Lipids 1

Lipid Structures and Properties

Digestion and AbsorptionDr. K. A. SukalskiFebruary 25, 2005

Major MacromoleculesCarbohydratesProteinsNucleic AcidsLipidsAll readings for the next 4 lectures:Chapter 9 (pp. 264 - 272, Sec.9.6) and Chapter 16 (pp. 490-517, 522-526)

Fig 9.1 Structural relationships of major lipid classes E - EnergyS - StructureM - MessengersMEES & MSMS & MSAll S, Some M

Most Important Lipids: My ChoiceFatty Acidsoxidized for energybuilding blocks for TAGS and PLprecursors to hormones - EicosenoidsTriacylglycerols - TAGSstorage form of fatty acids which are used for energyPhospholipids PLsFor membranesCholesterol

This is a fatty acid.Amphipathic contains both polar and nonpolar partsEven Number of carbonsMelting Pointchain lengthsaturation (=) Nomanclature: C1, , ,

Table 9.1

Note:oleate = oleic acidstearate = stearic acid

C18 stearateC18:1 oleateC18:3 linolenateMP = 70MP = -17MP = 13Saturated vs Unsaturated Fatty Acids

Triacylglycerols: a Family with Many MembersNote these are very hydrophobic!!!More so than the fatty acids from which they are madeEnergy Storage

Glycerolphos- pholipidsAmphipathic!!!Membrane Components

Fig 9.9 Phospholipases hydrolyze phospholipidsPLase A1 or A2 gives a lysophospholipid (lacking a fatty acid)PLase C gives a diacylglycerolPLase D gives a phosphatidate

Other Classes of LipidsLipids Important in Membrane StructureSphingolipids (Fatty acids and Carbohydrates)Steroids, especially cholesterolSpecialized functionsWaxesMessengers/Signaling (Prostaglandins)

Lipid Metabolism- Digestion and AbsorptionChapter 16

Chapter 16Fatty Acidsoxidized for energybuilding blocks for TAGS and PLprecursors to hormones - EicosenoidsTriacylglycerols - TAGSstorage form of fatty acids which are used for energy

Digestion and Absorption of Dietary LipidsComposition of lipid in the diet90% TAGsPLsCholesterolWe have a problem. Think oil and water or cutting a cow from a herdHow do we get from large fat globules to small molecules which can be absorbed across the intestinal cell?

The Answer:Break the hydrophobic TAGs down to a monoacylglycerol + 2 fatty acidsBreak Phospholipids down to lysophospholipid and a fatty acidCoat the particles with amphipathic molecules (bile salts or bile acids) to make micelleshydrophobic part meshes with the lipidhydrophilic part presents to the water compartment

Fig 16.2 Action of pancreatic lipase

Fig 16.3 Dietary phospholipids are degraded by phospholipases

Fig 16.1 Bile salts are amphipathic derivatives of cholesterol and amino acids.Taurocholate and glycocholate (cholesterol derivatives) are the most abundant bile saltsAmphipathic: hydrophilic (blue), hydrophobic (black) Synthesized in LiverStored in gall bladderSecreted into intestineForms micelles

Lipids are Absorbed from MicellesMonoacylglycerolFree fatty acidsLysophospholipidsCholesterol

In the Intestinal Cellmonoacylglycerol + 2 fatty acids--> TAGscholesterol + fatty acid -->cholesterol esterlysophospholipid + fatty acid --> PLapolipoprotein B addedAnd we have a special kind of lipoprotein called a chylomicron to transport lipids around the body

Lipids 2

Transport and Storage of Dietary Lipid Mobilization, Fatty Acid Oxidation and Energy ProductionDr. K. A. SukalskiFebruary 28, 2005

Why Do We Need Lipoprotein Particles to Transport Lipid?Solubility of cholesterol as an example of functional importance of lipoprotein particles. 0.2 mg/dL in water at 250.150 200 mg/dL in plasma30% free70% esterifiedIn Bile: Bile salts and phosphatidylcholine maintain cholesterol in solution.390 mg/dL in bile96% free4% esterified

Fig 16.5 Structure of a lipoprotein

Fig 16.6 Summary of lipoprotein metabolismLipoprotein Lipase

Major Classes of Lipoprotein ParticlesChylomicrons dietary TAGs to peripheral tissueVLDL TAGs from liver to peripheral tissueLDL Cholesterol to tissuesHDL Cholesterol from tissues to the liver for disposal

Storage of TAGs in Adipose Tissue

TAG Formation in Adipose TissueTAGs carried to adipose tissue on chylomicrons and VLDLs (Very Low Density Lipoproteins) Lipoprotein lipase works on the lipoprotein: TAG ---> 3FAs + glycerolFAs go into the adipose tissue3FAs + glycerol (not really) = TAGHold this thought for later!

Fig 16.7 Triacylglycerol Degradation in AdiposeAlbuminInsulin inhibits HSLLipoprotein Lipase

Fate of GlycerolFrom action of lipoprotein lipase and hormone-sensitive lipaseGlycerol returns to liver - gluconeogenesis converts it to glucoseFate of Fatty AcidsBound to albumin in the bloodGo to heart, skeletal muscle, liver and are used

Fatty Acid OxidationFA carried to tissue on albumin or Chylomicrons/VLDLsHeart, skeletal muscle, liverMitochondria-Oxidation2 Carbons at a time are removed from FAThree Stepsactivationtransport in mitochondriaoxidation

Activation of FAPreceeds almost ALL reactions involving fatty acidsCytosol of cellAcyl CoA synthetase2 ATP equivalents used

(Page 191 for structure of CoA)Discussed in PDH reaction

B. Transport of Fatty Acyl CoA into MitochondriaThe carnitine shuttle system transfers long-chain fatty acyl CoA from the cytosol into the mitochondria Fatty acyl CoA is first converted to acylcarnitine (which can enter the mitochondria) and then back to fatty acyl CoAThe b-oxidation cycle enzymes (mitochondrial) can then degrade the fatty acyl CoA

Carnitine shuttle systemFigure 16.8Regulation:Malonyl CoA inhibits CAT I.Malonyl CoA produced under the influence of insulin in the first step of FA synthesis.

C. The Reactions of b oxidation One round of b oxidation: 4 enzyme steps produce acetyl CoA from fatty acyl CoAEach round generates one molecule each of: QH2 NADH Acetyl CoA Fatty acyl CoA (2 carbons shorter each round)

Fig 16.9b-Oxidation of saturated fatty acidsFatty acyl CoANote:AcylCoA vs AcetylCoAQH2

Acyl CoA dehydrogenase: Several forms with different chain-length specificitiesTrans 2, 3 enoyl CoASCAD short chain acyl CoA DHMCAD medium chain acyl CoA DHLCAD long chainVLCAD very long chain

MCAD MCAD DeficiencyMedium Chain acyl CoA DHPrevalenceNorth Carolina - 1 in 10,000 live birthsSince 1997 515,000 babies screened106 diagnosed with metabolic disorders 38 with MCAD deficiency1 in 25, 000 nationallyInfants cant use fat for energyScenario: Interruption in feeding(illness, long sleep)Grow lethargic or go to sleep and dont wake up

Fig 16.9b-Oxidation of saturated fatty acidsFatty acyl CoANote:AcylCoA vs AcetylCoAQH2

ATP Generation from -Oxidation

Palmitoyl CoA + 7 HS-CoA + 7 Q + 7 NAD+ + 7 H2O 8 Acetyl CoA + 7QH2 + 7 NADH + 7 H+

Acetyl CoA (TCA Cycle)8 X 10 = 80In Oxidative Phosphorylation:QH2 - 1.5 ATP7 X 1.5 = 10.5NADH - 2.5 ATP7 X 2.5 = 17.5- 2 ATP for activation106 ATPs/ palmitate

-Oxidation of Odd Chain and Unsaturated Fatty AcidsVariations on -Oxidation - extra enzymes requiredUnsaturated FA (C18:1, C18:2, C18:3 and others)require two extra enzymes to get double bonds in the right place for enzymes of -oxidation to workskip one or more oxidation steps -SLIGHTLY LESS ENERGY DERIVED AS THESE MOLECULES ARE SLIGHTLY MORE OXIDIZED TO BEGIN WITH

16.3 b Oxidation of Odd-Chain Fatty AcidsOdd-chain fatty acids occur in bacteria and microorganisms Final cleavage product is propionyl CoA rather than acetyl CoAThree enzymes convert propionyl CoA to succinyl CoA (citric acid cycle intermediate)

Fig 16.10Conversion of propionyl CoA to succinyl CoA

Fig 16.10 (cont) Succinyl CoA --> oxalacetate--> glucose (gluconeogenesis)This is minor exception to the rule that NET CONVERSION OF FA TO GLUCOSE DOES NOT OCCUR

An anaplerotic reaction

Vitamin B12

ATP Generationfrom Glucose vs. Palmitate Glucose Palmitate 32ATP/6C vs. 106ATP/16C 5.3 vs. 6.6 ATP/C Carbons of glucose are more oxidized to start

Anhydrous storage of lipids more conservative of space (because of hydrophobicity) than storage of hydrated glycogen

Lipids 3

Ketones and Fatty Acid SynthesisDr. K. A. SukalskiMarch 2, 2005

Regulation of Fatty Acid Oxidation: Fed State - Insulin is Highinhibition of hormone sensitive lipase in adipose tissueFA are not releasedstimulates formation of malonyl CoA in FA synthesismalonyl CoA inhibits carnitine acyl transferase IFA dont go into mito for oxidation

Fasted State - Insulin is LowLipolysis is not inhibited at hormone-sensitive lipase stepFAs released from adipose into blood and go to tissuesglucagon stops malonyl CoA formationmalonyl CoA falls, transport of FA into mito increasesmore FA to oxidize

Glucose Sparing in FastingTCA full because of acetyl CoA from lipidslots of NADH and acetyl CoApyruvate dehydrogenase inhibited by NADH and acetyl CoAglucose not used - is spared (for brain and other nervous tissue)Glucose also spared by ketones which can be an energy providing substrate in nervous tissue

Lipid-Based Forms of Energy Fatty AcidsKetone Bodies-hydroxybutyrateacetoacetateacetoneEnergy Sources in StarvationBrain - ketone bodies + glucoseHeart, skeletal muscle - FA and ketones

Ketone BodiesFuel moleculesderived from excess acetyl CoAWater solublereadily and quickly transported to other tissues for energyMajor source of energy for brain in starvation (skeletal muscle and kidney, also)

Fig 16.12 Ketone bodies

Ketone Body SynthesisFig 16.13 Liver in mammalsMitochondrial matrixInsulin lowGlucagon elevated

HMGCoA Synthase - mito of liver only

Fig 16.13 (cont)

Ketone Bodies Oxidized in Mito-chondria of Extrahepatic TissuesNot found in liverTo the TCA cycle in heart, skeletal muscle, brain

-hydroxybutarate

Insulin-Dependent Diabetes Mellitus (IDDM)p. 504

Fatty Acid SynthesisQH2 from FADH2

Outline of Fatty Acid SynthesisEukaryotesacetyl CoA from mito to cytosolacetyl CoA (2C) carboxylated to malonyl CoA (3C)regulated stepassembly of FA from malonyl CoA by fatty acid synthase complex

Fig. 16.15 Citrate transport system

Supply of Cytosolic NADPH forFatty Acid SynthesisCitrate transport system (Figure 16-15)Pentose phosphate pathway (Earlier CHO lectures)

Carboxylation of Acetyl CoA by AcetylCoA Carboxylase Acetyl CoA carboxylase - regulated stepPhosphorylated form is inactiveGlucagon promotes phosphorylationCitrate inhibits phosphorylationInhibited by fatty acyl CoASynthesis - increased in high CHO diets; decreased in high fat diets

Fatty Acid Synthase ComplexHomodimerACP (Acyl Carrrier Protein)+7 enzymes for each dimerCytosolic

Fig 16.17 Biosynthesis of FA from acetyl CoA and malonyl CoA

Fig 16.17 (continued)

Fig. 16.17 (continued)

Condensation step for second round of FA biosynthesisFig 16.18

Final reaction of FA synthesis Rounds of synthesis continue until a C16 palmitoyl group is formedPalmitoyl-ACP is hydrolyzed by a thiolesterase

Acetyl CoA + 7 Malonyl CoA + 14 NADPH + 14 H+ Palmitate + 7 CO2 + 14 NADP+ + 8 HS-CoA + 6 H2OLots of Energy UsedATPs in acetyl CoAATPs to make malonyl CoANADPH if used in oxphos

Fig 16.19Organization of subunits in animal FA synthase

16.6 Fatty Acid Elongation and DesaturationFig 16.21Note: Double bonds arenot conjugated butinterrupted by methylenegroup

Fig 16.21 (cont)

Essentiality of Linolenic and Linoleic Acids Delta 9-desaturasesC:16 as substrate - results in omega 7C-C-C-C-C-C-C=C-C-C-C-C-C-C-C-COOHCant make omega 3 or 6 because palmitate is the shortest substrate for desaturaseC18:2 Linoleic is omega 6C18:3 Linolenic is omega 3

PalmitateLinoleate (C:18 9,12) & linolenate (C:18 9,12, 15)Are essential fatty acids, required in our diet.Why?

Palmitate is the shortest substrate that can be used by desaturases.We have delta 4, 5, 6, and 9 desaturases.

The best we can make is a C:16 9 which is elongated to C:18 11 . We cant place double bonds any closer to the methyl end of the molecule.

So the C:18 9,12 and C:18 9,12, 15 must be provided in the diet.

Lecture 4Synthesis of Triacylglycerol, Phospholipids and SterolsDr. K. A. SukalskiMarch 29, 2004

Fatty Acid SynthesisLiver, adipocytes, specialized tissues (mammary glands)ATP, NADPH, acetyl CoATransport of acetyl CoA to cytosol as citrateAcetyl CoA carboxylase - malonyl CoAFatty Acid Synthase Complex

Synthesis of Triacylglycerols (TGs) and Glycerophospholipids (GPLs)Most fatty acids in cells are found in esterified forms as TGs or GPLsPhosphatidic acid (phosphatidate) is an intermediate in the synthesis of TGs and GPLsGlycerol 3-phosphate is acylated by fatty acyl CoA molecules

Fig 16.23 Formation of phosphatidate

Fig 16.24 Synthesis of TGs and neutral phospholipids(Continued next 2 slides)

You will not be responsible for Figures 16-25, 16-26, 16-27, 16-28Synthesis of other membrane lipids

EicosanoidsArachidonate (C20:4) at C2 of a phospholipid in inner leaf of plasma membranePhospholipase A2 releases free C20:4Converted to local regulatory moleculesCyclooxygenase Tissue specificHalf life of seconds to minutesLipooxygenaseMediators of the allergic responseHalf life of many minutes to hours

Smooth muscle contractionpain, swelling, inflammationProstaglandin Biosynthesis

Aspirin inhibits cyclooxygenase

Leukotriene BiosynthesisAnd many moreMediators of inflammation

Synthesis of Cholesterol Cholesterol is a precursor of steroid hormones and bile acids, and an important component of many mammalian membranesMost cholesterol is synthesized in the liverLiver-derived and dietary cholesterol are both delivered to body cells by lipoproteinsCholesterol biosynthesis is regulated by hormones and blood cholesterol levels

Cholesterol BiosynthesisMost cells can synthesize cholesterol, liver is most importantCytosolic acetyl CoA2C 6C 5C 15C 30C Acetate (C2)Isoprenoid (C5) Squalene (C30) Cholesterol (C27) NADPHATP

Acetyl CoA to Isopentenyl PyrophosphateThe carbons of cholesterol come from cytosolic acetyl CoA (transported from mitochondria via citrate transport system)First step is a sequential condensation of three molecules of acetyl CoAIsopentenyl pyrophosphate is an important donor of isoprenyl groups for many synthetic reactions

HMG CoA Reductase

Catalyzes first committed step in pathwayPrimary site for regulating cholesterol synthesis - Covalent modification inactivated by phosphorylation - Repression of transcription - Control of degradation Cholesterol-lowering statin drugs (e.g. Lovastatin) inhibit HMG-CoA reductaseNo HMGCoA lyase in cytosol.

Formed inside mitochondriagoes to acetoacetate

C6

(continued from previous slide)C5

Fig 16.31 Isopentenyl Pyrophosphate to SqualeneC5C5

Fig 16.31 (cont)C15C10C5C15 C30

Squalene to Cholesterol

Lots of NADPH requiredC30 C27

End of Lipid MetabolismCheck Out Box 16.5Production and Utilization of Acetyl CoA