Fatty Acid Synthesis Lecture Notes 2024 PDF

Fatty acid and lipid biosynthesis Marc A. Ilies, Ph. D. Lehninger - Chapter 21 [email protected]; lab 517, office 517A (Tu, Fr 3-5) For questions, comments please use the discussion tool in Canvas © MAIlie...

Fatty acid and lipid biosynthesis Marc A. Ilies, Ph. D. Lehninger - Chapter 21 [email protected]; lab 517, office 517A (Tu, Fr 3-5) For questions, comments please use the discussion tool in Canvas © MAIlies 2024 1  Catabolism and Anabolism of Fatty Acids Proceed via Different Pathways Catabolism of fatty acids – produces acetyl-CoA – produces reducing power (NADH, FADH2) – takes place in the mitochondria Anabolism of fatty acids – requires acetyl-CoA and malonyl-CoA – requires reducing power from NADPH – 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, made by acetylCoAcarboxylase (ACC): ACC AcetylCoA + CO2 + ATP malonylCoA + ADP +P biotin Cytosol ACC also inhibited by epinephrine, palmitoylCoA, activated by citrate Formation of malonyl-CoA in the cytosol (FA synthesis) inhibits beta-oxidation  Fatty Acid Synthase makes palmitate Carried out in a repeating 4 step sequence that elongates the chain with 2 atoms each time, by the fatty acid synthase complex: single polypeptide chain in vertebrates, with several catalytic domains; dimer leads to single product: palmitate 16:0 uses NADPH as the electron donor groups Overall Palmitate (16C) Synthesis  Acyl Carrier Protein (ACP) Serves as a Shuttle in Fatty Acid Synthesis Contains a covalently attached prosthetic group 4’-phosphopantetheine – flexible arm to tether acyl chain while carrying intermediates from one enzyme subunit to the next Delivers acetate (in the first step) or malonate (in all the next steps) to the fatty acid synthase Shuttles the growing chain from one active site to another during the four-step reaction  Stoichiometry of synthesis of palmitate (16:0) 1. 7 acetyl-CoAs are carboxylated to make 7 malonyl-CoAs… using ATP. 7 acetyl-CoA + 7 CO2 + 7 ATP  7 malonyl-CoA + 7 ADP + 7 Pi 2. Seven cycles of condensation, reduction, dehydration, and reduction… using NADPH to reduce the -keto group and trans-double bond acetyl-CoA + 7 malonyl-CoA + 14 NADPH + 14 H+  palmitate (16-carbons) + 7 CO2 + 8 CoA + 14 NADP+ + 6 H2O 8 acetyl-CoA + 7 ATP + 14 NADPH + 14 H+  palmitate (16-carbons) + 8 CoA + 14 NADP+ + 7 ADP + 7 Pi + 6 H2O  Pathways for NADPH Production Major sources of cytosolic reducing power (NADPH) for fatty acid synthesis:  Acetyl-CoA Is Transported into the Cytosol for Fatty Acid Synthesis Acetyl-CoA is made in the mitochondria However, fatty acids are made in the cytosol So acetyl-CoA is transported into the cytosol as citrate, (after being combined with oxaloacetate) through a citrate transporter  FAS β-oxidation Anabolism Catabolism (see also slide 3) 10  Routes of synthesis of unsaturated fatty acids - Ratio of -6 to -3 fatty acids in diet important: if too high can lead to cardiovascular disease Essential Fatty Acids Fish Oils EPA AA DHA 11 Synthesis of Backbone of TAGs (Fats) and Phospholipids Most glycerol 3-phosphate comes from siphoning off dihydroxyacetone phosphate (DHAP) from glycolysis. – via glycerol 3-phosphate dehydrogenase Some glycerol 3-phosphate is made from glycerol. – via glycerol kinase – minor pathway in liver and kidney  Synthesis of Phosphophatidic Acid Occurs Before TAGs Phosphatidic acid is the precursor to TAGs and phospholipids. – fatty acids attached by acyl transferases – releases CoA Advantage of making phosphatidic acid: – It can then be made into triacylglycerol OR phospholipid. Phosphatidic Acid Can Be Modified to Form Phospholipids or TAGs Phosphatidic acid phosphatase (lipin) removes the 3-phosphate from the phosphatidic acid. – yields 1,2-diacylglycerol The third carbon is then acylated with a third fatty acid. – yields triacylglycerol Insulin stimulates conversion of dietary carbohydrates and proteins to fats. 15 Triacylglycerol Cycle: important in starvation 16  Glyceroneogenesis Glyceroneogenesis: similar to gluconeogenesis but active in adipose cells During lipolysis (stimulated by glucagon or epinephrine), glycolysis is inhibited. – So DHAP is not readily available to make glycerol 3-phosphate. And adipose cells don’t have glycerol kinase to make glycerol 3-phosphate on site. So cells make DHAP via glyceroneogenesis. 17  Thiazolidinediones: insulin sensitizers These agents known as “insulin sensitizers” decrease “insulin resistance” a phenomenon that occurs with high blood levels of free fatty acids and is one of the reasons obesity is related to Type 2 diabetes These agents are stimulators of PPAR (peroxisome proliferation activated receptor) a nuclear receptor which regulates the expression of genes involved in fat metabolism, specifically PEPCK but also affects other systems including GLUT-4 a glucose transporter in fat cells PPAR-peroxisome proliferation activated receptor stimulators 19 Biosynthesis of isoprenoids and cholesterol - occurs from acetate, via mevalonate, squalene: 20  Formation of Mevalonate from Acetyl- CoA Three acetyl-CoA are condensed to form HMG-CoA. HMG-CoA is reduced to form mevalonate. HMG-CoA reductase is a common target of cholesterol-lowering drugs statins inhibit  Fates of Cholesterol After Synthesis In vertebrates, most cholesterol is synthesized in the liver, then exported. - They are exported as bile acids, biliary cholesterol, or cholesteryl esters. - Bile is stored in the gall bladder and secreted into the small intestine after fatty meal. - Bile acids such as taurocholic acid emulsify fats; they surround droplets of fat, solubilizing them and increasing surface area for attack by lipases. Other tissues convert cholesterol into steroid hormones and so on.  Four Major Classes of Lipoprotein Particles Named based on position of sedimentation (density) in centrifuge: pure fat (TAG) is less dense than water - as TAG % increases, density decreases Composition varies between class of lipoprotein Includes four major classes: Synthesis Site Intestine Liver Liver Blood Liver LDL contains the highest amount of cholesterol 23 C = Cholesterol CE = Cholesterol Ester SEE PAGE 867-869 In general: LDL delivers C to tissues HDL delivers C to Liver 24 Cholesterol Uptake by Receptor-Mediated Endocytosis BLOOD LIPOPROTEINS: transport TG's and cholesterol from one tissue to another Chylomicrons and VLDL - main carriers of TG's -lipoprotein lipase in capillary walls (activated by apoC) degrades TGs the FFAs are oxidized or converted back to TGs and stored. Glycerol used for energy or stored as TGs. Chylomicron remnants are degraded and products (AA, FA, Cholesterol) reused LDL made from VLDL - provides cholesterol to tissues.- LDL receptor on cells take it up and release cholesterol this inhibits LDL receptor synthesis thus high levels of LDL will cause reduced cholesterol cellular uptake HDL - picks up cholesterol from tissues, transfers apoproteins and cholesterol. HDL is taken up by liver and cholesterol is released and may be repackaged as VLDL or converted into bile acids and secreted into bile. Atherosclerosis and Coronary Heart Disease correlated with serum cholesterol: deposition of cholesterol and its esters, as well as lipoproteins containing apo-B. Certain diseases increase VLDL and LDL including diabetes. - Elevated VLDL & normal LDL, or - Elevated LDL & normal VLDL, or - Both elevated An inverse relationship is observed between HDL and coronary heart disease! Low HDL or High LDL (VLDL) is bad !!! LDL:HDL cholesterol ratio best predictor. This is based on the roles of LDL in transporting cholesterol to tissues and HDL carrying cholesterol from the tissues Substitution of polyunsaturated and monounsaturated fatty acids helps lower blood cholesterol by the following mechanisms: Omacor- a combination of -3 PUFAs eicosapentaenoic (EPA) and docosahexanoic (DHA) acids; stimulate cholesterol excretion, increase LDL receptors, decrease formation of VLDL Hypolipidemics and Hypocholesterol Agents: Resins to absorb bile acids so conversion of cholesterol to bile acids increased - Cholestyramine (Questran®) HMG-CoA reductase inhibitors - Lovastatin (Mevacor®) / “statins- Rosuvastatin (Crestor ®) etc. Decrease VLDL synthesis - Fenofibrate (Tricor®) and Niacin (Nicotinic Acid) 26  Regulation of Cholesterol Metabolism Fate of Cholesterol 1.Biliary 2.Bile Acids (hydrophilic) 3.Esters to lipoproteins (hydrophobic) Steroid Hormones derived from cholesterol 28  Goals and Objectives Upon completion of this lecture at minimum you should be able to answer the following: ►Where is taking place the biosynthesis of fatty acids, what it requires, what enzymes and intermediates are involved? What is its outcome (product) and what is the stoichiometry of the process? ►What is the structure and the role of Acyl Carrier Protein ? Which are the major pathways for NADPH production? ►How is acetylCoA transported into the cytosol ? How is the anabolism of fatty acids controlled? Which are the major routes for unsaturated fatty acid biosynthesis and what is the impact of these acids on human health? ►Which is the key precursor of TAGs and phospholipids? What enzymes are involved in the biosynthesis of these lipid classes? ►What is the role of insulin in conversion of dietary carbohydrates and proteins to fats, what is the TAG cycle and what role is played by insulin sensitizers? ►Which are the key steps enzymes and intermediates involved into cholesterol biosynthesis and how we can inhibit and regulate them? ►Which are the fates of cholesterol after biosynthesis, how it is transported, internalized and what implications are for human health? ►Which steroid hormones are derived from cholesterol and where are they involved? 29  Drugs and Diseases ► Diseases: Hypercholesterolemia, Atherosclerosis and Coronary Heart Disease, Diabetes Type 2 with insulin resistance ► Drugs and supplements: linoleic, linolenic FAs (essential FAs), thiazolidinediones insulin sensitizers (Rosiglitazone (Avandia®), Pioglitazone (Actos®), statins (Lovastatin (Mevacor®), Rosuvastatin (Crestor®), etc.), 3 PUFAs eicosapentaenoic (EPA) and docosahexanoic (DHA) = Omacor®, Cholestyramine (Questran®), Fenofibrate (Tricor®), Niacin (B3, Nicotinic Acid) ► Blood analysis: cholesterol, cholesteryl esters, chylomicrons, VLDL, LDL, HDL