Dietary Lipid Metabolism Presented by Dr. Mohammad Saadeh The requirements for the Pharmaceutical Biochemistry II Philadelphia University Faculty of pharmacy
OVERVIEW Lipids are a heterogeneous group. water-insoluble (hydrophobic) organic molecules (Figure 15.1). Lipidsمهم transported in plasma in association with protein 1. as in lipoprotein particles 2. or on albumin. Lipids are a major source of energy. Provide the hydrophobic barrier that permits partitioning of the aqueous contents of cells and subcellular structures. Some fat-soluble vitamins have regulatory or coenzyme functions, and the prostaglandins and steroid hormones play major roles in the control of the body s homeostasis. Deficiencies or imbalances of lipid metabolism lead to clinical problems such as atherosclerosis and obesity.
Digestion, Absorption, Secretion, and Utilization of Dietary Lipids The average daily intake of lipids by U.S. adults is about 81 g, consists from: 90% is normally triacylglycerol (TAG), cholesterol, cholesteryl esters, phospholipids, and free fatty acids. A. Processing of dietary lipid in the stomach The digestion of lipids begins in the stomach. Triacylglycerols (TAGs) from milk contain short- to medium-chain length fatty acids that degraded in the stomach by two acid lipases: 1. lingual lipase and 2. gastric lipase. These are acid stable (ph 4-6) and important in neonates for milk fat digestion. Cystic Fibrosis: Cystic Fibrosis disease (CF): mutation in transmembrane conductance regulator (CFTR) protein that function as chloride channel on epithelium. Defective CFTR results in decreased secretion of chloride and increased reabsorption of sodium and water. In pancreas cause thickened secretion that clog the pancreatic ducts.
Digestion, Absorption, Secretion, and Utilization of Dietary Lipids Emulsification of dietary lipid in the small intestine Emulsification of dietary lipids occurs in the duodenum. Emulsification increases the surface area of the hydrophobic lipid droplets so the digestive enzymes can act effectively. The dietary lipids are emulsified in the small intestine using 1.mechanical peristaltic action, 2.and bile salts as detergent.
Degradation of dietary lipids by pancreatic enzymes Pancreatic secrete different enzymes 1.pancreatic lipase 2.Colipase 3.cholesteryl ester hydrolase (cholesterol esterase). 4.phospholipase A2. 5.Lysophospholipase
Degradation of dietary lipids by pancreatic enzymes 1. Triacylglycerol degradation: pancreatic lipase: 1. pancreatic lipase removes the fatty acids at carbons 1 and 3 from TAG, forming fatty acid and 2-Monoacylglycerol. 2. Pancreatic lipase consist 2%-3% of pancreatic secretion and colipase. Colipase: 1. Colipase binds the lipase at a ratio of 1:1, at the lipid-aqueous interface. 2. Colipase restores activity to lipase in the presence of inhibitory substances like bile acids that bind the micelles. 3. Colipase is activated in the intestine by trypsin.
Degradation of dietary lipids by pancreatic enzymes Cholesteryl ester degradation: cholesteryl ester hydrolase (cholesterol esterase): 1. Cholesteryl esters in diet are hydrolyzed by pancreatic cholesteryl ester hydrolase (cholesterol esterase), which produces cholesterol and free fatty acids. 2. cholesterol esterase activity is increased in the presence of bile salts.
Degradation of dietary lipids by pancreatic enzymes Phospholipid degradation: phospholipase A2: 1.phospholipase A2 is activated by trypsin and bile salts. 2.Phospholipase A2 removes one fatty acid from carbon 2 of a phospholipid, leaving a lysophospholipid. The remaining fatty acid at carbon 1 can be removed by lysophospholipase. leaving a glycerylphosphoryl base.
The digestion of dietary lipids is summarized in Figure 15.2.
Control of lipid digestion: lipid digestion controlled by two hormone: 1. cholecystokinin (CCK) and 2. Secretin When increase lipid in small intestine led to the cells in duodenum produce hormone called cholecystokinin (CCK). The effect of CCK: 1. gallbladder release bile salts 2. exocrine cells of the pancreas (release digestive enzymes). 3. Decrease gastric motility and contents. intestinal hormone is Secretin released in low ph, causes: 1. pancreas and liver release a solution rich in bicarbonate that helps neutralize the ph of the intestinal contents.
Absorption of lipids by intestinal mucosal cells (enterocytes) mixed micelles: 1. mixed micelles that facilitate the absorption of dietary lipids by intestinal mucosal cells (enterocytes). 2. Consist from 2-monoacylglycerol, cholesterol and free fatty acid with fat-soluble vitamins (A,D,E & K). Absorption of lipids contained in a mixed micelle by an intestinal mucosal cell. [Note: The micelle itself is not taken up.]
Resynthesis of triacylglycerol (TAG) and cholesteryl esters lipids absorbed by the enterocytes and the biosynthesis of complex lipids take place in endoplasmic reticulum. 1. fatty acyl-coa synthetase (thiokinase) convert long-chain fatty acids to fatty acyl-coa (need CoA and ATP). 2. 2-monoacylglycerols with fatty acyl-coa convert to TAG, Using two enzymes activities: acyl CoA:monoacylglycerol acyltransferase and acyl CoA:diacylglycerol acyltransferase. 3. Lysophospholipids are reacylated to form phospholipids by a family of acyltransferases. 4. Cholesterol is esterified to a cholesterol ester by acyl CoA:cholesterol acyltransferase.
Secretion of lipids from enterocytes: These serum lipoprotein particles such as Chylomicrons are released into the lymph, which carries them to the blood. Chylomicrons consist from two layers: very hydrophobic layer (Triglycerides, cholesterol ester. hydrophilic layer, pospholipids, cholesterol and apolipoprotein B-48. Explain: Chylomicrons consist from two layers. Because this layer stabilizes the particle and increases its solubility, thereby preventing multiple particles from coalescing.
Chylomicrons: 1. Chylomicrons synthesized in the intestinal. 2. Chylomicrons transport exogenous triglyceride to other tissues and transport cholesterol and fat soluble vitamin to the liver. Triacylglycerol in Chylomicrons removed by lipoprotein lipase (LPL) which activated by apocii as coenzyme to converted chylomicrons to fatty acids and glycerol. Remnant particles of Chylomicrons removed from the blood by the liver which bind to remnant receptor in liver and recognized by apo E). Fate of glycerol: TAG in intestinal Chylomicrons transporte the TAG to Produce energy, Reesterify to produce TAG Glycerol kinase Glycerol glycerol 3-phosphate Dihydroxyacetone phosphate G3-P enter glycolysis, DHAP enter gluconeogenesis glycolysis gluconeogenesis Fate of Free fatty acids: Fatty acids oxidize in cells to produce energy. Reesterify free fatty acids to produce TAG molecules in Adipocytes.
Fatty Acid, Ketone Body, and Triacylglycerol Metabolism Presented by Dr. Mohammad Saadeh The requirements for the Pharmaceutical Biochemistry II Philadelphia University Faculty of pharmacy
Overview Free Fatty acids exist in the body (unesterified), and are also found as fatty acyl esters such as triacylglycerols (TAGs). Low levels of free fatty acids occur in all tissues, but substantial amounts found in the plasma, particularly during fasting. Plasma free fatty acids transported on serum by albumin. Free fatty acids oxidized by many tissues particularly liver and muscle to provide energy. Biological membranes contain phospholipids and glycolipids. Fatty acids are attached to certain intra cellular proteins to associate those proteins with membranes. Fatty acids are also precursors of the hormone like prostaglandins. Triacylglycerols (esterified fatty acids ) stored in adipose cells, serve as the major energy.
metabolic pathways of fatty acid The figure illustrates the metabolic pathways of fatty acid synthesis, triacylglycerol synthesis and degradation, and their relationship to carbohydrate metabolism.
Structure of Fatty Acids A fatty acid consists of a hydrophobic hydrocarbon chain with a terminal carboxyl group (R COOH). A fatty acid has a pka of about 4.8. At physiologic ph, (R COOH) ionizes, becoming R COO, giving the fatty acid its amphipathic nature (having both a hydrophilic and a hydrophobic region). More than 90% of the fatty acids found in plasma as triacylglycerol, cholesteryl esters, and phospholipids contained in circulating lipoprotein particles. Unesterified, free fatty acids are transported in the circulation in association with albumin. Structure of fatty acid lipoprotein particles
Saturation of Fatty Acids Saturated fatty acid don't contain C-C double bond. Unsaturated fatty acid, contain two or more C-C double bond. C-C double bond always in the cis rather than in the trans configuration. Double bonds decreases the melting temperature (Tm) of a fatty acid. Increasing the chain length increases the Tm.
18:1(9) relative to ω-3 20:5(5,8,11,14,17) relative to ω-3 Essential fatty acids Inability to synthesize in humans, such as: 1. linoleic acid, precursor of ω-6 arachidonic acid, the substrate for prostaglandin synthesis. 2. α-linolenic acid, the precursor of ω-3 fatty acids important for growth and development. Omega (ω)= no of long FA chain no terminal double bond. Omega (ω)= 18 12 = ω-6 (relative to omega 6) Omega (ω)= 18 15 = ω-3 (relative to omega 3)
De novo synthesis of Fatty Acids fatty acids stored as triacylglycerols. fatty acid synthesis occurs in cytosol of liver and lactating mammary glands and, to a lesser extent, in adipose tissue. 1. Production of cytosolic acetyl CoA: Oxidation of Pyruvate produced acetyl CoA The acetyl CoA cannot cross the inner mitochondrial membrane by condensation of oxaloacetate with acetyl CoA by citrate synthase. transferring acetyl CoA (acetate units) from mitochondrial to cytosol. Increase ATP and citrate enhance fatty acid مهم synthesis. مهم See the figure
De novo synthesis of Fatty Acids 2. Carboxylation of acetyl CoA to form malonyl CoA: In cytosol, acetyl CoA carboxylated to malonyl CoA by acetyl CoA carboxylase. A. Short regulation of acetyl CoA carboxylase: This Carboxylation is both rate-limiting and regulated step in fatty acid synthesis. Regulation by two way: 1.Allosteric regulation Allosteric activation by citrate. Allosteric inhibition by long-chain fatty acyl CoA. 2. Covalent regulation by phosphorylation for acetyl CoA carboxylase. Inactivated (phosphorylation) of acetyl CoA carboxylase (ACC) by AMP-dependent kinase (AMPK) when increase glucagon and epinephrine. Allosteric regulation
De novo synthesis of Fatty Acids B. Long-term regulation of Acetyl CoA Carboxylase (ACC): Diet containing excess calories (particularly high calorie, high-carbohydrate diets) causes an increase in ACC synthesis therefore increase fatty acid synthesis. Note: Insulin enhance fatty acid synthesis. Glucagon and epinephrine inhibit fatty acid synthesis.
De novo synthesis of Fatty Acids, see the figure in the next slide Fatty acid synthesis are catalyzed by the multifunctional enzyme, fatty acid synthase, which produces palmitate from acetyl CoA and malonyl CoA. C. Fatty acid synthase: a multifunctional enzyme in eukaryotes x 1. Acetyl CoA bind to the SH group of the ACP. 2. Transferred acetyl CoA residue to thiol group of a cysteine. 3. ACP accepts a three-carbon malonate unit from malonyl CoA. 4. The acetyl group on the cysteine residue condenses with the malonyl group on ACP with release CO 2. 5. The keto group is reduced to an alcohol by NADPH. 6. A molecule of water is removed to introduce a double bond between carbons 2 and 3 (the α- and β-carbons). 7. The double bond is reduced by NADPH. 8. Repeat steps 2-7 five times more. 9. Palmitoyl thioesterase domain release PALMITATE.
x
De novo synthesis of Fatty Acids D. Major sources of the NADPH required for fatty acid synthesis NADPH produced from: two source of NADPH, 1. The pentose phosphate pathway is supplier of two NADPH for fatty acid synthesis. 2. Cytosolic NADPH produce from oxidize malate to pyruvate by NADP+-dependent malate dehydrogenase (malic enzyme). See the Figure 16.11. NADPH: nicotinamide adenine dinucleotide phosphate E. Further elongation of fatty acid chains in smooth endoplasmic reticulum. F. Desaturation of fatty acid chains: Enzymes (desaturases) also present in the SER are responsible for desaturating long-chain fatty acids.
x Figure 16.11: Interrelationship between glucose metabolism and palmitate synthesis.
There are two pathways for glycerol3-phosphate production: 1. glycerol3-phosphate From glycolysis 2. Phosphorylation of glycerol that produce from triacylglycerol hydrolysis that transported through the blood to the liver. Glycolysis Glycerol kinase in liver glycerol phosphate dehydrogenase Glycerol glycerol 3-phosphate dihydroxyacetone phosphate Figure 16.13: Pathways for production of glycerol phosphate in liver and adipose tissue.
Synthesis of a molecule of TAG from glycerol phosphate and
Mobilization of stored fats and oxidation of fatty acids Release of fatty acids from triacylglycerol: 1. Adipose tiglyceride lipase (ATGL) generate diacylglycerol and fatty acids. 2. Hormone-sensitive lipase (HSL) produce monoacylglycerol and fatty acid. 3. Monoacylglycerol lipase produce fatty acid and glycerol from monoacylglycerol. Regulation of Hormone-sensitive lipase (HSL): Epinephrine increase camp Activate protein kinase A Phosphorylation of Hormone-sensitive lipase (become active).
Mobilization of stored fats and oxidation of fatty acids Triacylglycerol glycerol + Fatty acids. Fate of glycerol: glycerol glycerol3-phosphate DHAP DHAP can participate in glycolysis and gluconeogenesis in liver. Fate of fatty acids: Activated to CoA. Oxidized for energy in mitochondria.
β-oxidation of fatty acids: Occur in mitochondrial two-carbon fragments are successively removed from the carboxyl end of the fatty acyl CoA, producing acetyl CoA, NADH, and FADH2.
β-oxidation of fatty acids: In cytosol, the first step is activation of long chain fatty acid converted to fatty acyl CoA by acyl CoA synthetase and consuming ATP. Inner mitochondria impermeable to fatty acyl-coa, therefor carnitine transported fatty acid, this process is called the carnitine shuttle. From fatty acyl CoA the Carnitine palmitoyl transferase I (CPT-I)produce CoA and acyl carnitine. acyl carnitine transported to mitochondrial matrix with exchange carnitine by carnitine-acylcarnitine translocase. Carnitine palmitoyltransferase II (CPT-II) in inner mitochondrial transfer acyl group from carnitine to CoA and regenerating free carnitine. acyl CoA synthetase
Inhibitor of the carnitine shuttle: Synthesis of fatty acids occurs in cytosol. β-oxidation of fatty acids occurs in mitochondrial. Malonyl CoA inhibits CPT-I, preventing the entry of long-chain acyl groups into the mitochondrial matrix. Therefore, palmitate which synthesis in cytosol can not transferred into mitochondria and oxidized. Medium and short Fatty acids (shorter than 12 carbons) can cross the inner mitochondrial membrane without the aid of carnitine or the CPT system. Therefor Malonyl CoA can not inhibit transported medium and short Fatty acids
Reactions of β-oxidation: It consists four reactions involving the β-carbon (carbon 3) so shortening the fatty acid chain by two carbons. 4 Steps of β-oxidation for saturated fatty acid: 1. Oxidation, produces FADH 2 by Acyl CoA dehydrogenase. 2. Hydration step, by Enoyl CoA hydratase. 3. Oxidation, produced NADH, by 3-Hydroxyacyl CoA dehydrogenase. 4. Thiolytic cleavage that releases a molecule of acetyl CoA, by β-ketoacyl-coa thilase. These four steps are repeated for saturated fatty acids. No of repeated steps: if even-numbered carbon chains = (n/2) 1 times. Each β-oxidation (4steps, shortening 2 carbon) = FADH 2 + NADH + acetyl CoA FADH2 = 2ATP; NADH= 3ATP; Acetyl CoA= 12 ATP
Oxidation of fatty acids with an odd number of carbons The β-oxidation of saturated fatty acid with an odd number proceeds by the same β-oxidation reaction, until the final three carbons are reached. This compound called propionyl CoA. Finally, propionyl CoA converted to Succinyl CoA which can enter the TCA cycle.
Oxidation of unsaturated fatty acids: The oxidation of unsaturated fatty acids provides less energy than saturated fatty acids. Unsaturated fatty acids bypass the first β-oxidation reaction, therefore unsaturated fatty acids don t generate FADH 2. Saturated unsaturated Explain:
KETONE BODIES: An alternate fuel for cells
KETONE BODIES: An alternate fuel for cells: Liver mitochondria convert acetyl CoA derived from fatty acid oxidation into ketone bodies (acetoacetate, 3-hydroxybutyrate, and acetone). In peripheral tissue, ketone bodies (acetoacetate and 3-hydroxybutyrate) can be reconverted to acetyl CoA, which can be oxidized by the TCA cycle. ketone bodies are important sources of energy because: 1. They are soluble in aqueous solution and so do not need to be incorporated into lipoproteins or carried by albumin as do the other lipids. 2. They are produced in the liver when the amount of acetyl CoA present exceeds the oxidative capacity of the liver. 3. They are used in extrahepatic tissues, such as the skeletal & cardiac muscle, renal cortex and brain. ketone bodies are important during prolonged periods of fasting (ketone bodies spare glucose).
Synthesis of ketone bodies by the liver (ketogenesis) Liver mitochondria convert acetyl CoA derived from fatty acid oxidation into ketone bodies (acetoacetate, 3-hydroxybutyrate, and acetone). Elevated NADH and acetyl CoA lead to activate ketone body synthesis because: inhibits pyruvate dehydrogenase. (pyruvate acetyl CoA) activates pyruvate carboxylase. (pyruvate Oxaloacetate) The OAA is used by the liver for gluconeogenesis. Therefore, acetyl CoA is channeled into ketone body synthesis. Note: during Fatty acid oxidation the NAD+ to NADH ratio is low, and the rise in NADH shifts OAA to malate. This pushes acetyl CoA away from gluconeogenesis and into ketogenesis.
Ketone bodies are synthesized by two steps: first step, acetyl CoA forming acetoacetyl CoA, by reversal of the thiolase reaction. Mitochondrial HMG CoA synthase combines a third molecule of acetyl CoA with acetoacetyl CoA to produce HMG CoA (precursor of cholesterol). HMG CoA synthase is the rate-limiting step in the synthesis of ketone bodies, and is present in liver. HMG CoA is cleaved to produce acetoacetate and acetyl CoA. Acetoacetate can be reduced to form 3- hydroxybutyrate with NADH as the hydrogen donor. Acetoacetate can decarboxylate in the blood to form acetone a volatile, nonmetabolized that can be released in the breath. NAD+/NADH ratio is low during fatty acid oxidation.
Use of ketone bodies by the peripheral tissues: ketolysis ketone bodies elevated during fasting when ketone bodies are needed to provide energy to the peripheral tissues. 3-Hydroxybutyrate is oxidized to acetoacetate by 3-hydroxy butyrate dehydrogenase, producing NADH. Acetoacetate is then provided with a CoA molecule taken from succinyl CoA by succinyl CoA: acetoacetate CoA transferase (thiophorase). Reversible reaction. Acetoacetyl CoA, is removed by its conversion to two acetyl CoA. Extrahepatic tissues, such as the brain oxidize acetoacetate and 3-hydroxybutyrate. Liver produces ketone bodies but it lacks thiophorase, therefore, is unable to use ketone bodies as fuel. 3-hydroxy butyrate dehydrogenase
Ketoacidosis occurs when the rate of formation of ketone bodies is greater than their rate of use, as is seen in cases of uncontrolled, type 1 diabetes mellitus. A frequent symptom of diabetic ketoacidosis is a fruity odor on the breath, which results from increased production of acetone.
Science Should be as simple as possible, but not simpler. Albert Einstein
References: Biochemistry. Lippincott's Illustrated Reviews. 6 th Edition by, Richard A Harvey, Denise R. Ferrier. Lippincott Williams and Wilkins, a Wolters kluwer business. 2014.