LIPID METABOLISM

Lipid Digestion, Absorption, and Transport Triglycerides ~ 90% of dietary lipids Metabolic NRG storage Oxidized to CO2 and H2O

oxidation state than glucose metabolism of fats yields ~9 kcal/gram Triglyerides, cont… oxidation state than glucose metabolism of fats yields ~9 kcal/gram vs ~4 kcal/gm Carbohydrates, proteins

Triglyceride Storage, cont… Stored in anhydrous state Non-polar Provide ~ 6 times energy of hydrated glycogen

Triglyceride storage, cont… Stored in large quantities in cells Non-reactive with other cell components Segregated into lipid droplets Do not affect osmolarity of cytosol

To be used as fuel: insoluble in H2O Triglycerides, cont… To be used as fuel: insoluble in H2O Must emulsify before lipid digestion in intestine Must be “carried” in blood (proteins)

Three cellular sources Triglycerides, cont… Sources of fat Three cellular sources Fat in diet Fat stored in cells Fat synthesized in one organ and transported to another

Fats obtained vary by organism Vertebrates Triglycerides, cont… Fats obtained vary by organism Vertebrates Fat in diet Fat in adipose tissue Convert excess carbohydrate to fat in liver for export

Humans Industrialized countries: Triglycerides, cont… Humans Industrialized countries: ~ 40% of daily calorie consumption is fat Should be < 30%! Atkins kills…

Humans, Triglycerides, cont… Used for > half the energy in: Liver Heart Resting skeletal muscle

Humans, Triglycerides, cont… Hibernating animals and migrating birds sole source of NRG Higher plants: Do not depend on fats for energy Germinating seeds

Triglyceride digestion Lipid digestion, cont… Digestion Triglyceride digestion takes place at lipid-water interfaces Rate is based on surface area at interfaces Peristalsis Emulsification by bile

Bile Synthesized from cholesterol, biliverdin By liver Lipid digestion, cont… Bile Synthesized from cholesterol, biliverdin By liver Stored in gallbladder Released after ingestion of fat

Lipid digestion, Bile, cont… Acts as digestive detergent Converts dietary fats into mixed micelles Micelles contain bile salts and triglycerides

Role of lipase Pancreatic lipase: Lipid digestion, cont… Role of lipase Pancreatic lipase: catalyzes hydrolysis of triglycerides At 1 and 3 positions sequentially Forms 1,2-diglycerol 2-glycerol Soap

Role of lipase, Lipid digestion, cont… Phospholipase A: Pancreatic enzyme Degrades phospholipids Hydrolysis at C(2)

Molecules diffuse into cells of intestine Facilitated by bile salts Absorption of Lipids Absorption Molecules diffuse into cells of intestine Facilitated by bile salts Micelles transport non-polar lipids across aqueous boundary layer

Bile salts essential Fatty Acids Fat soluble vitamins A,D,E & K Absorption, cont… Bile salts essential Fatty Acids Fat soluble vitamins A,D,E & K Biliary disease interferes

Lipid Uptake in Vertebrates

Reconversion: in mucosa cells Lipid digestive products  triglycerides Role of Lipase, cont… Reconversion: in mucosa cells Lipid digestive products  triglycerides Packaged into Chylomicrons Lipoprotein aggregates Triglycerides, cholesterol, protein

Chylomicron Aggregate of triglycerides, cholesterol, and proteins

Reconversion, Role of Lipase, cont… Released into bloodstream: Via lymphatic system Lacteals Triglycerides synthesized in liver Packaged in VLDL Directly released into blood

Form classes based on density Role of lipase, cont… Transport APOLIPOPROTEINS: lipid-binding blood proteins Transport between organs Form classes based on density

Proteins on outside of Chylomicron Are apoproteins Role of lipase, cont… Proteins on outside of Chylomicron Are apoproteins Act as cell surface receptors for recognition (apo C-II)

Lipid Uptake in Vertebrates

Transport, Role Chylomicrons, cont… Chylomicrons and VLDL: Move triglycerides, cholesterol To Skeletal muscle & adipose LIPOPROTEIN LIPASE an extracellular enzyme Activated by apo C II Hydrolyzes triglycerides

Chylomicron Carry triglycerides, cholesterol to tissues via lymphatic system (lacteals)

Transport, Role of lipase, cont… Deletes lipoprotein from triglyceride Forms IDL Then LDL LDL: (130-159 mg/dl) removed from plasma at liver, adrenals, adipose HDL: (above 35 mg/dl) moves cholesterol from tissues Sends to liver for excretion in bile salts Ratio LDL:HDL = 2:1

Lipid Metabolism, cont… Uptake by cells FA taken in by: muscle: oxidized for NRG adipose: stored as triglyceride

Lipid Uptake in Vertebrates

Transport, Role of lipase, cont… Glycerol Transported to liver or kidney Converted to dihydroxyacetone phosphate And then???

Pathway to Glycolosis Conversion of glycerol to dihydroxyacetone phosphate

Transport, Role of lipase, cont… Remnants of Chylomicrons triglycerides; cholesterol and apoproteins (apo E, apo B-48) To liver Uptake by endocytosis Triggered by apolipoproteins

Uptake in Liver via Endocytosis

Chylomicron Remnants, cont… Triglycerides Oxidized for energy or Converted to ketone bodies Excess fatty acid Convert to triglycerides Pack into VLDL To adipose for storage

Lipid Uptake in Vertebrates

Mobilization of Stored Fats Mobilization of stored triglycerides Triggered by hormones Stimulus is in blood glucose Hormones: Epinephrine Glucagon

Mobilization of Stored Fats, Hormone Action, cont… Activate adenylate cyclase in adipocyte plasma membrane Increases cAMP Activates protein kinase Activates triglycerol lipase Catalyzes hydrolysis of ester bonds in triglycerides

Mobilization of Stored Fats Action of second messenger hormones triggers mobilization of stored triglycerides

Free fatty acid released into bloodstream Role of lipase, cont… Free fatty acid released into bloodstream Binds to serum albumin Carried to tissues Released to diffuse into cells

NOTE: Glycerol  glycerol-3-phosphate dihydroxyacetone phosphate  Role of lipase, cont… NOTE: Glycerol  glycerol-3-phosphate dihydroxyacetone phosphate  glycolysis

II. Fatty Acid Oxidation FA activation: Free FA cannot enter Mitochondria Occurs in cytosol Acyl-CoA synthetases (thiokinases) 3 isozymes mitochondrial membrane

FA + CoA + ATP  fatty acyl-CoA + AMP + PPi FA activation, cont... FA + CoA + ATP  fatty acyl-CoA + AMP + PPi different acyl-CoA’s work on different FA Fatty acyl-CoA: high energy compound

Fatty acid oxidation, cont… Transport across mitochondrial membrane: Formation of fatty acyl-carnitine Carnitine acyl transferase I Outer face of inner membrane Fatty acyl from CoA  carnitine CoA released  cytoplasm

Transport across mitochondrial membrane, cont… Acyl-carnitine/carnitine transporter Carrier Fatty acyl-carnitine  matrix carnitine  to inter-membrane space Facilitated diffusion

F.A. Entry into Mitochondria

Fatty acid oxidation, cont… Transfer of acyl from Carnitine to intra-mitochondrial membrane Carnitine acyltransferase II Inner face of inner membrane Regenerates fatty acyl-CoA Frees carnitine  returns to cytosol

Carnitine Acyltransferase II, cont… Two separate pools of CoA (ATP, NAD+) Cytoplasm: makes FA Mitochondria: oxidative degradation Pyruvate FA AA

ß-oxidation ß-oxidation: FA dismembered to fatty acyl-CoA Mitochondrial oxidation of FA Stage One: ß-oxidation Remove 2-C chunks as acetyl-CoA Begins at carboxyl end of fatty acid chain

ß-Oxidation Fatty acids are dismembered into Acetyl-CoA subunits Each acetyl Co-A sends 4 H to NAD, FAD

Mitochondrial oxidation, cont… For example, 16C palmitic acid  8 acetyl CoA Stage Two: oxidation of acetyl-CoA  CO2 Stage Three: oxidative phosphorylation  ATP (+H2O)

ß oxidation, cont… ß oxidation: 4 reactions Formation of a trans – a, ß double bond Between C-2 (a) and C-3 (ß) Yields: Trans-∆2-enoyl-CoA Acyl-CoA dehydrogenase FAD  2 ATP Similar to succinate dehydration

ß-Oxidation First: formation of double bond Second: hydration of double bond Third: dehydrogenation Fourth: cleavage to Acetyl Co-A

Hydration of the double bond ß-oxidation, cont… Hydration of the double bond By enoyl-CoA hydrase Forms 3-L-hydroxyacyl-CoA Similar to fumarase reaction

Dehydrogenation By ß-hydroxyacyl-CoA dehydrogenase To ß-ketoacyl-CoA ß-oxidation, cont… Dehydrogenation By ß-hydroxyacyl-CoA dehydrogenase To ß-ketoacyl-CoA NAD  3 ATP Similar to malate dehydrogenase reaction

Ca -Cß cleavage (thiolysis) ß-oxidation, cont… Ca -Cß cleavage (thiolysis) Catalyzed by ß-ketoacyl-CoA thiolase (thiolase) Produces: acetyl-CoA fatty acyl-CoA (2 carbons short)

ß oxidation: Enzymes in Mitochondrial membranes 3 kinds of acyl-CoA dehydrogenase Specify short, medium, long chain fatty acid Medium-chain acyl-CoA dehydrogenase (MCAD) Deficient in ~10% of SIDS babies More common than PKU

Sites of Mitochondrial Enzymes

Fatty acid oxidation is highly exergonic Each round of ß-oxidation ß-oxidation, cont… Fatty acid oxidation is highly exergonic Each round of ß-oxidation 1 NADH 1 FADH2 1 acetyl-CoA

ß-oxidation, Energy Payoff Acetyl-CoA  citric acid cycle Example: PALMITOYL-CoA (16C) 7 rounds of ß -oxidation  7 NADH 7 FADH2 8 acetyl CoA So, 7 + 24 NADH = 31 x 3 = 93 ATP 7 + 8 FADH2 = 15 x 2 = 30 ATP + 8 GTP 131 hi-energy molecules for fatty acyl CoA - 2 ATP 129 total

ß-oxidation, cont… READ IN TEXT: Oxidation of unsaturated fatty acid: 2 additional reactions Oxidation of odd-chain fatty acid: 3 additional reactions

III. Ketone Bodies Acetyl-CoA from ß-oxidation Enters citric acid cycle Converted to KETONE BODIES Water-soluble “equivalent” of FA

Ketogenesis Occurs in liver: Primary ketone bodies Ketone bodies, cont… Ketogenesis Occurs in liver: acetyl CoA  ketone bodies Primary ketone bodies Acetoacetate - out of liver D-ß-hydroxybutyrate - out of liver Acetone - exhaled

Ketone Body Formation

Function: fuel for peripheral tissues Heart, skeletal muscle Brain Ketone bodies, cont… Function: fuel for peripheral tissues Heart, skeletal muscle Brain Normal fuel is glucose In starvation: ketone bodies Enzyme production adapts over time >40 days, provide 70% energy

Production and transportation Ketone bodies, cont… Production and transportation Determined by availability of oxaloacetate Combine with acetyl group enter TCA cycle

Oxaloacetate pulled from citric acid cycle Used in gluconeogenesis Ketone bodies, cont… In starvation Oxaloacetate pulled from citric acid cycle Used in gluconeogenesis [oxaloacetate] decreases therefore little  kreb’s cycle Production of ketone bodies is favored

Production and transportation, cont… Overproduction: starvation, diabetes Moved from liver to other tissues Allows fatty acid oxidation in liver In tissues: ketone bodies converted back to acetyl-CoA

Production and transportation, cont… Formation of ketone bodies from acetyl-CoA occurs in the mitochondrial matrix thiolase 2 Acetyl-CoA acetoacetyl-CoA Acetoacetyl-CoA + acetyl-CoA  ß -hydroxy- ß -methylglutaryl-CoA HMG-CoA  free acetoacetate + acetyl-CoA

Production and transportation, cont… Acetoacetate  D- ß -hydroxybutyrate dehydrogenase D-ß-hydroxybutyrate Acetoacetate decarboxylase Acetone High in uncontrolled diabetes

Production and transportation, cont… Ketosis Pathological Acetoacetate produced too fast for elimination Breath smells like acetone Blood pH decreases  acidosis

Lipid Biosynthesis “Reverse” of lipid catabolism Know differences Where in the cell does each occur? What are the e- transport molecules? What are the acyl group carriers? Where does acetyl Co-A “fit into” each set of reactions?

Pathway Differences

Fatty Acid Biosynthesis Read in Text For major steps, know Type of reaction Enzyme Product

Cholesterol Biosynthesis Read in Text Know Draw steps Intermediates