Chapter23SUMMARY - ‘ Summary 23.1 How Are Fats Mobilized...

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Unformatted text preview: ‘ Summary 23.1 How Are Fats Mobilized from Dietary Intake and Adi- Pose “$508? Triacylglycerols are a major source of fatty acids in the diet, and they are also our principal stored energy reserve in adipose tis- sue. Hormone messengers such as adrenaline, glucagon, and ACTH bind to receptors on the plasma membrane of adipose cells and lead to the activation of a triacylglycerol lipase that hydrolyzes a fatty acid from (3.1 or 0-3 of triacylglycerols. Subsequent actions of diacylglycerol lipase and monoacylglycerol lipase yield fatty acids and glycerol. The cell then releases the fatty acids into the blood, where they are carried to sites of utilization. Dietary triacylglycerols are degraded by lipases and esterases in the stomach and duodenum. Pancreatic lipase cleaves fatty acids from the C-1 and C—3 positions of triacylglycerols, and other lipases and esterases attack the 02 position. Bile salts act as detergents to emulsify the triacylglycerols and facilitate the hydrolytic activity of the lipases and esterases. 23.2 How Are Fatty Acids Broken Down? The process of [3— oxidation begins with the formation of a thiol ester bond between the fatty acid and the thiol group of coenzyme A, catalyzed by acyl-CoA syn- thetase. All of the other enzymes of the B-oxidation pathway are located in the mitochondrial matrix. Short-chain fatty acids are transported into the matrix as free acids and form the acyl-CoA derivatives there. However, long-chain fatty acyl-CoA derivatives must first be converted to acylcarni- tine derivatives, which are transported across the inner membrane by a translocase. On the matrix side of the inner membrane, a second acyl car- nitine transferase reforms the fatty acyl-CoA. The process of B—oxidation involves a recurring cycle of four steps. A double bond is formed, water is added across the double bond, and the resulting alcohol is oxidized to a carbonyl group. The fourth reaction of the cycle cleaves the resulting B-keto ester, producing an acetate unit and leaving a fatty acid chain that is two carbons shorter. 23.3 How Are Odd-Carbon Fatty Acids Oxidized? Humans and animals metabolize odd-carbon fatty acids via the B-oxidation pathway, with the final product being propionyl—CoA. Three specialized enzymes then convert propionyl—CoA to succinyl-COA, a TCA cycle intermediate. The pathway involves an initial carboxylation (by propionyl—COA car- boxylase) at the a-carbon of propionyl-COA to produce D—methylmalonyl- CoA, which is converted to the L—isomer by methylmalonyl-COA epimerase. The L-isomer is the substrate for methylmalonyl—CoA mutase, j which catalyzes a migration of a carbonyl-GOA group from one carbon to its neighbor, yielding succinyl-CoA. 23.4 How Are Unsaturated Fatty Acids Oxidized? Two addi- tional mitochondrial enzymes—an isomerase and a novel reductase—are required to handle the cis-double bonds of naturally occurring fatty acids. Consider the breakdown of oleic acid. The reactions of B-oxidation pro- ceed normally through three cycles, producing three molecules of acetyl- CoA and leaving the product cis-AS-dodecenoyl-COA. This intermediate is not a substrate for acyl-CoA dehydrogenase. Instead, enoyl-CoA isom- erase rearranges the alt-A” double bond to a trans-A2 double bond, which can proceed through the normal route of B—oxidation. 23.5 Are There Other Ways to Oxidize Fatty Acids? Organelles other than mitochondria, including peroxisomes and glyoxysomes, carry out B-oxidation processes. Peroxisomal B-oxidation is similar to mito- chondrial B-oxidation, except that the initial double bond formation is catalyzed by an FAD-dependent acyl—CoA oxidase, which transfers the liberated electrons directly to oxygen instead of the electron-transport chain. Short-chain products must be transferred to the mitochondria for further breakdown. Similar B-oxidation enzymes are also, found in glyoxysomes. Branched—chain fatty acids with alkyl branches at odd-numbered carbons are not effective substrates for B-oxidation. For such species, (Lt-oxidation is a useful alternative. Ruminants oxidize phytol to phy— tanic acid. The methyl group at G?) will block B—oxidation, but phytanic acid oz-hydroxylase places an —OH group at the a-carbon, and phy- tanic acid a-oxidase decarboxylates it to yield pristanic acid. The CoA ester of this metabolite can undergo B-oxidation in the normal man- ner. The terminal product, isobutyryl—CoA, can be sent into the TCA cycle by conversion to succinyl-CoA. 23.6 What Are Ketone Bodies, and What Role Do They Play in Metabolism? Acetone, acetoacetate, and fi-hydroxybutyrate are known as ketone bodies. These three metabolites are synthesized pri- -marily in the liver but are important sources of fuel and energy for many peripheral tissues, including brain, heart, and skeletal muscle. During periods of starvation, ketone bodies may be the major energy source for the brain. Acetoacetate and 3-hydroxybutyrate are the preferred and normal substrates for kidney cortex and for heart muscle. ...
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