Fatty Acid Catabolism2024.ppt

Fatty acid catabolism Marc A. Ilies, Ph. D. Lehninger - Chapter 17 [email protected]; lab 517, office 517A (Tu, Fr 3-5) For questions, comments please use the discussion tool in Canvas ©MAIlies...

Fatty acid catabolism Marc A. Ilies, Ph. D. Lehninger - Chapter 17 [email protected]; lab 517, office 517A (Tu, Fr 3-5) For questions, comments please use the discussion tool in Canvas ©MAIlies2024 1  Fatty acids and fats: overview - fats: esters of glycerin with fatty acids - highly reduced structures, similar to hydrocarbons → high energy/g; in liver and heart provide ~ 80% of the total energy consumed - hydrophobic, inert → segregate from water, thus easy to store (lipid droplets); do not raise the osmolarity of the cell → can be stored in large amounts in cells without risk of undesired chemical reactions; - sole source of energy for hybernating animals and migratory birds 2  Digestion, mobilization, and transport of fats (specialized tissue) Taurocholic acid → back 3  Structure of chylomicrons - Size : 100 – 500 nm - Composition: - triacylglycerols (80%), - phospholipids, - cholesterol, - cholesterol esters - apolipoproteins (lipid-binding proteins) - we distinguish various combinations of lipid and proteins: - chylomicrons - VLDL - VHDL - protein moieties of lipoproteins are recognized by receptors on cell surfaces 4  Mobilization of triacylglycerols stored in adipose tissue - binding of epinephrine on their receptors on the surface of adipocytes stimulates adenylyl cyclase; cAMP produced will activate PKA Epinephrine - PKA will phosphorylate perilipin on the surface of lipid droplets, making the fats accessible to a hormone- sensitive lipase (HSL), also activated by PKA; the lipase will start hydrolyzing the triglycerides into fatty acids and glycerol - Fatty acids will leave adipocytes and will be transported by serum proteins such as albumin to muscles, where will be used for energy generation 5  Metabolism of Glycerol - glycerol accounts to ~ 5% of total energy of fats; energy harvested through conversion into glyceraldehyde 3-phosphate, followed by normal glycolytic pathway: All phosphorylated species are negatively charged; they are trapped into cytoplasm 6  Fatty acid “activation” prior to oxidation - enzymes of fatty acid oxidation in animal cells are located in the mitochondrial matrix; in order to undergo oxidation fatty acids must be transported from cytoplasm into mitochondria; first step is their conjugation with CoA: - conjugation is made highly exothermic by the release of pyrophosphate, further hydrolyzed to two phosphates: ∆G’0total = -34 kJ/mol 7  Fatty acid transport into mitochondria - carnitine acts as a fatty acid shuttle between cytosol and interior of mitochondria : - links two separate acetylCoA pools: cytosolic and mitochondrial; conversion of a fatty acid into a carnitine ester commits the fatty acid molecule to oxidation in mitochondria: β-oxidation 8  Oxidation of fatty acids - in humans β-oxidation of fatty acids occurs primarily in the mitochondria and comprises three stages: Stage 1: sequential β-oxidation rounds to generate acetylCoA Stage 2: oxidation of acetylCoA to CO2, FADH2, NADH, using citric acid cycle Stage 3: transfer of electrons from FADH2, NADH to O2 with ATP generation 9 Stage 1: β-Oxidation of saturated fatty acids Must have a trans substrate 10  β-Oxidation of saturated fatty acids: energy balance - complete energy yield from palmitic (C16) acid to acetyl CoA: Rounds of -oxidation: (n/2) –1 where n = # of carbons 7 Rounds of -oxidation 7 NADH 21 ATP 7 FADH2 14 ATP 96 ATP 8 Acetyl CoA 131 ATP - 2 ATP (from step before carnitine) 129 ATP (net) This procedure works only for even numbered saturated fatty acids 11 12  Energy harvesting: fatty acid vs sugar - Complete Energy Yield From Hexanoic (C6) Acid to Acetyl CoA Rounds of -oxidation (n/2) –1 where n = # of carbons 2 Rounds of -oxidation 2 NADH 6 ATP 2 FADH2 4 ATP 36 ATP 3 Acetyl CoA 46 ATP - 2 ATP (from carnitine step) 44 ATP (net) A 6 carbon fatty acid yields 44 ATP while a 6 carbon sugar (glucose) can yields 38 ATP (at best). 13  14 21 24 24 8 16 24 131 Our conv: 1 NADH = 3ATP, 1 FADH2 = 2 ATP 14  Monounsaturated Fatty Acid Oxidation additional isomerisation step 15 Polyunsaturated Fatty Acid Oxidation * * Requires reducing power, thus less energy is produced compared to saturated fatty acids 16 Mono/polyunsaturated Fatty Acid Oxidation: summary Most naturally occurring unsaturated fatty acids have the cis conformation. This presents several problem for -oxidation: 1. Sometimes an intermediate with a double bond between C-3 and C-4 is produced (normally it’s between C-2 and C3 and required for enoyl CoA hydratase.) SOLUTION :Isomerase 2. If the cis bond of intermediate is between C-4 and C-5 a trans-2 cis –4 intermediate is produced by the dehydrogenase SOLUTION: A Reductase reduces the 2,4 bond , followed by the isomerase to produce the normal -oxidation substrate. 3.Thus, two additional enzymes are need in certain instances of beta oxidation. This occurs with a loss of total energy output compared to fully saturate acids because the reductase uses NADPH. 17  Oxidation of odd-number fatty acids - odd number fatty acids common in lipids from plants, marine organisms; they will undergo normal β-oxidation yielding propionyl-CoA in the end; this C3 fragment is then converted to a C4 one (succinyl-CoA): → cannot be used to produce glucose (C6) from ingested odd chained fatty acids. 18  Oxidation of odd-number fatty acids - coenzyme B12 catalyzes a key isomerization step: 19  B12 needed for: Can replace this 1. Methionine synthesis group with CN, OH or CH3 2. Succinyl-CoA synthesis from methylmalonyl-CoA mutase Vit B12: Cobalamine Synthesized by microbes Not by plants or animals 12/2006: FDA approved the use of hydroxocobalamin for cyanide poisoning 20  Regulation of fatty acid oxidation - high glucose levels results in the stimulation of the enzyme which synthesizes the first fatty acid precursor molecule, malonyl-CoA: Cytosol - the product of the first step in fatty acid synthesis (malonyl-CoA) inhibits beta-oxidation by blocking the transport of fatty acids into mitochondria 21  Ketone bodies – alternative fuel to sugars - acetyl-CoA formed in the liver in large amounts from fatty acid oxydation can enter citric acid cycle or can be transformed into “ketone bodies” for the use of other tissues (muscles, renal cortex, brain (when glucose is unavailable) Acetoacetate, acetone and - hydroxybutyrate are ketone bodies (acids) their accumulation results in ketoacidosis (ketosis). Spontaneous in humans 22  Ketone bodies – alternative fuel to sugars - use of ketone bodies for energy by conversion to acetyl CoA in the peripheral (non- hepatic) tissues: This enzyme does not exist in liver otherwise it could not supply ketone bodies to the periphery 23  Ketone bodies formation and export from the liver - starvation and untreated diabetes mellitus lead to overproduction of ketone bodies: 24  Ketone bodies - summary - liver acetyl-CoA from oxidation can take two paths: 1) Enter TCA or 2) Ketone Bodies - which path taken, 1 or 2, is ultimately determined by oxaloacetate (OAA). OAA levels drop in starvation: Why? OAA is drained off for gluconeogenesis !! If OAA is low then acetyl-CoA can’t enter TCA to produce energy. Instead, KBs are formed from acetyl-CoA; liver will perform fatty acid catabolism to acetyl-CoA and use all acetyl-CoA to produce KBs used to feed the brain and tissues. - normal hepatic production of ketone bodies is low but in prolonged starvation KB production increases. Diabetic Ketoacidosis: Untreated Diabetes Mellitus → no insulin → glucose can’t get into cells (no insulin = no GLUT2) and accumulates in blood while the cells starve. Although insulin is low, glucagon is normal and glycogen breaks down, further exacerbating hyperglycemia. Also, glucagon stimulates -oxidation so even more KBs formed!! Peripheral tissues can’t use up KBs fast enough with net result of acidosis. 25 26  Goals and Objectives Upon completion of this lecture at minimum you should be able to answer the following: ►What are fats, which are their main particularities and advantages for their use as energy storage compounds in cells? ►Which are the main steps of digestion, mobilization and transport of fats? ►Which are the main components of chylomicrons and what other fat complexes you know; what is their physiologic relevance? ►Which are the main steps, proteins, enzymes and intermediates involved in mobilization of fatty acids from adipose tissue? ►How is glycerol metabolized, which intermediates and enzymes are involved? ►Which are the main steps, processes and enzymes involved in fatty acid activation, transport, and oxidation (saturated (odd/even number of C atoms), unsaturated (mono/polyunsaturated)? ►What is the energetic balance of fatty acid oxidation as a function of chain length, (with practical examples)? How does fatty acid oxidation compare with oxidation of sugars of same backbone size? ►How is fatty acid oxidation regulated, what intermediates, enzymes, and hormones are involved? ►What are ketone bodies, representatives, how are they synthesized, where, from which intermediates and what enzymes are involved, what is their impact under normal and diseased states? 27  Drugs and diseases ►Diseases and conditions: diabetes, diabetic ketoacidosis (aka ketosis,= a metabolic acidosis) ►Drugs and vitamins: epinephrine (adrenaline), vitamin B12, hydroxocobalamin ►Hormones: epinephrine ►Metabolites and other blood components to be analyzed: chylomicrons, VLDL, LDL, HDL, ketone bodies 28