Chapter 5: Lipids
Chapter 5: Lipids Nutrition 344
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Date Created: 10/25/15
Chapter 5 Lipids Introduction to Lipids 0 Not soluble in water 0 Also part of membranes fat soluble vits hormones mediators 0 Simple lipids only contain carbon hydrogen and oxygen 0 Fatty acids 0 Triacylglycerols most important diacylglycerols and monoacylglycerols o Waxes esters of fas whigher EtOHs I Sterol esters cholesterol fa esters I Nonsterol esters vit A esters and so on 0 Compound lipids contain another element 0 Phospholipids I Phosphatidic acids I Plasmalogens I Sphingomyelins o Glycolipids carbohydratecontaining o Lipoproteins lipids in association wproteins o Derived Lipids o Derivatives such as sterols and straight chain EtOHs obtained by hydrolysis of those lipids in grps one and two that still possess general properties of lipids o Ethyl Alcohol 0 Though it is not a lipid per se it does supply dietary E and its metabolism resembles lipid metabolism Structure and Biological Importance o Lipids important in human nutrition 0 Fatty acids Triacylglycerols Sterols and steroids Phospholipids Glycolipids 0000 Fatty Acids 0 Straight H chain terminating wa carboxylic acid grp COOH 0 Fatty acid nomenclature 0 Two systems of notation have been developed to provide a shorthand way to indicate the chem structure of a fa Both systems are used regularly Delta A system length numberposition of double bonds Double bonds counted from omega methyl end Carboxylic end is hydrophobic methyl end is hydrophilic 0000 Two configurations cis and trans o Cis are most naturally occurring unsaturated fa I The cis isomerism form results in the folding back and kinking of the molecule into a Ulike orientation 0 Although the trans form does appear in some natural fats and oils and in dairy products most trans fatty acids are derived from partially hydrogenated fats and oils I The trans form has the effect of extending the molecule into a linear shape similar to that of saturated fas 0 Saturated Fatty Acids all bonds are saturated wH or C o Unsaturated Fatty Acids I Monounsaturated oleic acid I Polyunsaturated alpha linolenic 0 Essential fatty acids I Must consume in diet from plant foods I Linoleic acid omega 6 and alpha linolenic acid omega 3 I Flax seeds chia seeds walnuts o n3 Fatty Acids I Hypolipidemic and antithrombotic effects lowers blood lipid levels and makes blood thinner o Lowers the risk of heart attack and stroke I Fish oils are rich sources 0 Eicosapentaneoic acid important for cardiovascular health DHA important for mental health Triacylglycerols Triglycerides 0 Most stored body fat is in the form of triacylglycerols TAG which represent a highly concentrated form of E 0 Account for nearly 95 of dietary fat 0 Trihydroxy alcohol glycerol to which 3 fas are attached by ester bonds 0 Nomenclature stereospecific numbering sn 0 Exist as fats or oils depending on nature of fa components 0 Can exist as solids or liquids depending on room temp Sterols and Steroids 0 Sterols o Monohydroxy EtOHs w4ring core structure called cyclopentanoperhydrophenanthrene steroid nucleus 0 Cholesterol animal sterol I Component of cell membranes and stabilizes the cell membranes I Precursor for steroids bile acids sex hormones adrenocortical hormones vit D I Obtained from animal sources Phospholipids 0 Glycerophosphatides o Phosphatidic acid building block 0 SFA in position 1 and USFA in position 2 0 Common phospholipid is lecithin made out of soy Helps us to mix fat soluble and water soluble substances 0 Biological roles of phospholipids o Hydrophilic 0 Cell membranes source of cmpnds cell functions such as anchoring proteins 0 Sphingolipids o 18C amino EtOH sphingosine forms backbone sphingolipids o 3 subclasses Glycolipids Sphingomyelins sphingophosphatides o Myelin conduction of nerves Cerebrosides glycolipids Gangliosides glycolipids All deal with nerve tissues and CNS 0 Have CHO component in structure 0 Occur in medullary sheaths of nerves and in brain tissue 0 Cerebrosides o Ceramide linked to a monosaccharide unit 0 Gangliosides o Ceramide linked to an oligosaccharide 0 Provide CHO determinants of blood types 0 A B 0 AB Digestion of Lipids o Triacylglycerol Digestion o Lingual and gastric lipases Lingual lipase released by the serous gland which lies beneath the tongue Gastric lipase produced by the chief cells Account for much of the limited digestion about 1030 of TAG that occurs in the stomach Lipase activity is made possible by the enzymes particularly high stability at the low pH of the gastric juices Gastric lipase importance in penetrating fat globules in milk Wo substrate stabilization by bile salts This is important for fat digestion in suckling infants Both act preferentially on triacylglycerols containing medium and short chain fa 0 Added to formulas for infants or sports drinks Good source of E o Emulsification in the stomach 0 Presence of lipids in stomach delays gastric emptying Wfats therefore providing high satiety value I Caused by hormone secretin o Emulsification in sm intestine duodenum bile salts that have hydrophilic and hydrophobic ends called amphipathic properties 0 The role of colipase I Pancreatic lipase activation I 3 things need to be present for activation 0 The participation of the protein colipase o Formed by the hydrolytic activation by trypsin of procolipase 0 Contains approx 100 aa residues and possesses distinctly hydrophobic regions that are believed to act as lipid binding sites 0 Calcium ions 0 Bile salts o If any of the three are missing pancreatic lipase is inactive 0 Cholesterol and Phospholipid Digestion O O Esterified cholesterol undergoes hydrolysis to free cholesterol and fa are catalyzed by the enzyme cholesterol esterase C2 fa of lecithin hydrolytically removed to produce lysolecithin and a free fa Therapeutic Inhibition of Fat Absorption Orlistat o Orlistat o Interferes W digestion and absorption of dietary fat 200 kcal deficit o Semisynthetic derivative of lipstatin o Inhibits pancreatic lipase I Means fat cannot be absorbed ends up in lrg intestine o OTC form is Alli Rx form is Xenical Absorption of Lipids o Micelles interact at brush border and lipid contents diffuse out into enterocytes 0 FA gt 1012 C long reesterified 0 Will be used for lipoproteins 0 Short chain FA exit into portal blood 0 O 0 810 C Used as a source of E Present in breast milk added into sports drinks 0 Cholesterol 0 0 About half in intestines is excreted other half absorbed Cholesterol from diet must be hydrolyzed to free cholesterol to be absorbed I This is an independent process and is facilitated by specific transporter proteins Cholesterol is found in the intestinal micelle which must pass through the unstirred water layer which acts as a diffusion barrier at the lumen enterocyte membrane interphase 0 Chylomicron Formation O Lipids that are resynthesized in the ER of the enterocytes along Wfat soluble vits leave the enterocyte by exocytosis as chylomicrons or very low density lipoproteins 0 Large TAGrich spherical particles containing TAG cholesteryl esters phospholipids PL and vits A and E in the core and a monolayer of PL which contains free cholesterol and protein on the surface which is hydrophobic apo I The protein added to the particle surface is a large hydrophobic apolipoprotein B 48 apoB48 which is an abridged version of the apolipoprotein apoBlOO that is produced by the liver I The apoB48 protein is considered to be a constitution protein meaning it is synthesized at a constant rate 0 Another protein that is required for the formation of the chylomicron is a fatty acid building protein FABP which is involved in the synthesis of TAG from MAG and DAG Lipoproteins o Apolipoproteins 0 Protein components 0 Hydrophobic o Stabilizes the lipoprotein complexes 0 Makes it possible for lipids to be moved through the blood 0 Chylomicrons 0 Transport exogenous dietary lipids what you eat comes to the sm intestine than to the enterocyte and is finally transformed into chylomicron and is than absorbed into the body 0 Biggest particle 0 Mostly made out of triacylglycerol 0 Verylowdensity lipoprotein VLDL and lowdensity lipoproteins LDL 0 Transport endogenous lipids done by the liver producing those lipoproteins and using those proteins for transportation across the body 0 Similar composition to chylomicrons but mostly made out of protein or cholesterol Role of the Liver and Adipose Tissue in Lipid Metabolism 0 Liver o Synthesizes bile salts I Makes the micelles that can than emulsify fat 0 Synthesizes lipoproteins o Synthesizes new lipids from nonlipid precursors such as glucose and aa 0 Exogenous lipid metabolism following a fatty meal in the liver I Dietary nutrients enter the liver through the portal vein Glucose can be converted to glycogen or enter glycolysis I Amino acids enter the amino acid pool and sore are metabolized to produce pyruvate and oxaloacetate I Short chain FFA bound to albumin enter the fatty acid pool and are incorporated into TAG I CR attach to apoA B binding sites enter the hepatocyte by endocytosis and are taken up by a lysosome FFA MAG DAG and C are released The lipids are reformed to TAG and CE and packaged I TAG C and PL are packaged Wapolipoproteins and enter the circulation as VLDLs or HDLs I VLDLs deliver the meal s lipids to the nonhepatic tissue I HDL is involved in reverse cholesterol transport 0 Adipose tissue 0 Absorbs TAG and cholesterol from chylomicrons through lipoprotein lipase 0 Stores TAG I When there is more B than we need it is stored in adipose tissue and when there is a need for it it is than released 0 Metabolism of Triacylglycerol during Fasting o Adipocytes lipolysis release FA into blood I Used as a source of E 0 Liver produces ketone bodies continues to synthesize VLDL and HDL Metabolism of Lipoproteins 0 Lowdensity lipoprotein LDL o Transports cholesterol to tissues 0 Binds WLDL receptor on cells 0 The LDL receptor structure and genetic aberrations I Mutant cells can t bind ef ciently hypercholesterolemia high cholesterol 0 Fate of Chylomicrons o Chylomicrons deliver the TAG to tissues other than the liver particularly adipose and muscle 0 Adipose tissue and muscle cannot phosphorylate glycerol so they transfer it to the serum to be picked up by the liver or kidney 0 When much of the TAG are transferred from the chylomicrons they become chylomicron remnants o The chylomicron remnant transfers the apoA and apoC back to HDL o The chylomicron remnant attaches to the liver binding site containing hepatic lipase and the fatty acids cholesterol and cholesteryl esters are transferred to the liver 0 Fate of VLDLs and LDLs Both are produced by liver The fatty acids from TAG are hydrolyzed by lipoprotein lipase 0 As the TAG is removed from the VLDL the particle becomes smaller and becomes an IDL intermediate density lipoprotein 0 Further loss of TAG and it becomes a LDL o LDLs are taken by BlOO receptors found in the liver and nonhepatic tissue 0 Sequential steps in Endocytosis of LDL leading to synthesis of cholesteryl ester 0 LDL particle WapoB and apoE attaches to the LDL receptor 0 Endocytosis of LDL particle and receptor LDL particle fuses wlysosome LDL receptor returns to the membrane surface 0 HMGCoA reductase is involved in cholesterol synthesis When excess cholesterol is present HMGCoA reductase and synthesis of LDL receptors are inhibited o Highdensity lipoprotein HDL o Responsible for reverse cholesterol transport Removes unesterified cholesterol from cells and other lipoproteins Returns it to the liver for excretion in bile reverse cholesterol transport Binds to receptors on hepatic and extrahepatic cells OOOO HDL is synthesized primarily in the liver wa lesser amount from the intestine apoE and apoC are synthesized in the liver and added to HDL o The life span of HDL is about 2 days and during this time it actively works at the reverse transport of cholesterol to the liver for excretion via bile Lipids Lipoproteins and Cardiovascular Disease Risk 0 Of interest regarding CVD 0 Cholesterol 0 Saturated and unsaturated FAs 0 Trans FAs o Lipoprotein A o Apolipoprotein E 0 Cholesterol 0 High HDL low LDL healthy 0 Ratios of ApoA to ApoB used to assess CVD risk 0 Individuals respond differential to dietary cholesterol I Absorption or biosynthesis I Formation and receptormediated clearance of LDL I Rates of LDL removal and excretion I Potential genetic variations 0 People may predisposed genetically to cholesterol 0 Saturated and Unsaturated FAs 0 Positive correlation wCVD I Total fat I Saturated FAs 0 Twice as capable of increasing overall cholesterol levels than MUFA and PUFA I Cholesterol I Trans fat 0 Trans FAs o Lrg amounts created through hydrogenation of PUFA polyunsaturated o Raise LDL and cholesterol and lower HDL increased in ammation interleukin6 C reactive protein and dyslipidemia apoB triacylglycerols and lipoprotein a 0 Evidence for increased insulin resistance 0 Mostly found in fried foods and bakery products 0 Lipoprotein A 0 Genetic variant of LDL I Attached to a unique marker protein 0 Associated watherosclerosis 0 Apolipoprotein E o ApoE may be involved in atherogenesis o 3 isoforms apoE2 E3 E4 0 E4 phenotype associated wincreased CVD risk 0 A risk factor in early onset of Alzheimer s dz poorer outcomes following TBI and postoperative cognitive dysfunction Integrated Metabolism in Tissues 0 Catabolism of triacylglycerols and FAs o The complete hydrolysis of triacylglycerols yields glycerol and three FAs in the body this hydrolysis occurs largely through the activity of lipoprotein lipase of the vascular endothelium in nonhepatic tissue and through an intracellular lipase that is active both in the liver and in adipose tissue 0 FAs are a rich source of E o Mitochondrial transfer of acyl CoA I The oxidation of FAs occurs primarily within the mitochondrion and produces E through oxidative phosphorylation I Short chain FAs can pass directly into the mitochondrial matrix and form acyl CoA derivatives in the matrix I Long chain FAs and their CoA derivatives are incapable of crossing the inner mitochondrial membrane but can cross the permeable outer membrane so a membrane transport system is necessary 0 Betaoxidation of FAs I The oxidation of the activated FA in the mitochondrion occurs through a cyclic degradative pathway by which two carbon units in the form of acetylCoA are cleaved one acetylCoA at a time from the carboxyl end Energy Considerations in FA Oxidation 0 The complete beta oxidation of one palmitic acid including oxidation of the FADH2 and NADH produced during beta oxidation and of each of the acetylCoAs through the TCA cycle yields about 106 molecules of ATP 0 The activation of a FA requires 2 high energy bonds per molecule of FA oxidized 0 Each cleavage of a saturated CC bond yields 4 ATPs 15 by oxidation of FADH2 and 25 by oxidation of NADH by oxidative phosphorylation 0 The acetylCoAs are oxidized to C02 and water in the TCA cycle 0 For each acetylCoA oxidized 10 ATPs are produced 0 Cleavage of saturated CC yields 5 ATPs Formation of Ketone Bodies Over ow pathway for acetyl CoA use providing another way for the liver to distribute fuel to peripheral cells Ketone concentration rises during accelerated FA oxidation low CHO intake or impaired CHO use 0 Can be accelerated in persons wDM and can become deadly if not corrected immediately Formation of ketone bodies is called ketogenesis Catabolism of Cholesterol Does not produce E Structure remains intact 0 Four ring core structure remains intact in the course of its catabolism and is eliminated as such through the biliary system Cholesterol is primarily in the form of its ester and is delivered to the liver chie y in the form of chylomicron remnants as well as in the form of LDLC and HDLC The cholesterol that is destined for excretion either is hydrolyzed by esterases to the free form or it is first converted into bile acids before entering the bile o It is estimated that neutral sterol represents about 55 and bile acids and their salts about 45 of total sterol excreted Synthesis of FAs Basic process sequential assembly of starter acetyl CoA wunits of malonyl CoA Occurs in the cytosol In the end everything is carboxylated There is an elongation system that creates 18C Essential FAs o Humans cannot introduce double bond beyond A9 because we lack A12 and A15 desaturases o This is why linoleic acid omega 6 and alpha linolenic acid omega 3 are essential FAs 0 These FAs can be acquired from plant sources bc plant cells have the desaturase enzymes Eicosanoids FA Derivatives of Physiological Significance o Prostaglandins o Thromboxanes I Are hormone like in action 0 Leukotrienes I Potent biological actions 0 The Western diet favors linoleic acid and once it is acquired longer more highly unsaturated FAs can be formed from it by a combo of elongation and desaturation reactions I These can cause arrhythmias in the heart if we have too much in our diet I Therefore we need to consume more omega 3 Essential FA in Development o Deficiency sx for the n6 series that have been identified in adults include poor growth and scaly skin lesions 0 Deficiency sx for the n3 series include neurological and visual abnormalities 0 Human milk contains more of the essential FAs than most infant formulas do 0 There is evidence that n3 essential FAs are necessary for neural tissue and retinal photoreceptor membranes 0 Both term and preterm infants can convert n3 essential FAs to the long chain polyunsaturated FAs but whether they can convert them at an adequate rate to meet their needs is unclear 0 Impact of Diet on FA Synthesis 0 Diets high in simple CHOs and low in fats induce a set of lipogenic enzymes in the liver I This induction is exerted through the process of transcription leading to elevated levels of the mRNA for the enzymes 0 A very lowfat highsugar diet causes an increase in FA synthesis and in palmitic acid rich linoleic acidpoor VLDL triacylglycerols Synthesis of Cholesterol 0 Cytoplasmic sequence 0 Conversion of HMG CoA to squalene 0 Formation of cholesterol form squalene 0 The liver accounts for about 20 of endogenous cholesterol synthesis and the remaining 80 is exogenous cholesterol synthesis 0 The cholesterol production rate approximates l g day 0 The average intake of cholesterol is considered to be about 300mgday with only about half of it being absorbed Regulation of Lipid Metabolism o Linked to CH0 status 0 FAs formed in the cytosol of hepatocytes can either be converted into triacylglycerols and phospholipids or be transported via carnitine into the mitochondrion for oxidation 0 An increase in the concentration of malonylCoA promotes FA synthesis while inhibiting FA oxidation 0 MalonylCoA concentration increases whenever a person is well supplied wCHO 0 Excess glucose that cannot be oxidized through the glycolytic pathway or stored as glycogen is converted to triacylglycerols for storage 0 Insulin s presence or absence 0 Hyperglycemia triggers the release of insulin which promotes glucose transport into the adipose cell and promotes lipogenesis o Hypoglycemia results in a reduced intracellular supply of glucose thereby suppressing lipogenesis o Hormonesensitive triacylglycerol lipase mobilizes fat 0 Hormones that stimulate lipolysis o Epinephrine norepinephrine adrenocorticotropic hormone ACTH thyroid stimulating hormone TSH glucagon growth hormone and thyroxine o Acetyl CoA carboxylase Brown Fat Thermogenesis 0 Brown adipose tissue high vascularity abundant mitochondria 0 Special mitochondria promote thermogenesis at expense of ATP 0 Have H pores in inner membranes formed of uncoupling protein UCP 0 Thermogenesis triggered by ingestion of food or prolonged exposure to cold temps 0 Both of these events stimulate the tissue via sympathetic innervation via the hormone norepinephrine Ethyl Alcohol Ethanol Metabolism and Biochemical Impact 0 Resembles CHO but is metabolized like a FA 0 The alcohol dehydrogenase ADH pathway 0 ADH in liver cells NAD requiring dehydrogenase oxidizes ethanol to acetaldehyde o The microsomal ethanol oxidizing system MEOS or Cytochrome P450 o Tolerance ethanol induces synthesis of MEOS enzymes 0 Catalase in the presence of hydrogen peroxide 0 Alcoholism biochemical and metabolic alterations o Acetaldehyde toxicity 0 High NADHNAD ratio I Accumulation of lipids and lactate o Substrate competition 39 Vit A o Induced metabolic tolerance o EtOH in Moderation The Brighter Side 0 Elevated HDL o Lowers serum lipoprotein 0 May suppress proliferation of smooth muscle cells underlying the endothelium of arterial walls 0 Strongest correlation wAMOUNT not TYPE consumed beer wine spirits
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