Lipid Metabolism PDF

Ema Qurnianingsih 1 Lipids Definition The lipids are a large and heterogeneous group of substances of biological origin that are easily dissolved in organic solvents such as methanol, acetone, chloroform, and benzene. By contrast, they are either insoluble or only p...

Ema Qurnianingsih 1 Lipids Definition The lipids are a large and heterogeneous group of substances of biological origin that are easily dissolved in organic solvents such as methanol, acetone, chloroform, and benzene. By contrast, they are either insoluble or only poorly soluble in water. Derivatives of fatty acids or potentially capable of binding fatty acids. 2  Many kinds of lipids exist in nature, the biologically important ones are : – Fatty acids – Triacylglycerol – Phospholipids – Plasmalogen – Sphingolipids – Cholesterol 3 Lipids Classification Simple lipids, are esters of fatty acids with various alcohols). Include fats & waxes. Complex lipids, esters of fatty acids containing groups in addition to an alcohol and one or more fatty acids. Include phospholipids, glycolipids, lipoprotein. Derived lipids, are formed from hydrolysis of both simple and complex lipids. Include fatty acids, glycerol, steroids, ketone bodies, lipid-soluble vitamins, hormones, etc. 4 Lipids Classification 5  FATTY ACIDS (FA) = Acyl A FA is a carboxylic acid with a long aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain and an even number of carbon atoms, from 4 to 28 6 Fatty acids occur in the body mainly as esters in natural fats and oils, but are found in the unesterified form as free fatty acids, a transport form in plasma (in combination with albumin). O Basic chemical formula of Fatty acid : R – C – OH R is alkyl group O R – C – is acyl group CH3 – (CH2)n – CH2 – CH2 – COOH    1 7 8  Saturated fatty acids – Fatty acids with no double bond in their acyl groups – CnH2n+1COOH – Example : Butyric acid (C3H7COOH) Lauric acid (C11H23COOH) Palmitic acid (C15H31COOH) 9  Unsaturated fatty acids – Fatty acids containing one double bond are called monounsaturated fatty acid (MUFA) – Fatty acids containing two or more double bonds are called polyunsaturated fatty acids (PUFA) – Carbon atoms are numbered from carboxyl carbon (carbon no.1) – The carbon atom adjacent to carboxyl carbon (no.2,3,4) are also known as , , γ – Terminal methyl carbon is known as C  CH3 – (CH2)n – CH2 – CH2 – COOH    1 10  9 fatty acid indicates a double bond on the ninth carbon counting from methyl group or  carbon 9 indicates a double bond between carbon no. 9 and 10 of fatty acid Example : Palmitoleic acid (C16:1;9) or 9 → MUFA Linoleic acid (18:2 6) or (C18:2;9,12) → PUFA 11  Unsaturated fatty acid, may be further divided into : Monounsaturated fatty acid (MUFA) Polyunsaturated fatty acid (PUFA) Eicosanoids These compounds derived from eicosa (20-carbon) polyenoic fatty acid Comprise the prostanoids ( include prostaglandins, prostacyclins, thromboxanes), leukotrienes (LTs), lipoxins (LXs) 12 Unsaturated fatty acids have lower melting points than the saturated fatty acids Fatty Acids (elmhurst.edu). The reason is molecular geometries : – The tetrahedral bond angles on carbon in saturated fatty → relatively linear although with zigzags → allows many fatty acid molecules to be rather closely "stacked" together → high melting points. – Introduction of one or more double bonds in the hydrocarbon chain in unsaturated fatty acids → one or more "bends" in the molecule → do not "stack" very well → intermolecular interactions are much weaker → melting points are much lower. In human/animal body : TG (with 3 saturated fatty acids) → solid at body temperature. Membrane lipids (more unsaturated than storage lipids) → liquid at all enviromental temperature. Fig. Melting Point of Fatty Acids 13  Depend on the orientation of group or atoms around the axes of double bonds, the geometric isomerism of unsaturated fatty acids is divided into “cis” and “trans”. Double bonds in natural unsaturated fatty acids are “cis”→ molecules are bent 120˚ at double bond (V or U shape) → significant in molecular packing in cell membrane. “trans” fatty acids : – are present in certain foods, arising as by-product of fatty acids saturation during hydrogenation or “hardening” of natural oil in manufacture of margarine → associated with increased risk of diseases (CVD, DM). – Arising from alteration of dietary unsaturated fatty acids in the rumen by rumen bacteria (via the bio-hydrogenation pathway) → small number. (Cis & Trans Fatty Acids | Advice | Megalac) 14 Biological function of fatty acids 1. Source of biological energy – Some tissues, such as skeletal muscle and cardiac muscle, prefer to make ATP from fatty acids rather than from glucose. – FA are not a universal source of energy for some tissues, such as brain and Nervous tissue. 15  Why can’t the brain use fatty acids for fuel? Long standing beliefs: there is no efficient way of getting them there. Fatty acids are bound to albumin and albumin cannot cross blood brain barrier on its own or use transporter proteins to cross it → slow passage 16  Why can’t the brain use fatty acids for fuel? Corrected by later experiments : (1) ATP generation linked to β-oxidation of fatty acids demands more oxygen than glucose, thereby enhancing the risk for neurons to become hypoxic; (2) β-oxidation of fatty acids generates superoxide, which, taken together with the poor anti-oxidative defense in neurons, causes severe oxidative stress; (3) The rate of ATP generation based on adipose tissue- derived fatty acids is slower than that using blood glucose as fuel. → in periods of extended continuous and rapid neuronal firing, fatty acid oxidation cannot guarantee rapid ATP generation in neurons. Schonfeld and Reiser, 2013. Why does brain metabolism not favor burning of fatty acids to provide energy? - Reflections on disadvantages of the use of free fatty acids as fuel for brain. J Cereb Blood Flow Metab. 33(10): 1493–1499. 17  Biological function of fatty acids…. 2. Precursor of other lipids – Triacylglycerol, phospholipids, plasmalogens, sphyngolipids 3. Precursor of speciallized product – Eicosanoids (prostaglandins, leucotrienes, thromboxanes) Figure of Eicosanoids (source : Wikipedia) 18  fatty acids..... Level of free fatty acids (FFAs) are low in fully fed state and rise to 0,7 – 0,8 mEq/mL in starved state and to as much as 2 mEq/mL in uncontrolled DM. FFAs are removed from blood rapidly by tissues’s uptake and oxidation (to fulfill 25- 50% energy requirement during starvation) or esterified to form triacyl gliserol (TG). 19 Triacylglycerol (TG) Most abundant lipid in nature Commonly known as fat In human & animal → the most important depository energy source stored mainly in adipose tissue High energy content, low molecular weight Prolonged starvation → broken down into fatty acid (and glycerol) → release → fulfill energy requirement of most body tissues 20 Structure of a triglyceride. Triglycerides are composite molecules, made up of three fatty acid molecules bonded to a single glycerol molecule. This bonding takes place by dehydration synthesis. http://nutrition.jbpub.com 21 Biological Function of Triacylglycerol (TG) Depository energy source Thermal insulator – Preventing excessive heat loss to cold surroundings Mechanical dumper – TG in subcapsular adipose tissue that envelopes important organs provides padding that absorbs impact from potentially injurous mechanical force 22 Phospholipids A family of phosphate-containing, glycerol- backboned lipids Biological function : – Structural component of biological (cellular) membranes – Structural component of lippoproteins – Lung (alveolar) surfactant 23 Phospholipids as Lung (alveolar) surfactant 24  Phospholipids... Amphipathic lipids : – The phosphate-alcohol part of phospholipid molecule (head) is polar in nature, so it is hydrophilic. – The rest of the molecule (tail) is hydrophobic – In solution → lipid bilayer 25 26  Other amphipathic lipids Plasmalogen Sphingolipid Structurally, it The molecular backbone resembles of this lipid is not phosphatidylethanola glycerol but long chain mine, but posses ether alcohol link on the sn-1 carbon instead of ester link found in acylglycerol 27  Other amphipathic lipids... Cholesterol Is a specific lipid of cyclopentano perhydrophenantrene family that exists exclusively in human and animal The hydroxyl group is hydrophilic, while the four-ring cyclopentanoperhydrophenantrene is hydrophobic → amphipathic → structural component of cell membrane When hydroxyl group in position 3 is esterified → cholesteryl ester → not amphipathic 28 29  Biological function of cholesterol Structural component of (human and animal) cell membrane Precursor of steroid hormones, bile acids and vitamin D Very vital to human and animal but, prolonged elevation of its concentration is correlated with pathological conditions, e.g. atherosclerosis 30  31 Harper ilustrated biochemistry, 30th ed, 2015  Lipid Absorbtion in GI Tract TG is most abundant lipid in diet Digestion and absorbtion of lipid → require bile acid/salt → micelle. – Bile salts act as detergents, binding to globules of dietary fats (as they are broken up by intestinal peristaltic movement) → emulsified → has an increased surface area → attacked by digestive enzymes of pancreas. Mark’s basic medical biochemistry, A clinical approach, 5th ed, 2018 32  Lipid Absorbtion in GI Tract... The contraction of gallbladder and secretion of pancreatic enzymes are stimulated by cholecystokinin (CCK). Secretin is also released by small intestine (in response to acidic materials, from stomach that enters duodenum) → signals liver, pancreas and certain intestinal cells → secrete bicarbonate → raise the pH of intestine → optimum for Mark’s basic medical biochemistry, A clinical approach, 5th ed, 2018 action of intestinal enzymes. 33  Lipid Absorbtion in GI Tract... Pancreatic lipase (along with colipase) → TG is hydrolized & absorbed into fatty acids and these followings : – 2-monoacylglycerol : ± 72 % – 1-monoacylglycerol : ± 6 % – Glycerol : ± 22 % Fatty acids & monoacyglycerols (other dietary lipids as well) → packaged into micelles → travel to microvili → dietary lipids are absorbed, bile salts remain in gut lumen. 34 Harper ilustrated biochemistry, 30th ed, 35 2015 Figure of Resynthesis of triacylglycerols in intestinal epithelial cells. Fatty acids (FA) produced by digestion are activated in intestinal epithelial cells and then esterified to the 2-monoacylglycerol produced by digestion. The triacylglycerols are packaged in chylomicrons and secreted into the lymph→ enter the blood via thoracic duct. Mark’s basic medical biochemistry, A clinical approach, 5th ed, 2018 36  Transport of Lipid Basically insoluble in water In body, lipid must be transported interorgan → through circulatory system, which contains hydrophilic aqueous plasma → lipid must be packed & transported as lipoproteins Five major plasma lipoproteins : – Chylomicron – VLDL – IDL – LDL – HDL 37 Transport of Lipid... Integral apolipoproteins cannot be removed, whereas peripheral apolipoprotein are free to transfer to other lipoprotein, eg. apo C and E. Harper ilustrated biochemistry, 30th ed, 38 2015 Harper ilustrated biochemistry, 30th ed, 39 2015  Roles of apolipoprotein : – They form part of lipoprotein structure (ex. Apo B). – They serve as enzyme cofactors (apo CII for lipooprotein lipase, A1 for LCAT) or inhibitors (apo AII or CIII for lipoprotein lipase). – They act as ligands for interaction with lipoprotein receptors in tissues (ex. Apo B100 and apo E for LDL receptor) Harper ilustrated biochemistry, 30th ed, 40 2015  Transport of Lipid... Chylomicron – Is formed in the intestine to transport dietary lipids from intestine – The transported TG is delivered to the peripheral tissues after being hydrolized to fatty acid & glycerol (by capillary lipoprotein lipase) → chylomicron remnant → taken wholly by liver 41 TGs of chylomicron are digested by endothelial LPL which is produced by adipose cells, muscle cells, (cardiac) muscle cells, lactating mammary gland. → fatty acids (mostly) are taken up by cells → oxidated (in muscle or other tissues) or for storage in adipose tissues (major fate). → Glycerols are used for TG synthesis in liver (fed state) 42 Harper ilustrated biochemistry, 30th ed, 2015  Transport of Lipid... VLDL – Is lipoprotein that transports endogenous lipids made by liver and some of lipids brought into liver by chylomicron remnant, from the liver to various peripheral tissues – Transport route is resembles that of chylomicron, with involvement of lipoprotein lipase IDL – Is “VLDL remnant” analogoue of chylomicron remnant – It is formed during lipoprotein lipase lipolysis of VLDL TG – Short half life → directly converted into LDL 43  Transport of Lipid... LDL – Is lipoprotein from which the various tissues get most of its cholesterol – Cholesterol is major constitute of LDL HDL – Is smallest lipoprotein in plasma – It has important function totransport excess cholesterol in tissues back to liver → excreted via billiary system 44  Harper ilustrated biochemistry, 30th ed, 2015 The route of VLDL is mostly resemble of that in chylomicron. In humans, a large proportion of IDL forms LDL. 45 Harper ilustrated biochemistry, 30th ed, 2015 46  Harper ilustrated biochemistry, 30th ed, 2015 HDl is synthesized and secreted from liver and intestine (which the later lack of Apo E & C and gain them from liver HDL in plasma). LCAT (& Apo A-1) bind to discoid HDL → PL and chol are converted to chol Esthers and lysolecithin → chol Esthers move to interior, lysolecithin is transfered to albumin → forming sperical HDL → remove the excess of cholesterol from lipoproteins and tissues. 47  Harper ilustrated biochemistry, 30th ed, 2015 Reverse Cholesterol Transport (RCT)....also includes HDL cycle (which is explained in next slide) SR-B1 has dual role in HDL metabolism. In liver and steroidogenic tissues → binds HDL via Apo A-1 and cholesterol is delivered into cells. In other tissues → it mediates acceptance of chol efflux from tissues to HDL → transport to liver → excreted via bile. ABCA1 and ABCG1 ABCG1 mediates transport of chol from cells to HDL ABCA1 mediates efflux of chol to prebeta-HDL or Apo A-1 → discoidal HDL → 48 HDL3  Harper ilustrated biochemistry, 30th ed, 2015 HDL Cycle Discoidal HDL accepts chol from tissues → HDL3 → chol is esterified by LCAT → HDL2 → reformed : Selective delivery of HDL2 chol esthers to liver via SR-B1 Hydrolysis of HDL2 PL and TG by hepatic lipase and endhotelial lipase Free Apo-A1 is released by this process associated with minimum amount of PL and chol → forms prebeta – HDL Surplus of Apo-A1 is destroyed in kidney. 49 ATHEROSCLEROSIS The initial step : formation of fatty streak (accumulation of lipid-laden macrophages or foam cells in the subintimal space). Fatty streak is initiated when risk factors for atherosclerosis reach critical threshold → injure vascular endothelial cells. 50 ATHEROSCLEROSIS Risk factors : – Arterial hypertension – High levels of LDL, chylomicron remnants, and VLDL remnants; – low levels of circulating HDL, – cigarette smoking, – chronic elevations in blood glucose levels, etc 51 Hypothesis of Atherosclerosis pathophysiology : 52 53  Βeta (β) Oxidation fatty acid catabolism pathway → acetyl CoA, with the generation of ATP Aerobic process, takes place in mitochondria It occurs in three stages : – Activation of free fatty acid, takes place outside mitochondrial matrix – Translocation of acyl-CoA into mitochondrial matrix – Oxidation reactions 54  β Oxidation...... fatty acids (FA) are the major fuel for human and fullfil energy requirement between meal or increased demand (excercise). Between meals or fasting → insulin , glucagon → lipolysis → FA is transported to tissues (in serum/plasma, it is bound in hydrophobic binding pocket of albumin) is from lipolysis of TG in adipose tissues. – During overnight fasting → FA is major fuel for cardiac muscle, skeletal muscle and liver. – Liver converts FA into ketone bodies → used as fuel for other tissues, such as gut and brain (during prolonged fasting). 55  Βeta Oxidation.. First stage : Require energy from ATP In the presenceof ATP & CoA, the enzyme (Acyl-CoA synthetase/thiokinase) catalyzes conversion of free fatty acid into acyl-CoA Acyl-CoA synthetases/thiokinases are found in ER, peroxisomes, inside and outer membrane of mitochondria Harper ilustrated biochemistry, 30th ed, 2015 56 Second stage : Βeta Oxidation.. Harper ilustrated biochemistry, 30th ed, 2015 57  Βeta Oxidation.. Two carbons at a time are cleaved from acyl-CoA molecules, starting at the carboxyl end. The chain is broken between the α (2) and β (3) carbon atoms → β oxidation Harper ilustrated biochemistry, 30th ed, 2015 58  Βeta Oxidation.. Steps of β oxidation : 1. Removal of 2 H from 2(α)- and 3(β)- carbon atoms, catalyzed by acyl-coA dehydrogenase, requiring FAD → 2-trans-enoyl-CoA dan FADH2 → reoxidation of FADH2 yield 1.5 ATP/molecule of FADH2 Harper ilustrated biochemistry, 30th ed, 59 2015  Βeta Oxidation.. Steps of β oxidation : 2. Water is added to saturate double bond, catalyzed by 2-enoyl-CoA hydratase → 3-hydroxy-acyl-CoA Harper ilustrated biochemistry, 30th ed, 60 2015  Βeta Oxidation.. Steps of β oxidation : 3. L(+)-3-hydroxy-acyl-CoA undergoes further dehydrogenation on the 3- carbon, catalyzed by L(+)-3-hydroxy- acyl-CoA dehydrogenase, requiring NAD+ → 3-ketoacyl-CoA → Reoxidation of NADH in respiratory chain yield 2.5 ATP/molecule of NADH Harper ilustrated biochemistry, 30th ed, 61 2015  Βeta Oxidation.. Steps of β oxidation : 4. 3-ketoacyl-CoA is split at 2,3-position by thiolase → acetyl-CoA and new acyl-CoA two carbon shorter than the original acyl-CoA molecule The shorter acyl-CoA molecule reenter oxidative pathway again Long chain fatty acid with even number of carbon may be degraded completely to acetyl-CoA Acetyl-CoA → TCA cycle → CO2, water and ATP Harper ilustrated biochemistry, 30th ed, 62 2015  Βeta Oxidation.. The quantity of ATP produced in β-oxidation depends on the chain lenght of fatty acid molecule The longer the fatty acid molecule → the higher number of oxidation cycle → more abundant acetyl-CoA → more ATP is generated Harper ilustrated biochemistry, 30th ed, 63 2015 64  Βeta Oxidation.. Harper ilustrated biochemistry, 30th ed, 65 2015 66  Βeta Oxidation.. Regulation of β-oxidation Regulatory enzyme : carnitine palmitoyl transferase I Insulin inhibits enzyme indirectly, by increasing malonyl-CoA (allosteric inhibitor of carnitine palmitoyl transferase I) In starvation Insulin are low → β-oxidation rate is increased (except in erythrocyte and brain) Fatty acid is used as main energy source in preference to glucose Harper ilustrated biochemistry, 30th ed, 67 2015  Ketogenesis Ketone bodies : acetoacetate, D(-)-3-hydroxybutyrate (β- hydroxybutirate), acetone Occurs in liver, under metabolic condition with high rate of fatty acid oxidation Some of fatty acids, mobilized from adipose tissue → other tissue and liver Activated into acyl-CoA TG synthesis β-oxidation TCA cycle Acetyl-CoA Ketone bodies 68  Ketogenesis.. The immediate precursor for ketogenesis is acetyl- CoA mobilized from adipose tissue 3 molecules of acetyl-CoA to make acetoacetate Harper ilustrated biochemistry, 30th ed, 69 2015  Ketogenesis.. Harper ilustrated biochemistry, 30th ed, 70 2015 71  Ketogenesis.. Harper ilustrated biochemistry, 30th ed, 72 2015  Ketogenesis.. Harper ilustrated biochemistry, 30th ed, 73 2015  Ketogenesis.. Ketogenesis is mainly regulated through the availability of fatty acid (as a precursor of acetyl-CoA) The more fatty acids enter the liver, the more ketone bodies are made Factors that boost lypolisis → increase fatty acid delivery to liver → increase ketogenesis e.g. Prolonged fasting and starvation, diabetes mellitus Harper ilustrated biochemistry, 30th ed, 2015 type I 74 75 Figure of level of ketone bodies in the blood at various times during fasting. Mark’s basic medical biochemistry, A clinical approach, 5th ed, 2018 76  Mark’s basic medical biochemistry, A clinical approach, 5th ed, 2018 Fatty acids (FA) are used as fuel whenever its level is elevated in blood : During fasting or starvation High-fat low-carbohydrate diet During long term low-mild intensity excercise Decrease in Insulin, increased level of glucagon/epinephrine/other hormones Adipose tissue lipolysis Fatty acids level in blood increases ( begin at 3-4 hr after meal, up to 2-3 days of fasting) Rate of ketone bodies in liver increases as supply of FA increases. After 2-3 days of starvation, ketone bodies in blood are uptaken and utilized by Brain, hence it spares skeletal muscel protein as a major source of gluconeogenesis in this state. 77 78  Lipid Biosynthesis The goal for lipid synthesis are for obtaining a variety of lipids in the body with their respective biological functions. The body can synthesize many types of lipids, except for essential fatty acids (Only two fatty acids are known to be essential for humans: alpha-linolenic acid (an omega-3 fatty acid) and linoleic acid (an omega-6 fatty acid). The mechanism for the absorption of lipids from the gastrointestinal tract → the mechanism for transporting lipids between tissues (see: transporting lipids between tissues). The biosynthetic process is also a tool to convert excess calorie sources into lipids for storage in adipose tissue (On a high carbohydrate diet → The excess glucose is synthesized into FA 79 80 Fatty Acid Biosynthesis Fatty acid are synthesized by an extramitochondrial system, which is responsible for the complete synthesis of palmitate from acetyl-CoA in cytosol It is mainly used to convert excess energy to fatty acid → subsequently attached to glycerol backbone → triacylglycerol → stored Fatty acid synthesis : – De Novo Synthesis of fatty acid – Fatty acid chain elongation – Desaturation of fatty acid 81 De Novo Synthesis Is present in many tissues, e.g. Liver, kidney, brain, lung, mammary gland, adipose tissue Cofactor requirements : NADPH, ATP, Mn2+, biotin, HCO3- Immediate substrate : Acetyl-CoA – Precursor of acetyl-CoA : glucose, amino acid Involves malonyl-CoA as the twoo-carbon unit agent to elongate the original acetyl-CoA and acyl-CoA – Production of malonyl-CoA is the initial & controlling step 82  De Novo Synthesis The de novo synthesis of fatty acids involves a set of enzymes that can be grouped into three groups : 1. Acetyl CoA carboxylase 2. The Fatty Acid Synthase Complex 3. Enzymes Involved in the Citrate- Pyruvate Cycle During the process : acetyl, malonyl, other intermediates are bound to fatty acid synthase enzyme complex End product : free palmitate 83 84 85 86 Harper ilustrated biochemistry, 30th ed, 2015 De Novo Synthesis... 87  Harper ilustrated biochemistry, 30th ed, 2015 De Novo Synthesis... Steps of de Novo Synthesis : – after malonyl-CoA is formed, fatty acid synthase enzyme complex forms fatty acid – The reaction sequence begins with elongation of enzyme boound acetyl group with two carbon unit from malonyl-CoA, followed by hydrogenation, dehydration and second hydrogenation 88  Harper ilustrated biochemistry, 30th ed, 2015 De Novo Synthesis... Steps of de Novo Synthesis (2): – The resulting four-carbon acyl group is still bound to the enzyme and the process repeats seven times → acyl group has grown in lenght to become a sixteen-carbon moiety → released from its attachment to enzyme → palmitic acid – De Novo Synthesis is closely related to HMP shunt, the primary origin of NADPH, as reducing equivalent donor 89  De Novo Synthesis... Note : – Acetyl-CoA is formed from glucose via oxidation of pyruvate in mitochondria matrix require special mechanism involving citrate Process : condensation of acetyl-CoA with oxaloacetate in TCA cycle → citrate →translocated into cytosol, via tricarboxylate transporter → cleavage to acetyl-CoA & oxaloacetate (by ATP-citrate lyase, requires ATP & CoA) Oxaloacetate → malate → pyruvate + NADPH + H+ (by malic enzyme, requires NADP+) – NADPH, generated from : HMP shunt Malic enzyme Isocitrate dehydrogenase (cytosol) 90 Harper ilustrated biochemistry, 30th ed, 2015 De Novo Synthesis... 91 De Novo Synthesis... 92 Harper ilustrated biochemistry, 30th ed, 2015 93  Fatty Acid Desaturation Synthesis of monounsaturated fatty acid – Occurs in several tissues including the liver – Generate monoyunsaturated fatty acid from saturated fatty acid – The first double bond introduced is nearly always in 9 position – An enzyme system : 9 desaturase, in endoplasmic reticulum → converts palmitoyl-CoA to palmitoleoyl-CoA, stearoyl-CoA to oleoyl-CoA Synthesis of polyunsaturated fatty acid – Additional double bonds introduced into existing MUFA are always separated from each other by methylene group – Involves desaturase and elongase enzyme system – The desaturation and elongation system is greatly diminished in starving state, in response to glucagon & epinephrine, and in DM type I 94 Triacylglycerol (TG) Biosynthesis TG are synthesized by the body for storage, especially in adipose tissue and smaller amounts of TG are also stored in the muscles. The stored triacylglycerol must be synthesized by the tissues itself → because TG cannot penetrate cell membranes (including adipose tissue cells, intestinal mucosal cells and vascular endothelium). TG are also synthesized in the liver → transported by lipoproteins to body tissues. The mammary glands also synthesize TG → excreted with milk for infant nutrition. 95 96 97 FIGURE of Biosynthesis of triacylglycerol and phospholipids. (1) monoacylglycerol pathway; (2) glycerol phosphate pathway. Harper ilustrated biochemistry, 30th ed, 2015 98  Glycerol phosphate pathway (1) Two molecules of acyl- CoA (formed by the activation of fatty acids by acyl-CoA synthetase), combine with glycerol-3- phosphate to form phosphatidate (1,2- diacylglycerol phosphate). This takes place in two stages, catalyzed by glycerol-3-phosphate acyltransferase and 1- acylglycerol-3- phosphate acyltransferase. Harper ilustrated biochemistry, 30th ed, 2015 99  Glycerol phosphate pathway (1) Two molecules of acyl- CoA (formed by the activation of fatty acids by acyl-CoA synthetase), combine with glycerol-3- phosphate to form phosphatidate (1,2- diacylglycerol phosphate). This takes place in two stages, catalyzed by glycerol-3-phosphate acyltransferase and 1- acylglycerol-3- phosphate acyltransferase. Harper ilustrated biochemistry, 30th ed, 2015 100  Glycerol phosphate pathway 2. Phosphatidate is converted by phosphatidate phosphohydrolase and diacylglycerol acyltransferase (DGAT) to 1,2-diacylglycerol and then triacylglycerol.. Harper ilustrated biochemistry, 30th ed, 2015 101  Glycerol phosphate pathway 2. Phosphatidate is converted by phosphatidate phosphohydrolase and diacylglycerol acyltransferase (DGAT) to 1,2-diacylglycerol and then triacylglycerol.. Harper ilustrated biochemistry, 30th ed, 2015 102  Monoacylglycerol pathway In intestinal mucosa, monoacylglycerol acyltransferase converts monoacylglycerol to 1,2- diacylglycerol in the monoacylglycerol pathway Harper ilustrated biochemistry, 30th ed, 2015 103 Diacylglycerol acyltransferase (DGAT) catalyzes the only step specific for triacylglycerol synthesis and is thought to be rate limiting in most circumstances Harper ilustrated biochemistry, 30th ed, 2015 104 ADIPOSE TISSUES 105 Figure of lipolysis and esterification in adipose tissues In new state of eating : High blood glucose levels → high insulin high glucose levels in adipose tissue → glycerol 3-phosphate levels is increased AMP ↓ → hormone- sensitive lipase non active Esterification proceeds rapidly 106 Figure of lipolysis and esterification in adipose tissues Fasting / starvation : Blood glucose levels decrease → insulin secretion decreases, glucagon secretion increases Glucose entering adipocytes decreases Decreased synthesis/formation of glycerol-3P Esterification decreases, lipolysis increases Free FA from adipocytes increases → Mobilization to blood circulation increases. 107  Cholesterol Metabolism Cholesterol is biosynhesized from acetyl-coA – A little more than half of cholesterol in human body is biosynthesized (in all nucleated cells, liver & intestine account 10 - 15%), remainder from diet. – The biosynthesis occurs in endoplasmic reticulum and cytosol. Excretion of cholesterol: – Cholesterol is either excreted unchanged in bile or converted to bile acids and then excreted in intestine → mostly, undergo enterohepatic circulation. 108 Step 1. Biosynthesis of mevalonate from acetyl-coA, enzymes responsible are thiolase, HMG-coA synthase, HMG-CoA reductase (the later is regulatory enzyme) Step 2. Formation of isoprenoid units Step 3. Six isoprenoid units form squalene , by squalane synthetase & needs NADPH Step 4. Formation of lanosterol Step 5. Formation of cholesterol, takes place in endoplasmic reticulum membrane. 109 Harper ilustrated biochemistry, 30th ed, 2015  Regulation of Cholesterol Biosynthesis The cellular supply of cholesterol is maintained at steady level, by : Regulation of HMGR activity. – Feedback inhibition, Control of gene expression, rate of enzyme degradation, phosphorylation- dephosphorylation. Regulation of excess intracellular free cholesterol through activity of ratio acyl-CoA to cholesterol acyltransferase. Regulation of plasma cholesterol levels via LDL- receptor mediated uptake and HDL-mediated RCT. 110  Figure of HMG CoA reductase regulation. HMG CoA reductase is rate-limiting enzyme and subject to different kinds of metabolic control. (1) Sterol-dependent regulation of gene expression; (2) Sterol-accelerated enzyme degradation; (3) Sterol- independent phosphorylation/dephosphorylation; (4) Hormonal regulation, which Glucagon and the glucocorticoids have the opposite effect. 111 th Lippincot’s illustrated review : Biochemistry, 5 ed, 2011 112 113 114 115