CHAPTER 8

• DIGESTION AND ABSORPTION OF LIPIDS

  1. Triacylglycerols undergo physical changes in the stomach

  2. Chyme formation

  3. Lipid digestion begins in the stomach

  4. Bile release in small intestine

  5. Emulsification by bile

  6. Enzymatic hydrolysis by pancreatic lipase

  7. Fatty acid micelle formation

  8. Absorption by intestinal cells

  9. Chylomicron formation
• Triacylglycerols


  • Account for 98% of total dietary lipids
• Digestion in the mouth

  Emulsification by lingual lipase
• Digestion in the stomach

  1. Churning action breaks up triacylglycerol into small globules

  2. Formation of chyme
• High-fat foods


  Remain in the stomach longer than low-fat foods
• Lipid digestion also begins in the stomach under the action of gastric lipase enzymes
• Digestion in the small intestine

  1. Bile release

  2. Emulsification by bile

  3. Enzymatic hydrolysis by pancreatic lipase
• Fatty acid micelles


  Contain fatty acids, monoacylglycerols, and some bile
• Absorption in intestinal cells

  1. Free fatty acids and monoacylglycerols are reassembled into triacylglycerol

  2. Triacylglycerols are combined with membrane lipids and water-soluble proteins to form chylomicrons
• TRIACYLGLYCEROL STORAGE and MOBILIZATION


  1. Adipose tissue stores triacylglycerol

  2. Hormones like adrenaline and glucagon promote the use of TAGs stored in adipose tissue for energy production

  3. Hormone-sensitive lipase is phosphorylated to mobilize triacylglycerol
• Triacylglycerol mobilization


  The process of obtaining energy from the body's triacylglycerol energy reserves (adipose tissue)
• Dietary TAGs deposited in adipose tissue exhibit hydrolysis that is tuned to produce free fatty acids and/or monoacylglycerols, which are repackaged to re-form TAGs
• With enough water, the average person could endure famine for around 30 days thanks to triacylglycerol stores, while glycogen stores would be drained within a single day
• GLYCEROL METABOLISM


  1. Glycerol enters circulation and undergoes metabolism in the liver or kidneys, turning into dihydroxyacetone phosphate

  2. Glycerol phosphorylation

  3. Glycerol oxidation
• OXIDATION OF FATTY ACIDS

  1. Fatty acid activation

  2. Fatty acid transport into mitochondria

  3. Beta-oxidation
• Fatty acid activation


  Transformation of fatty acids into high-energy coenzyme-A derivative
• Fatty acid transport


  Acyl CoA enters the mitochondrial matrix through a shuttle mechanism involving carnitine
• Beta-oxidation


  Series of four metabolic processes in the mitochondrial matrix that remove two carbon units from the carboxyl end of the acyl CoA molecule
• Beta-oxidation


  • Degrades acyl CoA to acetyl CoA by removing two carbon atoms at a time, resulting in FADH and NADH
• Steps of Beta-oxidation


  1. First dehydrogenation

  2. Hydration

  3. Second dehydrogenation

  4. Thiolysis
• Oxidation of unsaturated fatty acids

  1. Second dehydrogenation

  2. Removal of two hydrogen atoms converts the B-hydroxy group to a keto group, with NAD* serving as the oxidizing agent

  3. The required enzyme exhibits absolute stereospecificity for the L isomer
• β-oxidation pathway


  The name for the series of reactions where the β-carbon atom has been oxidized from a --CH2 group to a ketone group
• Thiolysis


  1. The fatty acid carbon chain is broken between the a and β carbons by a reaction with a coenzyme A molecule

  2. Results in an acetyl CoA molecule and a new acyl CoA molecule that is shorter by two carbon atoms than its predecessor
• The chain cleavage reaction that occurs in Step 4 is called thiolysis by analogy with the process of hydrolysis, which also involves breaking a molecule into two parts
• The thiol group of coenzyme A undergoes reaction
• β-oxidation cycle


  1. The acyl CoA molecule, now shorter by two carbons, enters the cycle again

  2. Each cycle produces: 1. One acetyl CoA, 2. A two-carbon-shorter acyl CoA, 3.FADH, 4.NADH
• This recycling continues until the entire fatty acid is converted into acetyl CoA
• The fatty acid chain is sequentially degraded by two carbons per cycle
• Fatty acids in dietary triacylglycerols typically have an even number of carbon atoms
• During β-oxidation, the number of acetyl CoA molecules produced is half the number of carbon atoms in the fatty acid
• The β-oxidation pathway repeats one less time than the number of acetyl CoA molecules formed
• The final repetition of β-oxidation splits a C4 unit into two C2 units, yielding two acetyl CoA molecules
• Unsaturated fatty acids


  • Their oxidation through the ẞ-oxidation pathway requires two additional enzymes besides those needed for the oxidation of saturated fatty acids
  • These two epimerases that can change a D configuration to an L configuration and a cis-trans isomerase are needed
• The double bonds in naturally occurring unsaturated fatty acids are nearly always cis double bonds, which yield on hydration a D-hydroxy product rather than the L-hydroxy product needed for Step 3 of the pathway
• The epimerase enzyme affects a configuration change from the D form to the L form
• The double bonds in naturally occurring unsaturated fatty acids often occupy odd-numbered positions, and the hydratase in Step 2 of the pathway can affect hydration of only an even-numbered double bond
• The cis-trans isomerase produces a trans-(2,3) double bond from a cis-(3,4) double bond
• ATP production from fatty acid oxidation

  1. When stearic acid (18:0) undergoes eight rounds of the beta-oxidation pathway, it produces nine acetyl CoA molecules, eight FADH2 molecules, and eight NADH molecules

  2. These products are then processed through the citric acid cycle, electron transport chain, and oxidative phosphorylation to ultimately generate ATP
• Fatty acids provide 2.5 times more energy per gram than carbohydrates
• Fatty acids are superior energy-storing molecules, storing more than twice as much energy per gram