Lipid Metabolism PDF

Lipid Metabolism Dr Nesreen Elhelbawy  Intended learning outcomes (ILOs): By the end of the course, the student should be able to: 1. Point out digestion, absorption and secretion of lipids 2. Point out site of fatty acid synthesis, importance , steps and regulatory mechanisms of this...

Lipid Metabolism Dr Nesreen Elhelbawy  Intended learning outcomes (ILOs): By the end of the course, the student should be able to: 1. Point out digestion, absorption and secretion of lipids 2. Point out site of fatty acid synthesis, importance , steps and regulatory mechanisms of this pathway. 3. Point out site of fatty acid oxidation, importance, ,steps and regulatory mechanisms of this pathway. 4. Point out site of cholesterol & ketone bodies metabolism, importance ,steps and regulatory mechanisms of these pathways. 5. Point out types, structures and metabolism of various lipoproteins.  B) Intellectual skills By the end of the course, the student should be able to: 1. Interpret symptoms, signs and biochemical laboratory findings of some inborn errors of metabolic disorders. 2. Point out the clinical significance of determination of plasma levels of cholesterol. 3. Point-out the etiology of metabolic disturbance in a given case study report.  Digestion, absorption and secretion of lipids DIGESTION OF LIPIDS The major lipids in the diet are ❑ Triacylglycerol ❑ Phospholipids to a lesser extent ❑ The fat-soluble vitamins—A, D, E, and K ❑ and a variety of other lipids (including cholesterol, cholesterol esters and unesterified fatty acids). A- Hydrolysis of triacylglycerol (TAG) is initiated by: ❑ Lingual and gastric lipases in mouth and stomach: that attack the sn-3 ester bond, forming 1, 2-diacylglycerols and free fatty acids. ❑ Pancreatic lipase is secreted into the small intestine and requires pancreatic protein, colipase, for activity. It is specific for positions 1 and 3 in triacylglycerol—resulting in 2- monoacylglycerols and free fatty acids. ❑ Intestinal lipase:- acts on 1-monoacylglycerol converting it into glycerol and FA. B- Digestion of cholesterol esters:- ✓ Cholesterol esters are digested by cholesterol esterase enzyme into cholesterol and FA. ✓ Free cholesterol is absorbed as such. C- Digestion of phospholipids (PLs):- ✓ PLs may be absorbed without digestion. ✓ The pancreatic phospholipase A2 (in the presence of Ca+2 ions as activator) gives lysophospholipids and FAs.  ABSORPTION OF LIPIDS The end products of lipid digestion are (monoacylglycerols, FAs , glycerol, PLs and cholesterol). They are absorbed from the jejunum and ileum as follow:- ❑ Short chain FAs and glycerol:- They are water soluble and pass via portal circulation to the liver. ❑ Other lipids are water insoluble, so they combine with bile salts to form a water soluble complex called micelles, which enter the mucosal cells. ❑ They are reacylated to TAG in intestinal cells ❑ Finally, TAGs, PLs and cholesterol are bound to protein called apolipoprotein B-48 to form chylomicrons (lipoproteins) which are secreted into the lymphatics, then into the thoracic duct entering the blood stream.  Lipid malabsorption Steatorrhea: Disturbances in lipid digestion and / or absorption lead to increased lipid in the feces. Causes: ❑ Bile salts deficiency (due to obstruction of bile duct). ❑ Pancreatic lipase deficiency (due to atresia or obstruction of pancreatic duct e.g by tumors) ❑ Cystic fibrosis (poor digestion due to insufficient pancreatic secretion). ❑ Short bowel (decreased absorption).  FATTY ACID SYNTHESIS (LIPOGENESIS) The basic strategy of fatty acid synthesis includes the following: 1. Synthesis of Palmitate(16C) from Acetyl-CoA (2C) 2. Chain Elongation of Palmitate (long chain fatty acids) 3. Fatty Acid Desaturation - Two systems are responsible for fatty acid synthesis: 1. Cytoplasmic (Extramitochondrial system) 2. Microsomal system (chain elongation system) - Cytoplasmic (Extramitochondrial) System "De novo synthesis" 1. Intracellular site: cytoplasm 2. Tissues: lipogenesis is active mainly in liver, lactating mammary gland and adipose tissue and to lesser extent in kidney, brain and lung. 3. It converts acetyl CoA (the starting substrate) to palmitate (the end product). Requirements: ✓ Enzymes: acetyl-CoA carboxylase and fatty acid synthase complex. ✓ Coenzymes: NADPH, Mn2+, biotin and pantothenic acid ✓ CO2 : source of CO2is HCO3- ✓ ATP for energy A- Acetyl CoA carboxylase: It is a rate-limiting-enzyme. It catalyzes the carboxylation of acetyl-CoA to malonyl-CoA B -Fatty Acid Synthase Complex ❑ It is a multienzyme complex. This complex is a dimer of 2 identical polypeptide monomers: 1&2, each consisting of 7 enzyme activities and the acyl carrier protein (ACP). ❑  Steps of Fatty Acid Synthesis If propionyl CoA acts as primer instead of acetyl CoA, long chain fatty acids having an odd number of carbon atoms result. Steps of de novo synthesis of fatty acids  Sources of NADPH The pentose phosphate pathway is the main source. Both fatty acid synthesis and pentose phosphate pathway occur in the cytoplasm, with no membranes or permeability barriers against transfer of NADPH. Sources of acetyl CoA 1. Carbohydrate: Via the oxidative decarboxylation of pyruvate in the mitochondria. 2. Fat: Oxidation of fatty acids is the richest source of active acetate (acetyl CoA) e.g. palmitate gives 8 active acetate while glucose gives two. 3. Protein: Ketogenic amino acid is converted to active acetate or acetoacetate. Glucogenic amino acid can be converted to pyruvate, which gives active acetate.  Acetyl-CoA cannot diffuse into the cytosol to be used for lipogenesis. It is converted to citrate, which diffuses into the cytosol via tricarboxylate transporter, then by the action of ATP-citrate lyase, acetyl COA and oxaloacetate is produced. Fate of acetyl CoA 1. Oxidation in citric acid cycle 2. Lipogenesis 3. Ketogenesis 4. Steroids formation: cholesterol, steroid hormones and bile acids 5. Formation of acetyl choline.  Regulation of lipogenesis 1. Nutritional state (Availability of substrates) 2. Short term control by allosteric and covalent modification of enzymes 3. Long term control by changes in gene expression controlling rates of enzymes synthesis.  1- Nutritional state (Availability of substrates) 1. The nutritional state is the main factor regulating the rate of lipogenesis. Thus, the rate is high in the well-fed animal whose diet contains a high proportion of carbohydrate. 2. While it is depressed in restricted caloric intake, or a high fat diet, or diabetes mellitus  2- Short term control Acetyl-CoA carboxylase is the key enzyme and regulated by; 1-Allosteric regulation: citrate activates this enzyme. While long chain fatty acyl-CoA(the end product of the FA synthesis pathway) inactivates it (negative feedback inhibition). 2-Covalent modification: Phosphorylation inactivates acetyl-CoA carboxylase , while in the presence of insulin, acetyl-CoA carboxylase is dephosphorylated and activated.  3-Long term control The Fatty Acid Synthase Complex & Acetyl-CoA Carboxylase Are Adaptive Enzymes ❑ These enzymes adapt to the body’s physiologic needs by increasing in total amount in the fed state and by decreasing in starvation, feeding of fat, and in diabetes. ❑ Insulin is an important hormone causing gene expression and induction of enzyme biosynthesis, and glucagon antagonizes this effect. MICROSOMAL SYSTEM (chain Elongation system) 1. This pathway (the “microsomal system”) elongates saturated and unsaturated fatty acyl-CoAs (from C10 upward) 2. Importance: increases rapidly in brain during myelination Desaturation of fatty acid chain Mammals cannot synthesize double bonds in fatty acids beyond the ninth carbon, so linoleic acid (double bonds at carbons 9 and 12) and linolenic Acid (double bonds at carbons 9,12, and 15) must be provided in mammalian diets.  Fatty Acid oxidation Oxidation of fatty acids gives the maximum amount of energy among food stuffs (where 1 gm fat gives ≈ 9 Kcal). Sources of FA in body:- 1. Diet. 2. From adipose tissue by lipolysis. Methods by which fatty acids are oxidized are as follows: ❖ β-oxidation: the principal method of oxidation of FA ❖ α-oxidation ❖ ω-oxidation ❖ peroxisomal fatty acid oxidation β-oxidation of fatty acids 1. Site: in the mitochondria 2. Tissues: liver, kidney and heart. Brain can not use fatty acids as source of energy because they can not cross blood brain barrier and also there is deceased expression of beta oxidation enzymes in brain 3. Steps of β-oxidation:- ✓ It includes 3 steps 1-Activation of FA to form acyl CoA. Occurs in the cytoplasm and is catalyzed by acyl-CoA synthetase (also called thiokinase). Two high energy bonds are utilized 2-Transport of Acyl CoA by carnitine shuttle into the mitochondria. 3-Oxidation ( palmitate give 129 ATP) 3-Steps of β Oxidation  Regulation of fatty acid oxidation: ❖ The availability of free fatty acids (FFA) regulates the net utilization through beta-oxidation. The level of FFA, in turn, is controlled by glucagon: insulin ratio. Glucagon increases FFA level and insulin has the opposite effect. ❖ Carnitinepalmitoyl transferase-I (CPT-I) is the regulator of entry of fatty acids into mitochondria( gate). Malonyl CoA inhibits CPT-I activity. Thus, during de novo synthesis of fatty acid, the beta-oxidation is inhibited.  METABOLISM OF KETONE BODIES During high rates of fatty acid oxidation, primarily in the liver, large amounts of acetyl-CoA are generated. These exceed the capacity of the TCA cycle, and one result is the synthesis of ketone bodies (ketogenesis). 1. The ketone bodies include 3 substances: Acetoacetic acid, β- hydroxybutyric acid and acetone. 2. They are synthesized exclusively by the liver in the mitochondria. 3. Ketone bodies are important fuels in extrahepatic tissues. The liver can't utilize ketone bodies because it doesn't contain enzymes of ketone bodies oxidation( ketolysis)  Ketosis Ketosis is the presence of increased quantities of ketone bodies in the blood (ketonemia) and in urine (ketonuria). Causes of ketosis Uncontrolled Diabetes mellitus, starvation, high fat diet Effects of ketosis 1. Metabolic acidosis 2. Compensatory hyperventilation. 3. Smell of acetone in patient's breath. 4. Osmotic diuresis induced by glycosuria and ketonuria 5. Loss of cations: they are excreted in urine as their sodium or potassium salts. 6. Dehydration: the sodium loss further aggravates the dehydration. 7. Coma  Metabolism of adipose tissue Adipose tissue is the main store of triacylglycerol in the body. The triacylglycerol stored in adipose tissue are continually undergoing lipolysis ( hydrolysis) and reestrification. Triacylglycerol is synthesized from acyl CoA and glycerol 3- phosphate. The latter must be supplied by glucose via glycolysis because the enzyme, glycerol kinase is not expressed in adipose tissue. Triacylglycerol undergoes hydrolysis by a hormone-sensitive lipase forming free fatty acids and glycerol. This enzyme is inhibited by insulin and activated by glucagon ,epinephrine and norepinephrine  Cholesterol Metabolism Function: It enters in the structure of cell membrane, synthesis of bile acids and bile salts, steroid hormones and vitamin D3. Clinical Significance: The abnormal deposition of cholesterol and cholesterol- rich lipoproteins in the coronary arteries leads to atherosclerosis, which is the leading contributory factor in diseases of the coronary arteries. Structure: Synthesis of Cholesterol The major sites for synthesis are liver, adrenal cortex, testis, ovaries and intestine. All nucleated cells can synthesize cholesterol. Cholesterol synthesis occurs in the cytoplasm and endoplasmic reticulum from acetyl-CoA. The key regulatory enzyme is HMG- CoA reductase Regulation of cholesterol synthesis 1. Long-term regulation: when sufficient cholesterol is present in the cell, transcription of HMG-CoA reductase gene is suppressed. 2. Short-term regulation: (HMG-CoA reductase ) Covalent modification: Activated by dephosphorylation Feedback inhibition :by cholesterol. Hormonal control : insulin and thyroid hormone increases HMG-CoA activity, whereas glucagon and glucocorticoids decrease it. Drugs: statin drugs are competitive inhibitors of HMG-CoA reductase. Cholesterol transport ❑Cholesterol is transported in the plasma predominantly as cholesterol esters associated with lipoproteins. Degradation of Cholesterol ❑Cholesterol is eliminated from the body by conversion to bile acids and bile salts which are excreted in the feces.About 1 gm of cholesterol is eliminated from the body per day  LIPID TRANSPORT Plasma lipids Since lipids are insoluble in water, they need carriers in plasma. Therefore, they are complexed with proteins to form lipoproteins. Free fatty acids also known as nonesterified fatty acid (NEFA) are complexed with albumin in plasma Classification of lipoproteins Depending on the density (by ultracentrifugation) lipoproteins are classified into: 1. Chylomicrons 2. Very low density lipoproteins(VLDL) 3. Intermediate-density lipoproteins (IDL) 4. Low-density lipoproteins (LDL 5. High-density lipoproteins (HDL) 6. Lipoprotein (a) (Lpa)  ❑ Structure of a typical lipoprotein: 1. A typical lipoprotein consists of a lipid core of mainly non-polar triacylglycerol and cholesteryl ester surrounded by a single surface layer of phospholipid and cholesterol. The protein part of a lipoprotein constitutes 1% (in chylomicron) to 60% (HDL). Some of which are integral and cannot be removed, whereas others are free to transfer to other lipoproteins. Lipoprotein structure Functions of apolipoproteins The protein part of lipoprotein is called apolipoprotein or apoprotein. 1. Promote the solubility and stability of lipids in plasma 2. Act as enzyme activator e.g. C-II for lipoprotein lipase 3. Act as enzyme inhibitors eg. ApoC-III for lipoprotein lipase. 4. Ligands to specific lipoprotein receptors in tissues to regulate tissue uptake e.g. apo B100 and apoE for the LDL receptor.  Metabolism of plasma lipoproteins A-Chylomicrons Site of synthesis: the intestinal mucosal cells Function: transport dietary lipids from intestine to peripheral tissues and liver Structure: lipids: mainly triacylglycerols Protein part: apo B-48, apo C-II and apo E. Metabolism: Loading of apoB-48 with lipid in intestinal cells. The particles are released to the blood as nascent chylomicron Nascent chylomicron receives apo E and C-II from HDL forming mature chylomicron. Apo C-ll activates Lipoprotein lipase ( LpL).The LpL hydrolyses triglycerides (TG) present in chylomicrons into fatty acids and glycerol. As the TG content is progressively decreased, the chylomicrons decreases in size and apo C-II is returned to HDL (apo E is retained). The remaining particles are called chylomicron remnants, bind through apo E to specific receptors on the liver where they are endocytosed.  B-Very Low Density Lipoprotein (VLDL) Site of Synthesis: liver Function: transport of endogenously synthesized lipids (mainly TG) from the liver to peripheral tissues. Structure: Lipids: mainly triacylglycerols Protein part: apo B-100, apo C-II and apo E. Metabolism: ▪ Liver secretes nascent, endogenously synthesized TG-rich VLDL particles that contain apo B-100. In the circulation, apo C-II and apo- E are transferred from HDL to nascent VLDL. ▪ apo C-ll activates LpL which hydrolyses TG liberating fatty acids that are taken up by adipose tissue and muscle. As the TG content is progressively decreased, the VLDL decreases in size and apo C-II is returned to HDL (apo E is retained). The remaining particles are called VLDL remnants (or IDL), bind through apo E to specific receptors on the liver where they are endocytosed. ▪ The major fraction of IDL further loses triglycerides and apo E so as to be converted to LDL (low density lipoprotein)  C-low Density Lipoprotein (LDL) Site of synthesis: in circulation from VLDL Function: LDL transports cholesterol from liver to peripheral tissues. LDL concentration in blood has positive correlation with incidence of cardiovascular diseases. Structure: Lipids: cholesterol, cholesterol ester and phospholipids. Protein part: apo B-100. Metabolism: LDL receptors are present on all cells but most abundant in hepatic cells (70%). LDL are taken up via LDL receptors by endocytosis (the apo B-100 receptor).When the apo B-100 binds to the apo B-100 receptor, the receptor-LDL receptor complex is internalised by endocytosis. LDL and Clinical Applications: A fraction of cholesterol infiltrates through arterial walls, and is taken up by macrophages. This is the starting event of atherosclerosis leading to myocardial infarction. When these cells become engorged with cholesterol, foam cells are formed, that get deposited in the subendothelial space triggering formation of atheromatous plaque,resulting in increased chances of thrombosis and coronary artery disease. So LDL is called "bad cholesterol“ Familial hypercholesteromeia: There is defective LDL receptors or mutation in ligand region of apo B- 100. This leads to elevated LDL levels and hypercholesterolemia, resulting in atherosclerosis and coronary disease in young age.  D-Lipoprotein (a) Lp(a) It is synthesized in the liver and has the same lipid composition as LDL. It contains an additional apolipoprotein molecule, apo Lp(a). Lp(a) is associated with an increased risk of coronary heart disease when present in large quantities in plasma.  E-HIGH DENSITY LIPOPROTEIN (HDL): Site of synthesis:-liver and intestine Function: 1. Act as a reservoir of apo CII which is transferred to chylomicrons and VLDL needed for their metabolism and to activate the lipoprotein lipase. 2. Removes free cholesterol from peripheral tissue to liver (reverse cholesterol metabolism) (good cholesterol) Structure :- Lipids:- Contains mainly phospholipids together with free or esterified cholesterol. Proteins:- apo A, apoC and apoE. Metabolism:- Nascent HDL (newly synthesized HDL) is discoid in shape and consists of phospholipid bilayers containing apo A and free cholesterol. The transport of cholesterol from tissues to the liver is known as reverse cholesterol transport and is mediated by HDL cycle as follow; Nascent HDL accepts cholesterol from the tissues and form The smaller HDL3 HDL3 is converted to HDL2 (spherical shape) by accepting more free cholesterol and esterifying it by LCAT. The cycle is completed either by:- 1. Selective delivery of cholesterol ester by HDL receptor in the liver and in steroidogenic tissues where HDL binds the receptor via apo A-1 and cholesterol ester is selectively delivered to the cells, but the particle itself is not taken up(reformation of HDL3 ) 2. Or by hydrolysis of HDL2 by the liver Clinical Significance of HDL The level of HDL in serum is inversely related to the incidence of myocardial infarction. As it is "anti-atherogenic" or "protective" in nature, HDL is known as "good cholesterol" in common language. It is convenient to remember that"H" in HDL stands for "Healthy". HDL level below 35 mg/dl increases the risk, while level above 60 mg/dl protects the person from coronary artery diseases. Thank you