatty acids are not as hydrated as monosaccharides and polysaccharides meaning they can stack more closely in storage tissues I Modern diets are often high in fat This is due to the fact that the meat we eat today comes from less mobile animals meaning they are raised to ensure a high fat content because of flavor II Triacylglycerols are a major form of stored energy in animals Fat fatty acids is mainly stored as triacylglycerides in animals Fat acts as the major contributor to total energy because more energy is released per molecule of fat whereas with glucose carbohydrates and protein less energy is released when they are oxidized Triacylglycerols are stored in animals in specialized cells called adipose cells III Hormones trigger the release of fatty acids from adipose tissue The release of fatty acids from triacylglycerols either from adipose cells or from the diet is due to the release of certain hormone messengers that stimulate its breakdown These hormones include glucagon adrenocorticotropic hormone ACTH and adrenaline How The hormone messengers bind to the receptors on the adipose cells This binding stimulates the release of adenylyl cyclase which forms cyclic AMP from ATP cAMP in adipose tissues then activates protein kinase A which leads to the phosphorylation and activation of many triacylglycerol lipases including triacylglycerol lipase diacylglycerol lipase and monacylglycerol lipase each which breaks down triacylglycerols into fatty acids and glycerol The fatty acids are then released into the blood where they are bound to serum albumin which transports them to their site of utilization IV Degradation of Dietary Fatty Acids occurs primarily in the DUODENUM A small portion of the dietary triacylglycerols is broken down by lipases in the acidic environment of the stomach Most passes all the way to the duodenum however before being broken down and released The duodenum is an alkaline environment which increases the pH of the digestive mixtures allowing the hydrolysis of triacylglycerols by pancreatic lipase and also some esterases hydrolyze fatty acid ester linkages Pancreatic lipase hydrolyzes at C 1 and C 3 position Other esterases and lipases hydrolyze them at the C 2 position Both of these processes depends on the presence of bile salts used to emulsify the triacylglycerols and facilitate the hydrolytic activity of these lipases and esterases Short chain fatty acids are absorbed into the villi of the intestinal mucosa Long chain fatty acids are rather form micelles with the bile salts and are transported to the epithelial cells that cover the villi Here they become condensed with glycerol to give triacylglycerol which then combines with lipoproteins to form chylomicrons Chylomicrons then are transported to the various parts of the body via the lymphatic system 23 2 How are fatty acids broken down I Knoop Elucidated the essential feature of oxidation Knoope s experiment Fed modified fatty acids to his dogs He showed that fatty acids must be degraded by oxidation at the carbon followed by the cleavage of the Alpha Carbon Beta Carbon bond He showed that in order for fatty acids to be degraded there had to be a removal of 2 C units Albert Lehninger showed that this degradative process took place in the mitochondria F Lynen and E Reichart showed that the two carbon unit relased is acetyl CoA not acetate like Knoope originally believed Since the entire process begins with the oxidation of the carbon that is to the carboxyl carbon this process is known as oxidation II Coenzyme A Activates Fatty Acids for Degradation The initial part of the reaction involves the attack by the carboxyl group on the fatty acid to the phosphate group of ATP This reaction produces pyrophosphate and an acyladenylate intermediate A subsequent attack by CoASH to the carboxyl carbon then produces a thiol ester bond between the fatty acid and the thiol group of CoA while kicking off the phosphate obtained from ATP During this process it requires the consumption of ATP to produce AMP and pyrophosphate as stated which is rapidly hydrolyzed to produce two molecules of phosphate This entire reaction has a net Gibbs free energy of 0 8 kJ mol and so is favorable and easily reversible This creation of the bond between CoA and the fatty acid is what activates the fatty acid for the reaction in the oxidation pathway III Carnitine carries fatty acyl groups across the inner mitochondrial membrane All other enzymes are located in the mitochondrial matrix SCFA easily enter the matrix and then form their acyl CoA derivatives there But LCFA need to first be turned into a acylcarnitine derivative before it can enter the matrix How this happens the fatty acid CoA complex first approaches the carnitine acyltransferase I located in the outer membrane of the mitochondria This complex catalyzes the production of O acylcarnitine which is created via the removal of the CoASH and the addition of the molecule carnitine It is now located in the intermembrane space where acylcarnitine translocase transports O acylcarnitine across the inner membrane into the mitochondrial matrix It is then transported to carnitine acyltransferase II which catalyzes the reverse of I and adds the CoASH to the fatty acid with the removal of Carnitine The created carnitine then may go back across the matrix and membranes and be used again III BETA OXIDATION 4 REACTIONS The first three reactions are focused on turning the Beta C into a carbonyl by oxidizing the alpha beta C bond to form an olefin The second reaction involves the cleaving of the beta keto ester in a reverse Claisen condensation which produces an acetate unit and a fatty acid chain that is two carbon units less than the initial molecule The enzymes involved are separated into two different groups On is for the LCFA 14 C and longer and is located as a membrane bound complex SLFA rather are focused on in the matrix via soluble enzymes So once multiple oxidations have occurred and the main chain continues to shorten once it gets below 14 it goes from the membrane bound complex to the matrix Acyl CoA Dehydrogenases Each contains a noncovalently bound FAD that is reduced to FADH2 during the oxidation and hence

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FSU BCH 4054 - Chapter 23

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