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Biochemistry-II - Fatty Acid Oxidation

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    Study Guide Biochemistry - II – Fatty Acid Oxidation 1. Dietary Fat Absorption 1. 1 How Dietary Fats Are Broken Down Most of the fat we eat is in the form of triacylglycerols (TAGs) . To use this fat for energy, the body must first break it down. This process begins with hydrolysis , a reaction carried out by enzymes called lipases . Lipases split one triacylglycerol molecule into: • Three free fatty acids • One glycerol molecule The glycerol is water - soluble, so it is easily absorbed by intestinal cells. Later, it can recombine with fatty acids inside the cells. 1. 2 Why Fat Digestion Is More Complicated Fatty acids are not very soluble in water . Because of this: • Fat molecules stick together • They form large lipid droplets • Digestive enzymes (which are water - soluble) cannot easily reach them

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    Study Guide To solve this problem, fats must be broken into smaller droplets with a larger surface area. This process is called emulsification . 1. 3 Role of Bile Salts in Fat Digestion The molecules responsible for emulsification are called bile salts (bile acids) . Where do bile salts come from? • They are made in the liver • Stored in the gall bladder • Released into the duodenum (first part of the small intestine) Why are bile salts effective? • They are made from cholesterol • They act like detergents • One end is hydrophobic (attracts fat) • The other end is charged and water - soluble At a certain concentration, called the critical micelle concentration (CMC) — usually 2 – 5 mM — bile salts group together to form micelles .

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    Study Guide 1. 4 Mixed Micelles and Fat Absorption Figure 1 In the intestine, bile salts combine with: • Fatty acids • Triacylglycerols • Cholesterol This structure is called a mixed micelle . Structure of a mixed micelle: • Bile salts form the outer edge • Some bile salts are scattered inside • Lipids form a bilayer in the center

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    Study Guide Mixed micelles are essential because: • They greatly increase the surface area for digestion • They help fatty acids move through the watery intestinal environment Important note: Fat - soluble vitamins A, D, E, and K cannot be absorbed without bile salts . 1. 5 Action of Pancreatic Lipase The main digestive enzyme for fats is pancreatic lipase . • It works on the surface of mixed micelles • It needs a helper protein called colipase • Colipase helps lipase bind to the micelle surface Lipase breaks fats into free fatty acids , which are slightly soluble in water. What happens next? • Most fatty acids are absorbed by cells of the small intestine • Any remaining fatty acids may be used by bacteria in the large intestine • Bile salts are reabsorbed in the final third of the small intestine 1. 6 Bile Acids, Fiber, and Cholesterol Control Bile acids are reused many times — often five or more cycles between the liver and intestine. How does fiber help? • Soluble fiber (like oat bran) binds bile acids • Fiber cannot be absorbed • Bound bile acids are lost in feces Because bile acids are made from cholesterol , losing them forces the body to: • Use more cholesterol to make new bile acids • This lowers blood cholesterol levels • Reduces the risk of heart disease

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    Study Guide However, fiber cannot cancel out a very high - fat, high - cholesterol diet. Eating large amounts of fatty meat and expecting fiber alone to fix it is unrealistic. 1.7 Lipids in the Bloodstream Transport of Lipids in Blood Different lipids travel in the blood in different ways: • Free fatty acids bind to serum albumin • Other lipids are carried in lipoproteins What are lipoproteins? • Spherical protein - lipid complexes • Proteins on their surface are called apolipoproteins • Lipoproteins are classified by density Types of Lipoproteins Chylomicrons • Largest and least dense • Over 95% lipid • Carry dietary fat • Contain mostly triacylglycerols • Include several apolipoproteins Very - Low - Density Lipoproteins (VLDL) • Slightly denser than chylomicrons • 90 – 95% lipid • Carry fats made in the liver • Rich in Apolipoprotein B Low - Density Lipoproteins (LDL)

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    Study Guide • About 85% lipid • High in cholesterol • Deliver cholesterol to cells • High LDL levels are linked to heart disease • Often called “bad cholesterol” High - Density Lipoproteins (HDL) • About 50% protein • Contain Apolipoprotein A • Carry cholesterol away from tissues • Help reduce heart disease risk • Known as “good cholesterol” Summary of Fat Digestion and Transport • Dietary triacylglycerols are digested by lipase • Bile salts form mixed micelles • Fatty acids are absorbed in the small intestine • Lipids travel through the lymph system to the liver • They enter the bloodstream as lipoproteins • These lipoproteins deliver fats and cholesterol to tissues 1.8 Lipids in Body Tissues Use of Lipids by Cells • Chylomicrons carry dietary fats • LDL carries liver - made fats • Cellular lipases break triacylglycerols into: o Fatty acids o Glycerol Special carrier proteins transport fatty acids into cells. Different carriers exist for different chain - length fatty acids .

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    Study Guide 2. Hydrolysis of Triacylglycerols

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    Study Guide 2. 1 Starting Energy Production from Fats The body stores most of its fat as triacylglycerols (TAGs) . To use this stored fat for energy, the first and most important step is hydrolysis . Hydrolysis breaks one triacylglycerol molecule into: • Free fatty acids • Glycerol This reaction is catalyzed by an enzyme called lipase .

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    Study Guide 2.2 Hydrolysis in Adipose (Fat - Storing) Tissue In adipose tissue , which stores fat, a cellular lipase carries out the hydrolysis of triacylglycerols. As a result: • Fatty acids are released into the bloodstream • Glycerol is released separately This step allows stored fat to be mobilized whenever the body needs energy. 2. 3 Transport of Fatty Acids and Glycerol Once released: • Free fatty acids are not very soluble in water , so they travel through the blood attached to serum albumin • Glycerol is water - soluble and travels freely to the liver This separation ensures that each molecule reaches the tissue where it can be used most effectively. 2. 4 Fate of Glycerol in the Liver In the liver, glycerol enters energy - producing pathways with the help of two key enzymes : 1. Glycerol kinase o Converts glycerol into glycerol - 3 - phosphate 2. Glycerol - 3 - phosphate dehydrogenase o Converts glycerol - 3 - phosphate into glyceraldehyde - 3 - phosphate 2. 5 Metabolic Uses of Glyceraldehyde - 3 - Phosphate Glyceraldehyde - 3 - phosphate is a very versatile molecule. It can: • Enter the glycolytic pathway to produce energy • Be used to form glucose through gluconeogenesis • Be converted into phosphoenolpyruvate (PEP) and then used to supply intermediates for the tricarboxylic acid (TCA) cycle

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    Study Guide This flexibility allows glycerol from fat breakdown to support multiple metabolic needs. Key Takeaway s • Triacylglycerols are broken down by lipase • This releases fatty acids and glycerol • Fatty acids travel in blood bound to serum albumin • Glycerol goes to the liver • In the liver, glycerol enters glycolysis, gluconeogenesis, or the TCA cycle Together, these steps connect fat storage directly to the body’s energy - producing pathways. 3. β - Oxidation of Fatty Acids 3.1 Overview: What Is β - Oxidation? β - Oxidation is the main pathway by which fatty acids are broken down to produce energy . This process occurs in target tissues such as muscle and liver cells. During β - oxidation: • Fatty acids are broken down two carbons at a time • Each cycle releases a 2 - carbon unit called acetyl - CoA • These acetyl - CoA molecules enter the TCA (citric acid) cycle to produce energy 3.2 Structure and Transport of Fatty Acids

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