Outline 24.1 Digestion of Triacylglycerols 24.2Lipoproteins for Lipid Transport 24.3Triacylglycerol Metabolism: An Overview 24.4Storage and Mobilization.

Outline

24.1 Digestion of Triacylglycerols

24.2 Lipoproteins for Lipid Transport

24.3 Triacylglycerol Metabolism: An Overview

24.4 Storage and Mobilization of Triacylglycerols

24.5 Oxidation of Fatty Acids

24.6 Energy from Fatty Acid Oxidation

24.7 Ketone Bodies and Ketoacidosis

24.8 Biosynthesis of Fatty Acids

Goals1. What happens during the digestion of triacylglycerols?

Be able to list the sequence of events in the digestion of dietary triacylglycerols and their transport into the bloodstream.

2. What are the various roles of lipoproteins in lipid transport? Be able to name the major classes of lipoproteins, specify the nature and function of the lipids they transport, and identify their destinations.

3. What are the major pathways in the metabolism of triacylglycerols? Be able to name the major pathways for the synthesis and breakdown of triacylglycerols and fatty acids, and identify their connections to other metabolic pathways.

4. How are triacylglycerols moved into and out of storage in adipose tissue?

Be able to explain the reactions by which triacylglycerols are stored and mobilized, and how these reactions are regulated.

5. How are fatty acids oxidized, and how much energy is produced by their oxidation?Be able to explain what happens to a fatty acid from its entry into a cell until its conversion to acetyl-CoA.

• What is the function of ketogenesis? Be able to identify ketone bodies, describe their properties and synthesis, and explain their role in metabolism.

• How are fatty acids synthesized? Be able to compare the pathways for fatty acid synthesis and oxidation, and describe the reactions of the synthesis pathway.


• The pathway of dietary triacylglycerols is not as straightforward as that of carbohydrates.

• Triacylglycerols are not water-soluble but must enter an aqueous environment.

• They are packaged in lipoproteins, which consist of droplets of hydrophobic lipids surrounded by phospholipids and other molecules.

• Lipoproteins are special forms of micelles.



• When food leaves the stomach, it enters the duodenum, triggering the release of pancreatic lipases.

• The gallbladder releases bile, a mixture of cholesterol, phospholipids, and bile acids that is manufactured in the liver and stored in the gallbladder until needed.

• It is the job of bile acids and phospholipids to emulsify the triacylglycerols by forming micelles.


• The major bile acid is cholic acid.

• It resembles soaps and detergents because it contains both hydrophilic and hydrophobic regions.


• Pancreatic lipase partially hydrolyzes the emulsified triacylglycerols, producing mono- and diacylglycerols plus fatty acids and a small amount of glycerol.


• Small fatty acids and glycerol are absorbed through the villi that line the small intestine, then carried by the blood to the liver (via the hepatic portal vein).

• Water-insoluble acylglycerols and larger fatty acids are again emulsified, then absorbed by the cells lining the intestine.

• To enter the aqueous bloodstream for transport, they are packaged into lipoproteins known as chylomicrons.


• Chylomicrons are absorbed into the lymphatic system through lacteals, small vessels analogous to capillaries within villi.

• They are carried to the thoracic duct, where the lymphatic system empties into the bloodstream. At this point, the lipids are ready to be used either for energy generation or storage.

• From the thoracic duct the chylomicrons are carried directly to the liver.



• The lipids used in the body’s metabolic pathways have three sources. They enter the pathways: 1. From the digestive tract,

2. From adipose tissue, where excess lipids have been stored, and

3. From the liver, where lipids are synthesized.

• Whatever their source, these lipids must eventually be transported in blood.



• Fatty acids released from adipose tissue associate with albumin, a protein found in blood plasma that binds up to 10 fatty acid molecules per protein molecule.

• All other lipids are carried by lipoproteins.

• Because lipids are less dense than proteins, the density of lipoproteins depends on their ratio of lipids to proteins.

• Lipoproteins can be arbitrarily divided into five major types.

24.2 Lipoproteins for Lipid Transport• Chylomicrons are devoted to transport of lipids from the diet.

They carry triacylglycerols through the lymphatic system into the blood and to the liver for processing. These are the lowest-density lipoproteins.

• Very-low-density lipoproteins (VLDLs) carry triacylglycerols from the liver (where they are synthesized) to tissues for storage or energy generation.

• Intermediate-density lipoproteins (IDLs) carry remnants of the VLDLs from peripheral tissues back to the liver for use in synthesis.

• Low-density lipoproteins (LDLs) transport cholesterol from the liver to peripheral tissues, where it is used in cell membranes or for steroid synthesis.

• High-density lipoproteins (HDLs) transport cholesterol from dead or dying cells back to the liver, where it is converted to bile acids.

24.2 Lipoproteins for Lipid TransportLipids and Atherosclerosis

• According to the U.S. Food and Drug Administration (FDA), there is “strong, convincing, and consistent evidence” for the connection between heart disease and diets high in saturated fats and cholesterol.

• A diet rich in saturated animal fats leads to an increase in blood-serum cholesterol, while a diet low in saturated fat and higher in unsaturated fat can lower the serum cholesterol level.

• High levels of cholesterol are correlated with atherosclerosis, and an increased risk of coronary artery disease and heart attack or stroke.

• Risk factors for heart disease: High blood levels of cholesterol and low levels of high-density lipoproteins, cigarette smoking, high blood pressure, diabetes, obesity, low level of physical activity, family history of early heart disease.

• The ideal ratio of total cholesterol/HDL is considered to be 3.5. A ratio of 4.5 indicates an average risk, and a ratio of 5 or higher shows a high and potentially dangerous risk.



Dietary Triacylglycerols• Hydrolysis occurs when chylomicrons encounter

lipoprotein lipase anchored in capillary walls. • When energy is in good supply, they are converted

back to triacylglycerols for storage in adipose tissue. • When cells need energy, the fatty acid carbon atoms

are activated then oxidized as acetyl-CoA.• Acetyl-CoA generates energy via the citric acid cycle

and oxidative phosphorylation. • Acetyl-CoA serves as the starting material for

lipogenesis, ketogenesis and the synthesis of cholesterol, from which all other steroids are made.


Triacylglycerols from Adipocytes• When stored triacylglycerols are needed as an

energy source, lipases within fat cells are activated by hormone level variation—low insulin and high glucagon.

• Stored triacylglycerols are hydrolyzed to fatty acids, and free fatty acids and glycerol are released into the bloodstream.

• These fatty acids travel in association with albumins (blood plasma proteins) to cells (primarily muscle and liver cells), where they are converted to acetyl-CoA for energy generation.


Glycerol from Triacylglycerols• Glycerol produced from triacylglycerol hydrolysis

is carried to the liver or kidneys, where it is converted to glyceraldehyde 3-phosphate and dihydroxyacetone phosphate (DHAP).

• DHAP enters the glycolysis/gluconeogenesis pathways, linking lipid and carbohydrate paths.


Fate of Dietary Triacylglycerols

Triacylglycerols undergo hydrolysis to fatty acids and glycerol.

Fatty acids undergo:– Resynthesis of triacylglycerols for storage– Conversion to acetyl-CoA

Glycerol is converted to glyceraldehyde 3-phosphate and DHAP, which participate in:– Glycolysis—energy generation– Gluconeogenesis—glucose formation– Triacylglycerol synthesis—energy storage

Acetyl-CoA participates in:– Triacylglycerol synthesis– Ketone body synthesis (ketogenesis)– Synthesis of sterols and other lipids– Citric acid cycle and oxidative phosphorylation


Fat Storage: A Good Thing or Not? • Mammals store excess dietary calories as triacylglycerols in

adipocytes (fat cells, the cells that make up adipose tissue). • Excessive storage of triacylglycerols is a predictor of serious health

problems, and has been associated with increased risk of Type II diabetes, colon cancer, heart attack, or stroke.

• Leptin, a peptide hormone, is synthesized in adipocytes and acts on the brain to stop eating—it suppresses appetite. Grehlin stimulates intense sensations of hunger. Other hormones, including insulin, are also apparently involved in appetite and satiety regulation.

• The bodies of mammals are seemingly “programmed” to conserve extra calories as fat against a time when calories might be scarce.

• Metabolic pathways exist to convert carbohydrate and protein into fat for storage; it is not only dietary fat that is stored.

• Scientists do not yet understand all the hormonal and metabolic connections in the storage process, but a sensible diet combined with exercise will sustain a stable weight without fat accumulation.


Triacylglycerol Synthesis

• After a meal, blood glucose levels increase, insulin levels rise, and glucagon levels drop.

• Glucose enters cells, the rate of glycolysis increases, and insulin activates the synthesis of triacylglycerols for storage.


Triacylglycerol Synthesis• The reactants in triacylglycerol synthesis are glycerol 3-

phosphate and fatty acid acyl groups carried by coenzyme A.

• Triacylglycerol synthesis proceeds by transfer of first one and then another fatty acid acyl group from coenzyme A to glycerol 3-phosphate.

• The reaction is catalyzed by acyl transferase, and the product is phosphatidic acid.


Triacylglycerol Synthesis• Next, the phosphate group is removed from phosphatidic

acid to produce 1,2-diacylglycerol. The third fatty acid group is then added to give a triacylglycerol:


• Adipocytes cannot synthesize glycerol 3-phosphate from glycerol.

• Glycerol 3-phosphate can be synthesized from dihydroxyacetone phosphate (DHAP), so adipocytes can synthesize triacylglycerols as long as there is available DHAP.

• This pathway is called glyceroneogenesis, and it supplies the DHAP necessary to become glycerol 3-phosphate.


• When digestion of a meal is finished, blood glucose levels return to normal; insulin levels drop and glucagon levels rise.

• The lower insulin level and higher glucagon level activate triacylglycerol lipase, the enzyme that controls hydrolysis of stored triacylglycerols.

• When glycerol 3-phosphate is in short supply, fatty acids and glycerol produced by hydrolysis of stored triacylglycerols are released to the bloodstream for transport to energy-generating cells.

• Otherwise, the fatty acids and glycerol are cycled back into new TAGs for storage.



Activation• The fatty acid must be activated by conversion

to fatty acyl-CoA.• This serves the same purpose as the first few

steps in oxidation of glucose by glycolysis. • Some energy from ATP must be invested in

converting the fatty acid to fatty acyl-CoA, a form that breaks down more easily.


Transport• Fatty acyl-CoA must be transported from the

cytosol into the mitochondrial matrix, where energy generation will occur.

• Carnitine undergoes an ester formation exchange reaction with the fatty acyl-CoA, resulting in a fatty acyl-carnitine ester that moves into the mitochondria by facilitated diffusion.

• There, another ester formation exchange reaction regenerates the fatty acyl-CoA and carnitine.


Oxidation:• Fatty acyl-CoA must be oxidized in the

mitochondrial matrix to produce acetyl-CoA plus the reduced coenzymes used in ATP generation.

• The oxidation occurs by repeating four reactions, which make up the -oxidation pathway.

• Each repetition of these reactions cleaves a 2-carbon acetyl group from the end of a fatty acid acyl group and produces one acetyl-CoA.

• The acyl group must continue to return to the pathway until each pair of carbon atoms is removed.



The -Oxidation Pathway

STEP 1: The first -oxidation • Acyl-CoA dehydrogenase and FAD remove

hydrogen atoms from the carbon atoms and to the carbonyl group in the fatty acyl-CoA, forming a carbon–carbon double bond.

• These hydrogen atoms and their electrons are passed directly from FADH2 to coenzyme Q so that the electrons can enter the electron transport chain.


The -Oxidation Pathway

STEP 2: Hydration

• Enoyl-CoA hydratase adds a water molecule across the newly created double bond to give an alcohol with the —OH group on the carbon.

STEP 3: The second -oxidation

• The coenzyme NAD+ is the oxidizing agent for conversion of the —OH group to a carbonyl group by -hydroxyacyl-CoA dehydrogenase.


The -Oxidation PathwaySTEP 4: Cleavage to remove an acetyl group • An acetyl group is split off by thiolase (acyl-CoA

acetyltransferase) and attached to a new coenzyme A molecule, leaving behind an acyl-CoA that is two carbon atoms shorter.

• For a fatty acid with an even number of carbon atoms, all of the carbons are transferred to acetyl-CoA molecules through the -oxidation spiral. Additional steps are required to oxidize fatty acids with odd numbers of carbon atoms and double bonds.


• The number of acetyl-CoA molecules produced from a fatty acid is its number of carbon atoms divided by 2.

• These acetyl-CoA molecules proceed to the citric acid cycle, where each one yields 1 ATP, 3 NADH molecules and 1 FADH2.

• Using the estimates of 3 ATP molecules produced for each NADH and 2 ATP molecules produced for each FADH2, each acetyl-CoA generates 11 ATP molecules from reduced coenzymes.

• Adding the single ATP molecule generated in the citric acid cycle to the 11 obtained from the reduced coenzymes, we get a total of 12 ATP molecules per acetyl-CoA.


• With 2 ATP molecules produced per FADH2 and 3 ATP molecules produced per NADH, 5 ATP molecules are produced for each -oxidation.

• The number of repetitions is always one fewer than the number of acetyl-CoA molecules produced because the last -oxidation cleaves a 4-carbon chain to give 2 acetyl-CoA molecules

• 2 ATP molecules are spent in activation of a fatty acid.


• Best estimates show that 1 mol of glucose (180 g) generates 38 mol of ATP.

• 1 mol of lauric acid (200 g) generates 95 mol of ATP.

• Fatty acids yield nearly three times as much energy per gram as carbohydrates.

• Carbohydrates yield 4 Cal/g (16.7 kJ/g), whereas fats and oils yield 9 Cal/g (37.7 kJ/g).


• -oxidation produces several acetyl-CoA from each molecule of fatty acid, and the enzymes in the -oxidation pathway catalyze reactions more rapidly than the enzymes in the citric acid cycle.

• Excess acetyl-CoA is converted by liver mitochondria to 3-hydroxybutyrate and acetoacetate. Acetoacetate spontaneously decomposes to acetone.


• 3-hydroxybutyrate, acetoacetate, and acetone are known as ketone bodies.

• They are water-soluble, so once formed they are available to all body tissues.

• Ketogenesis occurs in four enzyme-catalyzed steps plus the spontaneous decomposition of acetoacetate.


Steps 1 and 2 of Ketogenesis: Assembly of 6-Carbon Intermediate

• Step 1 is the reverse of the final step of -oxidation: two acetyl-CoA molecules combine in a reaction catalyzed by thiolase to produce acetoacetyl-CoA.

• In Step 2, a third acetyl-CoA and a water molecule react with acetoacetyl-CoA to give 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA).


Steps 3 and 4 of Ketogenesis: Formation of the Ketone Bodies

• In Step 3, removal of acetyl-CoA from the product of Step 2 produces acetoacetate.

• Acetoacetate is the precursor of 3-hydroxybutyrate and acetone.

• In Step 4, the acetoacetate produced in Step 3 is reduced to 3-hydroxybutyrate by 3-hydroxybutyrate dehydrogenase.

• As acetoacetate and 3-hydroxybutyrate are synthesized, they are released to the bloodstream. Acetone is then formed in the bloodstream by the decomposition of acetoacetate and is excreted primarily by exhalation.



• When energy production from glucose is inadequate, the body must respond by providing other energy sources, and the production of ketone bodies accelerates.

• During the early stages of starvation, heart and muscle burn acetoacetate, preserving glucose for the brain.

• In prolonged starvation, the brain can switch to ketone bodies to meet up to 75% of its energy needs.


• Ketone bodies are produced faster than they are utilized in diabetes. This is indicated by acetone on the patient’s breath and ketone bodies in the urine (ketonuria) and blood (ketonemia).

• Because two of the ketone bodies are carboxylic acids, continued ketosis leads to ketoacidosis.

• The blood’s buffers are overwhelmed and blood pH drops. Dehydration due to increased urine flow, labored breathing (acidic blood is a poor oxygen carrier), and depression ensue. Untreated, the condition leads to coma and death.

24.7 Ketone Bodies and KetoacidosisThe Liver, Clearinghouse for Metabolism

• The liver is the largest reservoir of blood in the body and the largest internal organ, making up about 2.5% of the body’s mass.

• Blood carrying the end products of digestion enters the liver through the hepatic portal vein before entering general circulation. The liver is ideally situated to regulate the concentrations of substances in the blood.

• The liver synthesizes glycogen from glucose, glucose from non-carbohydrate precursors, triacylglycerols from mono- and diacylglycerols, fatty acids, cholesterol, bile acids, plasma proteins, and blood clotting factors, and can catabolize glucose, fatty acids, and amino acids.

• The liver stores glycogen, certain lipids and amino acids, iron, and fat-soluble vitamins; only liver cells have the enzyme needed to convert glucose 6-phosphate to glucose that can enter the bloodstream.

• A number of pathologic conditions are based on excessive accumulation of various metabolites. One example is cirrhosis, another is Wilson’s disease, a genetic defect in copper metabolism.


• Lipogenesis provides a link between carbohydrate, lipid, and protein metabolism.

• Because acetyl-CoA is an end product of carbohydrate and amino acid catabolism, using it to make fatty acids allows the body to divert the energy of excess carbohydrates and amino acids into storage as triacylglycerols.

• Fatty acid synthesis and catabolism are similar in that they both proceed two carbon atoms at a time and in that they are both recursive, spiral pathways.



The stage is set for lipogenesis by two reactions:

1. Transfer of an acetyl group from acetyl-CoA to a carrier enzyme in the fatty acid synthase complex (S-enzyme 1) and

2. Conversion of acetyl-CoA to malonyl-CoA, followed by transfer of the malonyl group to acyl carrier protein (ACP) and regeneration of coenzyme A.



• The result of the first cycle in fatty acid synthesis is the addition of two carbon atoms to an acetyl group to give a 4-carbon acyl group still attached to the carrier protein in fatty acid synthase.

• The next cycle adds two more carbon atoms to give a 6-carbon acyl group by repeating the four steps of chain elongation shown here up to sixteen carbon palmitoyl groups.

Chapter Summary

1. What happens during the digestion of triacylglycerols?

• Triacylglycerols from the diet are broken into droplets in the stomach and enter the small intestine, where they are emulsified by bile acids and form micelles.

• Pancreatic lipases partially hydrolyze the triacylglycerols in the micelles. Small fatty acids and glycerol from triacylglycerol hydrolysis are absorbed directly into the bloodstream at the intestinal surface.

• Insoluble hydrolysis products are carried to the lining in micelles, where they are absorbed and reassembled into triacylglycerols.

• These triacylglycerols are then assembled into chylomicrons (which are lipoproteins) and absorbed into the lymph system for transport to the bloodstream.

Chapter Summary, Continued

2. What are the various roles of lipoproteins in lipid transport?

• In addition to chylomicrons, which carry triacylglycerols from the diet into the bloodstream, there are VLDLs (very-low-density lipoproteins), which carry triacylglycerols synthesized in the liver to peripheral tissues for energy generation or storage; LDLs (low-density lipoproteins), which transport cholesterol from the liver to peripheral tissues for cell membranes or steroid synthesis; and HDLs (high-density lipoproteins), which transport cholesterol from peripheral tissues back to the liver for conversion to bile acids that are used in digestion or excreted.


3. What are the major pathways in the metabolism of triacylglycerols?

• Dietary triacylglycerols carried by chylomicrons in the bloodstream undergo hydrolysis to fatty acids and glycerol by enzymes in capillary walls. Triacylglycerols in storage are similarly hydrolyzed within adipocytes.

• The fatty acids from either source undergo -oxidation to acetyl-CoA or resynthesis into triacylglycerols for storage. Acetyl-CoA can participate in resynthesis of fatty acids (lipogenesis), formation of ketone bodies (ketogenesis), steroid synthesis, or energy generation via the citric acid cycle and oxidative phosphorylation.

• Glycerol can participate in glycolysis, gluconeogenesis, or triacylglycerol synthesis.


4. How are triacylglycerols moved into and out of storage in adipose tissue?

• Synthesis of triacylglycerols for storage is activated by insulin when glucose levels are high.

• The synthesis requires dihydroxyacetone phosphate (from glycolysis or glycerol) for conversion to glycerol 3-phosphate, to which fatty acyl groups are added one at a time to yield triacylglycerols.

• Hydrolysis of triacylglycerols stored in adipocytes is activated by glucagon when glucose levels drop.


5. How are fatty acids oxidized, and how much energy is produced by their oxidation?

• Fatty acids are activated (in the cytosol) by conversion to fatty acyl coenzyme A, a reaction that requires the equivalent of two ATPs in the conversion of ATP to AMP.

• The fatty acyl-CoA molecules are transported into the mitochondrial matrix and are then oxidized two carbon atoms at a time to acetyl-CoA by repeated trips through the -oxidation spiral.


6. What is the function of ketogenesis? • The ketone bodies are 3-hydroxybutyrate,

acetoacetate, and acetone. They are produced from two acetyl-CoA molecules.

• Their production is increased when energy generation from the citric acid cycle cannot keep pace with the quantity of acetyl-CoA available. This occurs during the early stages of starvation and in unregulated diabetes.

• The ketone bodies are water-soluble and can travel unassisted in the bloodstream to tissues where acetyl-CoA is produced from acetoacetate and 3-hydroxybutyrate.

• In this way, acetyl-CoA is made available for energy generation when glucose is in short supply.


7. How are fatty acids synthesized? • Fatty acid synthesis (lipogenesis), like -

oxidation, proceeds two carbon atoms at a time in a four-step pathway. The pathways utilize different enzymes and coenzymes.

• In synthesis, the initial four carbons are transferred from acetyl-CoA to the malonyl carrier protein. Each additional pair of carbons is then added to the growing chain bonded to the carrier protein, with the final three steps of the four-step synthesis sequence the reverse of the first three steps in -oxidation.

Outline 24.1 Digestion of Triacylglycerols 24.2Lipoproteins for Lipid Transport 24.3Triacylglycerol Metabolism: An Overview 24.4Storage and Mobilization.

Documents

fatty acid synthesis

larger fatty acids

metabolism of triacylglycerols

emulsified triacylglycerols

fatty acid oxidation24

breakdown of triacylglycerols

oxidation of fatty acids24

job of bile acids