Fatty Acid Metabolism - Lipid Metabolizm PDF

Fatty acid metabolism; Structure and classification; Essential fatty acids; de novo synthesis of fatty acids; Triacylglycerols Lali Shanshiashvili  Lipids have diverse biological functions as well as diverse structures. triacylglycerols glycerophospholipids...

Fatty acid metabolism; Structure and classification; Essential fatty acids; de novo synthesis of fatty acids; Triacylglycerols Lali Shanshiashvili  Lipids have diverse biological functions as well as diverse structures. triacylglycerols glycerophospholipids sphingolipids. Lipids containing phosphate groups are called phospholipids and lipids containing both sphingosine and carbohydrate groups are called glycosphingolipids.  Low levels of free fatty acids occur in all tissues, but substantial amounts can sometimes be found in the plasma, particularly during fasting. Fatty acids are precursors of the hormone-like prostaglandins. Esterified fatty acids, stored in the adipose cells, serve as the major energy reserve of the body.  Free fatty acids can be oxidized by many tissues — particularly liver and muscle—to provide energy. Fatty acids are structural components of membrane lipids, such as phospholipids and glycolipids. STRUCTURE OF FATTY ACIDS The simplest lipids are the fatty acids that have the general formula R—COOH, where R represents a hydrocarbon chain composed of various lengths of —CH -(methylene) units.  At physiologica pH, the terminal carboxyl group (–COOH) ionizes, becoming -COO- This anionic group gives the fatty acids an amphipathic nature (having both a hydrophilic and a hydrophobic region). In the molecule of long-chain fatty acids (LCFAs), the hydrophobic portion is predominant. They are highly water insoluble, and must be transported in the circulation in association with albumin.  Saturation of fatty acids Fatty acid chains may be saturated - contain no double bonds,—or be mono- or polyunsaturated - contain one or more double bonds. Double bonds are nearly always in the cis configuration. The introduction of a cis double bond causes the fatty acid to bend or “kink” at that position.  The configuration of the double bonds in unsaturated fatty acids can be either cis or trans. The configuration is usually cis in naturally occurring fatty acids.  The addition of double bonds decreases the melting temperature – Tm of a fatty acid. Increasing the chain length increases the Tm Membrane lipids typically contain LCFA, the presence of double bonds in some fatty acids helps maintain the fluid nature of those lipids.  Chain lengths of fatty acids More than 100 different fatty acids have been identified in various species. Fatty acids differ from one another in the length of their hydrocarbon tails, the number of carbon–carbon double bonds, the positions of the double bonds in the chains, and the number of branches. Fatty acids can be referred to by either International Union of Pure and Applied Chemistry (IUPAC) names or common names. Common names are used for the most frequently encountered fatty acids.  The carbon atoms are numbered, beginning with the carboxyl carbon as carbon 1 Carbon 2, the carbon to which the carboxyl group is attached, is also called the α-carbon, carbon 3 is the β-carbon, and carbon 4 is the γ-carbon. The carbon of the terminal methyl group is called the ω-carbon regardless of the chain length. Linolenate is an omega-3 (ω - 3) fatty acid since the last double bond is three carbon atoms from the tail end of the molecule. Omega-3 fatty acids are very popular dietary supplements. They are enriched in fish oils. Linolenate is an essential fatty acid so your diet must include an adequate supply of this omega-3 fatty acid.  Blood rheology is the scientific field working on the biophysical properties and flow properties of blood. One of the well-known hemorheological parameter is blood viscosity. The most important benefit is protection against cardiovascular disease (remnant lipoproteins).  Essential fatty acids Mammals require certain dietary polyunsaturated fatty acids that they cannot synthesize, such as: linoleic acid, which is the precursor of ω-6 arachidonic acid, the substrate for prostaglandin and α-linolenic acid, the precursor of other ω-3 fatty acids important for growth and development. Note: Arachidonic acid becomes essential if linoleic acid is deficient in the diet.  Essential fatty acid deficiency can result in scaly dermatitis (ichthyosis) and visual and neurologic abnormalities. Essential fatty acid deficiency, however, is rare.  DE NOVO SYNTHESIS OF FATTY ACIDS In adult humans, fatty acid synthesis occurs primarily in the liver and lactating mammary glands and, to a lesser extent, in adipose tissue. This cytosolic process incorporates carbons from acetyl coenzyme A (CoA) into the growing fatty acid chain, using adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH).  Production of cytosolic acetyl CoA Mitochondrial acetyl CoA is produced by : the oxidation of pyruvate the catabolism of fatty acids ketone bodies certain amino acids. The CoA portion of acetyl CoA cannot cross the inner mitochondrial membrane; Only the acetyl portion enters the cytosol. It does so as part of citrate produced by the condensation of oxaloacetate (OAA) and acetyl CoA. Transport of acetyl units from the mitochondrion to the cytoplasm. The inner mitochondrial membrane has carriers for citrate, pyruvate, and malate but not for acetyl-CoA and oxaloacetate. CoA, Coenzyme A; NADP, nicotinamide adenine dinucleotide phosphate.  Translocation of citrate from the mitochondrion to the cytosol, occurs when the mitochondrial citrate concentration is high. This is observed when isocitrate dehydrogenase is inhibited by the presence of large amounts of ATP, causing citrate and isocitrate to accumulate. In cytosol citrate is cleaved by ATP- citrate lyase to produce cytosolic acetyl CoA and OAA  So cytosolic citrate may be viewed as a high- energy signal. As a large amount of ATP is needed for fatty acid synthesis, the increase in both ATP and citrate enhances this pathway.  Carboxylation of acetyl CoA to form malonyl CoA The carboxylation of acetyl CoA to form malonyl CoA is catalyzed by acetyl CoA carboxylase and requires CO2 and ATP. The coenzyme is the vitamin - biotin, which is covalently bound to a lysyl residue of the carboxylase.  Two types of regulation: Short-term regulation of acetyl CoA carboxylase (ACC). The inactive form of ACC is a dimer. The enzyme undergoes allosteric activation by citrate, which causes dimers to polymerize, and allosteric inactivation by long-chain fatty acyl CoA which causes its depolymerization  Two types of regulation: Short-term regulation of acetyl CoA carboxylase (ACC). AMP-activated protein kinase (AMPK) phosphorylates and inactivates ACC. AMPK itself is allosterically activated by AMP and covalently activated by phosphorylation via several kinases. At least one of these AMPK kinases is activated by cAMP-dependent protein kinase A (PKA). In the presence of epinephrine and glucagon, ACC is phosphorylated and, thereby, inactivated. In the presence of insulin, ACC is dephosphorylated and, thereby, activated  Two types of regulation: Long-term regulation of acetyl CoA carboxylase. Prolonged consumption of a high-carbohydrate diets causes an increase in ACC synthesis, thus increasing fatty acid synthesis. A low calorie or a high-fat diet causes a reduction in fatty acid synthesis by decreasing the synthesis of ACC. Synthesis of the carboxylase is upregulated by insulin via a sterol response element binding protein - SREBP-1.  Reactions that build up the fatty acid chain in eukaryotes is catalyzed by the multifunctional, dimeric enzyme, fatty acid synthase (FAS). Fatty acid synthase: a multifunctional enzyme in eukaryotes Each segment of the disc represents one of the six enzymatic activities of the complex: acetyl-CoA-ACP transacetylase (AT) malonyl-CoA-ACP transferase (MT) b-keto-ACP synthase (KS) containing a critical Cys-SH residue b -ketoacyl-ACP reductase (KR) b -hydroxyacyl-ACP dehydratase (HD) enoyl-ACP reductase (ER). At the center is acyl carrier proteins (ACP), with its phosphopantetheine arm (Pn) ending in another -SH.  First, the acetyl group of acetyl-CoA is transferred to the Cys -SH group of the β- ketoacyl ACP synthase (KS). This reaction is catalyzed by acetyl-CoA-ACP transacetylase. The second reaction, transfer of the malonyl group from malonyl-CoA to the - SH group of ACP, is catalyzed by malonyl- CoA-ACP transferase (MT), also part of the complex. In the charged synthase complex, the acetyl and malonyl groups are very close to each other and are activated for the chain-lengthening process, which consists of four steps.  Condensation of the activated acetyl and malonyl groups. The result is a four-carbon unit attached to the ACP domain.  The keto group is reduced to an alcohol. Domain: b - ketoacyl-ACP reductase (KR)..  A molecule of water is removed to introduce a double bond between carbons 2 and 3 (the α- and β-carbons). Domain: b -hydroxyacyl-ACP dehydratase (HD).  The double bond is reduced. Domain enoyl-ACP reductase (ER).  The result of these seven steps is production of a four- carbon compound (butyryl) whose three terminal carbons are fully saturated, and which remains attached to the ACP.  These seven steps are repeated, beginning with the transfer of the butyryl chain from the ACP to the Cys residue, the attachment of a molecule of malonate to the ACP and the condensation of the two molecules liberating CO2. Major sources of the NADPH required for fatty acid synthesis  Further elongation of fatty acid chains Palmitate, a 16-carbon, fully saturated fatty acid (16:0), is the primary end product of fatty acid synthase activity. Palmitate further is elongated by the addition of two-carbon units in the smooth endoplasmic reticulum (SER). The process of elongation requires separate enzymes. Malonyl CoA is the two-carbon donor and NADPH supplies the electrons. The brain produces the very long-chain fatty acids (over 22 carbons) that are required for the synthesis of brain lipids. For this brain has additional elongation capabilities.  Desaturation of fatty acid chains Desaturation of fatty acid chains occures in the smooth endoplasmic reticulum (enzymes desaturases). The human fatty acid desaturase systems can desaturate various chain lengths at Δ4, Δ5, Δ6, and Δ9 positions. However, humans cannot introduce double bonds beyond carbons 9 and 10 and must have the polyunsaturated fatty acids linoleic (18:2 cis-Δ9,12), linolenic (18:3 cis-Δ9,12,15), and arachidonic (20:4 cis-Δ5,8,11,14) acids provided in the diet. These fatty acids are thus essential fatty acids in humans.  Storage of fatty acids as components of triacylglycerols In the molecule of TAG the fatty acid on carbon 1 is typically saturated, on carbon 2 is typically unsaturated and on carbon 3 can be either. The presence of the unsaturated fatty acids decrease the melting temperature (Tm) of the lipid.  TAGs coalesce within adipocytes to form oily droplets that are nearly anhydrous. These lipid droplets are the major energy reserve of the body.  Glycerol phosphate is the initial acceptor of fatty acids during TAG synthesis. Glycerol phosphate can be produced from glucose, during glycolytic pathway as in the liver, as well in adipose tissue.  Conversion of a free fatty acid to its activated form Reaction is catalyzed by a family of fatty acyl CoA synthetases (thiokinases). Synthesis of a molecule of TAG from glycerol phosphate and fatty acyl CoA: Different fates of TAG in the liver and adipose tissue In white adipose tissue, TAG is stored as fat droplets in the cytosol of the cells. It serves as “depot fat,” ready for mobilization. Little TAG is stored in the liver. Others are packaged with other lipids and apoproteins to form lipoprotein particles - very-low density lipoproteins (VLDL). VLDL are secreted directly into the blood where they mature and function to deliver the endogenously derived lipids to the peripheral tissues. DIGESTION, ABSORPTION, SECRETION, AND UTILIZATION OF DIETARY LIPIDS DIGESTION, ABSORPTION, SECRETION, AND UTILIZATION OF DIETARY LIPIDS The average daily intake of lipids by U.S. adults is about 81 g, of which more than 90% is normally triacylglycerol.  The remainder of the dietary lipids consists primarily of cholesterol, cholesterol esters Phospholipids and non-esterified (“free”) fatty acids. A. Processing of dietary lipid in the stomach The digestion of lipids begins in the stomach, catalyzed by an acid- stable lipase - lingual lipase - that originates from glands at the back of the tongue. TAG molecules, particularly those containing fatty acids of short- or medium-chain length (fewer than 12 carbons) are the primary target of this enzyme.  These same TAGs are also degraded by a separate gastric lipase, secreted by the gastric mucosa. Both enzymes are relatively acid-stable, with pH optimums of pH 4 to pH 6. These “acid lipases” play a particularly important role in lipid digestion in neonates, for whom milk fat is the primary source of calories.  They also become important digestive enzymes in individuals with pancreatic insufficiency, such as those with cystic fibrosis. Lingual and gastric lipases help these patients in degrading TAG molecules despite a near or complete absence of pancreatic lipase.  This autosomal recessive disorder is caused by mutations to the gene for the cystic fibrosis transmembrane conductance regulator - CFTR protein - that functions as a chloride channel on epithelium.  Delta F508 never reaches the cell membrane. People who are homozygous for delta F508 mutation tend to have the most severe symptoms of cystic fibrosis due to critical loss of chloride ion transport. This upsets the sodium and chloride ion balance needed to maintain the normal, thin mucus layer that is easily removed by cilia lining the lungs and other organs. The sodium and chloride ion imbalance creates a thick, sticky mucus layer that cannot be removed by cilia and traps bacteria, resulting in chronic infections.  out in R-domain N ATP ATP cAMP-activated C protein kinase  Cl- out in N O O ADP O- O P ADP O O- P O O R-domain O C -O O O P P O O -O O  Defective CFTR results in decreased secretion of chloride and increased reabsorption of sodium and water. In the pancreas, the decreased hydration results in thickened secretions such that pancreatic enzymes are not able to reach the intestine, leading to pancreatic insufficiency. Treatment includes replacement of these enzymes and supplementation with fat-soluble vitamins. Note: CF also causes chronic lung infections with progressive pulmonary disease.  B. Emulsification of dietary lipid in the small intestine The critical process of emulsification of dietary lipids occurs in the duodenum. Emulsification increases the surface area of the hydrophobic lipid droplets so that the digestive enzymes, which work at the interface of the droplet and the surrounding aqueous solution, can act effectively.  Emulsification is accomplished by two complementary mechanisms, namely: use of the detergent properties of the bile salts and mechanical mixing due to peristalsis. These emulsifying agents interact with the dietary lipid particles and the aqueous duodenal contents, thereby stabilizing the particles as they become smaller.  Bile salts, are made in the liver and stored in the gallbladder. They are derivatives of cholesterol. They consist of a sterol ring structure with a side chain to which a molecule of glycine or taurine is covalently attached by an amide linkage.  C. Degradation of dietary lipids by pancreatic enzymes The dietary TAG, cholesteryl esters, and phospholipids are degraded (“digested”) by pancreatic enzymes, whose secretion is hormonally controlled. 1. TAG degradation: TAG degradation: TAG molecules are too large to be taken up efficiently by the mucosal cells of the intestinal villi. They are, therefore, acted upon by an esterase, pancreatic lipase, which preferentially removes the fatty acids at carbons 1 and 3. The primary products of hydrolysis are thus a mixture of 2- monoacylglycerol and free fatty acids. This enzyme is found in high concentrations in pancreatic secretions (2–3% of the total protein present), and it is highly efficient catalytically, thus ensuring that only severe pancreatic deficiency, such as that seen in cystic fibrosis, results in significant malabsorption of fat.  A second protein, colipase, also secreted by the pancreas, binds the lipase at a ratio of 1:1, and anchors it at the lipid-aqueous interface. Note: Colipase is secreted as the zymogen, procolipase, which is activated in the intestine by trypsin. Orlistat, an antiobesity drug, inhibits gastric and pancreatic lipases, thereby decreasing fat absorption, resulting in loss of weight. 2. Cholesteryl ester degradation: Most dietary cholesterol is present in the free (nonesterified) form, with 10–15% present in the esterified form. Cholesteryl esters are hydrolyzed by pancreatic cholesteryl ester hydrolase - cholesterol esterase, which produces cholesterol plus free fatty acids. Cholesteryl ester hydrolase activity is greatly increased in the presence of bile salts. 3. Phospholipid degradation: Pancreatic juice is rich in the proenzyme phospholipase A2 that, like procolipase, is activated by trypsin and, like cholesteryl ester hydrolase, requires bile salts for optimum activity. Phospholipase A 2 removes one fatty acid from carbon 2 of a phospholipid, leaving a lysophospholipid. For example, phosphatidylcholine (the predominant phospholipid during digestion) becomes lysophosphatidylcholine.  4. Control of lipid digestion: Cells in the mucosa of the lower duodenum and jejunum produce a small peptide hormone, cholecystokinin (CCK), in response to the presence of lipids and partially digested proteins entering these regions of the upper small intestine. Additionally!!! CCK is also produced in the neurons of the central nervous system (CNS), where it acts as a neurotransmitter involved in regulating satiety and anxiety.  CCK acts on the gallbladder (causing it to contract and release bile—a mixture of bile salts, phospholipids, and free cholesterol), and on the exocrine cells of the pancreas causing them to release digestive enzymes. It also decreases gastric motility, resulting in a slower release of gastric contents into the small intestine.  Other intestinal cells produce another small peptide hormone, secretin, in response to the low pH of the chyme entering the intestine. Secretin causes the pancreas and the liver to release a solution rich in bicarbonate that helps neutralize the pH of the intestinal contents, bringing them to the appropriate pH for digestive activity by pancreatic enzymes.  D. Absorption of lipids by intestinal mucosal cells (enterocytes) Free fatty acids, free cholesterol and 2- monoacylglycerol are the primary products of lipid digestion in the jejunum. These, plus bile salts and fat-soluble vitamins (A, D, E, and K), form mixed micelles—disc-shaped clusters of amphipathic lipids that coalesce with their hydrophobic groups on the inside and their hydrophilic groups on the outside.  The primary site of lipid absorption is the brush border membrane of the mucosal cell. The hydrophilic surface of the micelles facilitates the transport of the hydrophobic lipids through the unstirred water layer to the brush border membrane where they are absorbed. Bile salts are absorbed in the ileum.  E. Resynthesis of TAG and cholesteryl esters The mixture of lipids absorbed by the enterocytes migrates to the endoplasmic reticulum where the biosynthesis of complex lipids takes part. Fatty acids are first converted into their activated form by fatty acyl-CoA synthetase (thiokinase). Using the fatty acyl CoA derivatives, the 2-monoacylglycerols absorbed by the enterocytes are converted to TAGs by the enzyme complex, TAG synthase.  All long-chain fatty acids entering the enterocytes are used in this fashion to form TAGs, phospholipids, and cholesteryl esters. Short- and medium-chain length fatty acids are not converted to their CoA derivatives and are not reesterified to 2- monoacylglycerol. Instead, they are released into the portal circulation, where they are carried by serum albumin to the liver. F. Lipid malabsorption Lipid malabsorption, resulting in increased lipid (including the fat-soluble vitamins and essential fatty acids) in the faeces (that is, steatorrhea), can be caused by disturbances in lipid digestion and/or absorption. Such disturbances can result from several conditions, including CF (causing poor digestion) and shortened bowel (causing decreased absorption). The effects of unabsorbed substances, especially in global malabsorption, include diarrhoea, steatorrhea, abdominal bloating, and gas. Other symptoms result from nutritional deficiencies. Patients often lose weight despite adequate food intake. Some of the other causes of malabsorption include: Cystic fibrosis, chronic pancreatitis, and other diseases that affect the pancreas Lactose intolerance or other enzyme-related conditions Intestinal disorders such as celiac disease (when the gluten protein from wheat, barley, and rye triggers your immune system to attack your body) Severe congestive heart failure which causes the bowel wall to become swollen with fluid (edema) and doesn't absorb nutrients well.  The ability of short- and medium-chain length fatty acids to be taken up by enterocytes without the aid of mixed micelles has made them important in dietary therapy for individuals with malabsorption disorders.  G. Secretion of lipids from enterocytes The newly resynthesized TAGs and cholesteryl esters are very hydrophobic, and aggregate in an aqueous environment. It is, therefore, necessary that they be packaged as particles of lipid droplets surrounded by a thin layer composed of phospholipids, unesterified cholesterol, and a molecule of the characteristic protein, apolipoprotein B-48. This layer stabilizes the particle and increases its solubility, thereby preventing multiple particles from coalescing.  The particles are released by exocytosis from enterocytes into the lacteals (lymphatic vessels originating in the villi of the small intestine). The presence of these particles in the lymph after a lipid-rich meal gives it a milky appearance. This lymph is called chyle (as opposed to chyme—the name given to the semifluid mass of partially digested food that passes from the stomach to the duodenum), and the particles are named chylomicrons.  Chylomicrons follow the lymphatic system to the thoracic duct and are then conveyed to the left subclavian vein, where they enter the blood. Triacylglycerol contained in chylomicrons is broken down primarily in the capillaries of skeletal muscle and adipose tissues, but also those of the heart, lung, kidney, and liver. Triacylglycerol in chylomicrons is degraded to free fatty acids and glycerol by lipoprotein lipase. This enzyme is synthesized primarily by adipocytes and muscle cells. It is secreted and becomes associated with the luminal surface of endothelial cells of the capillary beds of the peripheral tissues.  Familial lipoprotein lipase deficiency Familial lipoprotein lipase deficiency (Type I hyperlipoproteinemia - is a rare, autosomal recessive disorder caused by a deficiency of lipoprotein lipase or its coenzyme, apolipoprotein C-II. The result is fasting chylomicronemia and hypertriacylglycerolemia. Fate of free fatty acids: The free fatty acids derived from the hydrolysis of TAG may either directly enter adjacent muscle cells or adipocytes, or they may be transported in the blood in association with serum albumin until they are taken up by cells.  Most cells can oxidize fatty acids to produce energy. Adipocytes can also reesterify free fatty acids to produce TAG molecules, which are stored until the fatty acids are needed.  Fate of glycerol Glycerol that is released from TAG is used almost exclusively by the liver to produce glycerol 3-phosphate, which can enter either glycolysis or gluconeogenesis by oxidation to dihydroxyacetone phosphate.  Fate of the remaining chylomicron components: After most of the TAG has been removed, the chylomicron remnants (which contain cholesteryl esters, phospholipids, apolipoproteins, fat-soluble vitamins, and some TAG) bind to receptors on the liver and are then endocytosed. The remnants are then hydrolyzed to their parts. If the removal of remnants by the liver is decreased due to impaired binding to their receptor, they accumulate in the plasma. This is seen in Type III hyperlipoproteinemia (rare, also called familial dysbetalipoproteinemia). If you use in Argus uploaded pdf file - pp.425-463