Lipid Metabolism PDF

Lipid Metabolism Oxidation of fatty acids is a major energy source in many organisms About one-third of our energy needs comes from dietary triacylglycerols About 80% of energy needs of mammalian heart and liver are met by oxidation of fatty acids!! Many hibernating animals, s...

Lipid Metabolism Oxidation of fatty acids is a major energy source in many organisms About one-third of our energy needs comes from dietary triacylglycerols About 80% of energy needs of mammalian heart and liver are met by oxidation of fatty acids!! Many hibernating animals, such as grizzly bears, rely almost exclusively on fats as their source of energy (and water during their long-term sleep)  Fats provide efficient fuel storage The advantage of fats over polysaccharides: – Fatty acids carry more energy per carbon because they are more reduced – Fatty acids carry less water along because they are nonpolar (aggregate in lipid droplets and are unsolvated) Glucose and glycogen are for short-term energy needs, quick delivery Fats are for long-term (months) energy needs, good storage, slow delivery Different Fatty Acid Chain Composition Animal fat contains more saturated fatty acids than vegetable oils – Solid versus liquid – Only coconut oil (from vegetable oils) is rich in saturated fatty acids – Vegetable oils are good sources of PUFAs Natural fatty acids are even numbered Fatty acids seldom contain less than C16 – Most fatty acids contain C18 chains  Dietary fatty acids are absorbed in the small intestine Remaining chylomicrons go to liver and enter by RME  used for ketone bodies synthesis. When diet contains more f.a. than needed, liver converts them to TAG and packages them into VLDL to be transported to adipocytes Used for energy (muscles) or reesterified for storage (adipose) Emulsification 2nd by biological breakdown detergents of TAG (bile) Breakdown of Bloodstream to TAG to DAG, target tissues MAG, FFA and glycerol Chylomicrons Uptake by intestinal cells (lipoproteins)  Lipids are transported in the blood as chylomicrons Apoliporpotein + lipids particles = lipoprotein Lipoproteins range in density: VLDL to VHDL  Plasma Lipoproteins Composed of a neural lipid core (TAG and cholesteryl esters) surrounded by a shell of apolipoproteins, phospholipids and unesterified cholesterol Soluble in aqueous medium In constant state of synthesis and breakdown Keep lipids soluble in plasma for transport and delivery to tissues Not very efficient delivery in humans  gradual deposition of lipids (cholesterol) in arteries (atherosclerosis)  Plasma Lipoproteins 1. Chylomicrons Fat derived from intestinal absorption (99% fat, 1% protein) Carry dietary fats 2. VLDL Carry endogenously made TAG (high) and cholesterol (low) 3. LDL Carry cholesterol and cholesteryl esters; synthesized in liver 4. HDL Low percentage of TAG and cholesterol and high protein Reverse cholesterol transport 5. VHDL 99% albumin, 1% fatty acids Greatest amount of transported lipid in plasma  Hormones trigger mobilization of stored triacylglycerols Hydrolysis of TAGs is catalyzed by lipases - can produce MAGs, DAGs, FFA and glycerol Some lipases are regulated by hormones glucagon and epinephrine Recall: Epinephrine means: “We need energy now” Glucagon means: “We are out of glucose”  Hormones trigger mobilization of stored triacylglycerols Perilipins – proteins that coat lipid droplets and restrict access to lipids to prevent premature mobilization [glc]blood  glucagon cAMP PKA  phosphorylation of hormone-sensitive lipase & perilipin  dissociation of CGI and activation of adipose triacylglycerol lipase Monoacylglycerol lipase Serum albumin binds up to 10 f.a. noncovalently hydrolyzes MAGs  Glycerol from fats enters glycolysis Only 5% of biologically-active energy of TAG is in glycerol Glycerol kinase activates glycerol at the expense of ATP Subsequent reactions recover more than enough ATP to cover this cost Allows limited anaerobic catabolism of fats Fatty Acid Transport into Mitochondria Fats are degraded into fatty acids and glycerol in the cytoplasm of adipocytes Fatty acids are transported to other tissues for fuel -oxidation of fatty acids occurs in mitochondria Small (< 12 carbons) fatty acids diffuse freely across mitochondrial membranes Larger fatty acids (most free fatty acids) are transported via carnitine shuttle Stages of Fatty Acid Oxidation  The -Oxidation Pathway Each pass removes one acetyl moiety in the form of acetyl-CoA. activation Palmitate (C16) undergoes seven passes through the oxidative sequence  Fatty Acid Catabolism for Energy For palmitic acid (C16) – Repeating the above four-step process six more times (7 total) results in eight molecules of acetyl-CoA FADH2 is formed in each cycle (7 total) NADH is formed in each cycle (7 total) Acetyl-CoA enters citric acid cycle and further oxidizes into CO2 – This makes more GTP, NADH, and FADH2 Electrons from all FADH2 and NADH enter ETC (1.5 ATP/FADH2; 2.5 ATP/NADH) Transfer of e–s from FADH2 and NADH to O2 yields 1 H2O per pair (camels and hibernating animals!) Palmitoyl-CoA + 7CoA + 7O2 + 28Pi+ 28ADP  8 acetyl-CoA + 28ATP + 7H2O ( oxidation) Palmitoyl-CoA + 23O2 + 108Pi+ 108ADP  CoA + 108ATP + 16CO2 + 23H2O (full oxidation) NADH and FADH2 serve as sources of ATP  Formation of Ketone Bodies Entry of acetyl-CoA into citric acid cycle requires oxaloacetate When oxaloacetate is depleted, acetyl-CoA is converted into ketone bodies (acetone, acetoacetate and D-- hydroxybutyrate) – Frees Coenzyme A for continued β-oxidation – Acetone is exhaled – Acetoacetate and -HB are transported in the blood Under starvation conditions, the brain can use ketone bodies for energy The first step is reverse of the last step in the - oxidation: thiolase reaction joins two acetate units  Release of Free Coenzyme A The reactions of ketone body formation occur in the matrix of liver mitochondria Another condensation with acetyl-CoA yields HMG-CoA This frees 2 CoA molecules from 3 acetyl CoA  Formation of Ketone Bodies Cleaved into Specific for the D- acetoacetate and isomer; don’t confuse acetyl-CoA it with L-- hydroxyacyl-CoA DH of  oxidation Untreated diabetes  [acetoacetate] is high  more acetone produced  exhaled (odor)  Ketone Bodies as fuel In extrahepatic tissues: Ketone bodies can be used as fuels in all tissues except the liver Found in all  CAC tissues except the liver The liver is a producer, not a consumer, of ketone bodies  Liver is the source of ketone bodies Production of ketone bodies increases during starvation (and diabetes) Ketone bodies are released by liver to bloodstream Organs other than liver can use ketone bodies as fuels High levels of acetoacetate and - hydroxybutyrate lower blood pH dangerously (acidosis) Acidosis due to ketone bodies - ketoacidosis  Catabolism and anabolism of fatty acids proceed via different pathways Catabolism of fatty acids (excergonic and oxidative) – produces acetyl-CoA – produces reducing power (NADH and FADH2) – activation of fatty acids by CoA – takes place in the mitochondria Anabolism of fatty acids (endergonic and reductive) – requires acetyl-CoA and malonyl-CoA – requires reducing power from NADPH – activation of fatty acids by 2 different –SH groups on protein – takes place in cytosol in animals, chloroplast in plants  Overview of Fatty Acid Synthesis Fatty acids are built in several passes, processing one acetate unit at a time. The acetate is coming from activated malonate in the form of malonyl-CoA. Each pass involves reduction of a carbonyl carbon to a methylene carbon. Malonyl-CoA is formed from acetyl-CoA and bicarbonate Catalyzed by acetyl-CoA carboxylase (ACC)  Synthesis of fatty acids is catalyzed by fatty acid synthase (FAS) FAS system: – Catalyzes a repeating four-step sequence that elongates the fatty acyl chain by two carbons at each step – Uses NADPH as as the electron donor – Uses two enzyme-bound -SH groups as activating groups  Fatty Acid Synthesis Overall goal: attach two-C acetate unit from malonyl-CoA to a growing chain and then reduce it Reaction involves cycles of four enzyme-catalyzed steps – Condensation of the growing chain with activated acetate – Reduction of carbonyl to hydroxyl – Dehydration of alcohol to trans-alkene – Reduction of alkene to alkane The growing chain is initially attached to the enzyme via a thioester linkage During condensation, the growing chain is transferred to the acyl carrier protein (ACP) After the second reduction step, the elongated chain is transferred back to fatty acid synthase The General Four-Step Fatty Acid Synthase I Reaction in Mammals (1) Prep: Malonyl CoA and acetyl CoA (or longer fatty acyl chain) are bound to FAS I - bind via thioester terminus of a Cys of the FAS - activates the acyl group Step 1: Condensation rxn attaches two C from malonyl CoA to the attached acetyl-CoA (or longer fatty acyl chain) - also releases CO2 from malonyl-CoA - the decarboxylation facilitates the rxn - creates -keto intermediate Step 1 of FAS I: Elongation The General Four-Step Fatty Acid Synthase I Reaction in Mammals Step 2: 1st Reduction: NADPH reduces the - keto intermediate to an alcohol Step 3: Dehydration: OH group from C-2 and H from neighboring CH2 are eliminated, creating double bond (trans-alkene) Step 4: 2nd Reduction: NADPH reduces double bond to yield saturated alkane Step 5: Translocation: The growing chain is moved from ACP to –SH on FAS Steps 2-4 of the FAS I rxn Overall Palmitate Synthesis  Stoichiometry of Synthesis of Palmitate (16:0) 1) 7 acetyl-CoAs are carboxylated to make 7 malonyl-CoAs… using ATP 7 AcCoA + 7 CO2 + 7 ATP  7 malCoA + 7 ADP + 7 Pi 2) Seven cycles of condensation, reduction, dehydration and reduction…using NADPH to reduce the -keto group and trans-double bond AcCoA + 7 malCoA + 14 NADPH + 14 H+ Palmitate + 7 CO2 + 8 CoA + 14 NADP+ + 6 H2O Note: Eukaryotes have one additional energy cost. (Next slide)  Acetyl-CoA is transported into the cytosol for fatty acid synthesis In nonphotosynthetic eukaryotes… Acetyl-CoA is made in the mitochondria But fatty acids are made in the cytosol So Acetyl-CoA is transported into the cytosol with a cost of 2 ATPs Therefore, cost of FA synthesis is 3 ATPs per 2-C unit  Regulation of Fatty Acid Synthesis and Breakdown Occurs only when need for energy requires it When the diet provides a ready source of carbohydrate as fuel, β oxidation of fatty acids is unnecessary and is therefore downregulated 2 pathways for f.a.CoA in liver: TAG synthesis in cytosol or f.a. oxidation in mito Transfer into mito is rate limiting, once f.a. are in mito they WILL undergo oxidation [NADH]/[NAD+]  Acetyl-CoA  Concn increases when CHO is well-supplied Inhibition of shuttle ensures oxidation of f.a. is inhibited when energy Cytosol  Clinical significance 2 mammalian isoforms ACC1 and ACC2 Mice without ACC2 (null mice) consume more food but show continuous fatty acid oxidation, reduced body fat mass, and reduced body weight These mice are protected from diabetes ACC1 null mice are embryonically lethal Research of new drugs specific to ACC2 but not to ACC1 as weight loss drugs  Palmitate can be lengthened to longer-chain fatty acids Elongation systems in the endoplasmic reticulum and mitochondria create longer fatty acids As in palmitate synthesis, each step adds units of 2 C Stearate (18:0) is the most common product  Phospholipids must be transported from the ER to membranes Phospholipids are: – synthesized in the smooth ER – transported to Golgi complex for additional synthesis Must be inserted into specific membranes in specific proportions but can’t diffuse because they are nonpolar So transported in membrane vesicles that fuse with target membrane Details of the process are not well-understood  Four Steps of Cholesterol Synthesis 1) Three acetates condense to form 6-C mevalonate 2) Mevalonate converts to phosphorylated 5-C isoprene 3) Six isoprenes polymerize to form the 30-C linear squalene 4) Squalene cyclizes to form the four rings that are modified to produce cholesterol  Statin drugs inhibit HMG-CoA reductase to lower cholesterol Statins resemble HMG-CoA and mevalonate  competitive inhibitors of HMG-CoA reductase First statin, lovastatin, was found in fungi Lowers serum cholesterol by ~20 – 40% Also reported to improve circulation, stabilize plaques by removing chol from them, reduce vascular inflammation Most circulating chol comes from internal manufacture rather than the diet Conversion of Squalene to Cholesterol Fates of Cholesterol After Synthesis In vertebrates, most cholesterol synthesized in the liver, then exported: - As bile acids, biliary cholesterol or cholesteryl esters Other tissues convert cholesterol into steroid hormones, etc. Bile Acids Assist in Emulsification of Fats Bile is stored in the gall bladder, secreted into small intestine after fatty meal Bile acids such as taurocholic acid emulsify fats – Surround droplets of fat, increase surface area for attack by lipases Cholsteryl esters are more nonpolar than cholesterol Contain a fatty acid esterified to the oxygen – Comes from a fatty acyl-CoA – Makes the cholesterol more hydrophobic, unable to enter membranes Transported in lipoproteins to other tissues or stored in liver  Five Modes of Regulation of Cholesterol Synthesis and Transport 1) Covalent modification of HMG-CoA reductase 2) Transcriptional regulation of HMG-CoA gene 3) Proteolytic degradation of HMG-CoA reductase 4) Activation of ACAT, which increases esterification for storage (Acyl-CoA cholesterol acyl transferase) 5) Transcriptional regulation of the LDL receptor  HMG-CoA reductase is most active when dephosphorylated 1) AMP-dependent protein kinase -when AMP rises, kinase phosphorylates the LOW enzyme  activity , cholesterol synthesis  Energy 2) Glucagon, epinephrine Level - cascades lead to phosphorylation,  activity 3) Insulin - cascades lead to dephosphorylation, activity Covalent modification provides short-term regulation.