Biochemistry Fatty Acid Synthesis 2023 PDF

Fatty Acid Synthesis Lecture 35 Reference: Lieberman and Peet, Chapter 31 (pp 679 – 688) Bluefield University – VCOM Campus MABS Program Biochemistry Jim Mahaney, PhD 1 Lecture Objectives a. Recall the physiological role of fatty acid synthesis and relate the cellular conditions that favor lipoge...

Fatty Acid Synthesis Lecture 35 Reference: Lieberman and Peet, Chapter 31 (pp 679 – 688) Bluefield University – VCOM Campus MABS Program Biochemistry Jim Mahaney, PhD 1 Lecture Objectives a. Recall the physiological role of fatty acid synthesis and relate the cellular conditions that favor lipogenesis. b. Recall the mechanism by which mitochondrial acetyl CoA moves out to the cytoplasm to initiate fatty acid synthesis and relate how cytoplasmic oxaloacetate is converted back to pyruvate. Include the key enzymes citrate lyase and malic enzyme, and the products they produce. c. Recall the conversion of acetyl CoA to malonyl CoA as the committed, rate-limiting step for fatty acid synthesis. d. Recall the five stages of fatty acid synthesis, relate the role of the fatty acid synthase complex and the central role of the acyl carrier protein (ACP). A detailed e. Relate the process of fatty acid elongation. f. Relate the process by which double bonds are introduced into fatty acids. g. Relate how human fatty acid desaturase enzymes can only producing fatty acids with double bonds at three specific places. Interpret why humans must consume two essential fats for optimal health. h. Compare and contrast fatty acid synthesis and fatty acid oxidation and relate how cells keep these two processes separated so that newly synthesized fatty acids are not immediately re-oxidized after synthesis. i. Recall the mechanisms of regulation of fatty acid synthesis, and specifically the regulation of Acetyl CoA carboxylase. j. Relate the function of the AMP-kinase (AMPK) and interpret why the activated AMPK inhibits biosynthesis reactions and activates catabolic reactions. 2 2 Objective A Lipogenesis: Fatty Acid Synthesis • Dietary lipid provides lipid species for cells and excess is stored. • When more lipid is needed or blood glucose exceeds body’s needs, lipids are synthesized by the body: lipogenesis • Liver is the major site of lipogenesis. • • Glucose is the major starting material, fatty acids à triglycerides Fats exported to blood as very low density lipoprotein (VLDL) • FAs synthesized in the cytosol by a different pathway than FA oxidation. • Lipogenesis activity is greatest under an insulin signal • • • High blood glucose Plenty of energy from glycolysis / OxPhos TCA Cycle intermediate citrate will leave the mitochondria to start • During fasting / glucagon signaling, lipogenesis activity is greatly diminished. • Liver is moving glucose out to the blood and not using glucose for glycolysis 3 3 Objective B Conversion of Glucose to Cytosolic Acetyl CoA • Under and insulin signal, glycolysis / OxPhos gives the liver sufficient energy. Excess glucose à pyruvate is directed away from the TCA cycle by citrate efflux. • Acetyl CoA is the starting material for FA synthesis, but acetyl CoA cannot cross the inner mitochondrial membrane. • Acetyl CoA condenses with oxaloacetate (OAA) to form citrate (first reaction of the TCA cycle) • Citrate is transported out of the mitochondrion 4 4 Objective B Regeneration of Pyruvate • In the cytosol, citrate lyase converts citrate back to acetyl CoA and OAA • OAA is reduced to malate by cytosolic malate dehydrogenase using NADH • Malate is oxidatively decarboxylated by malic enzyme forming NADPH, pyruvate and CO2. • Pyruvate reenters the mito matrix and can be reconverted to OAA by pyruvate carboxylase or to acetyl CoA by PDH as needed. • Notice the induced enzymes…patients with chronically high carbohydrate intake will need this pathway more and will have higher levels of these enzymes. 5 5 Objective B Sources of NADPH for FA Synthesis • NADPH provides the reducing power needed to synthesize hydrocarbon chains • In addition to malic enzme, NADPH is also provided by the pentose phosphate pathway. • This is a key clinical point…PPP deficiency? Malic enzyme can help supply some NADPH. 6 6 Objective C Conversion of Acetyl CoA to Malonyl CoA • The condensation of acetyl groups is endergonic so the first step in FA synthesis is the “activation” of acetyl CoA by is carboxylation to form malonyl CoA, the major intermediate for fatty acid chain formation. • This carboxylation is catalyzed by acetyl CoA carboxylase – a metabolically irreversible reaction and the rate limiting step of FA synthesis: Why? • Commits acetyl CoA to FA synthesis versus other paths. • Consumes energy • Malonyl CoA contributes the 2-carbon unit to the growing FA chain and the extra carboxyl group is a leaving group (CO2) 7 7 Objective D The Fatty Acid Synthase Complex • Palmitate is synthesized by the repetitive addition of two carbon units • derived from malonyl CoA to a growing acyl chain • followed by the reduction of each condensation product by NADPH • Fatty Acid Synthase: a single multifunctional protein with 7 different enzyme activities in a single polypeptide chain which forms a homodimer • A protein cofactor called acyl carrier protein (ACP) is also required. It contains phosphopantetheine, a prosthetic group very similar to coenzyme A. • Malonyl CoA attaches to –SH on ACP 8 8 Objective D FA Synthesis Occurs in 5 Stages • Loading: one acetyl CoA then one malonyl CoA transferred onto ACP forming acetyl ACP then malonyl ACP • Acetyl CoA only used at the beginning to start process • Condensation: acetyl group transfers onto malonyl group, displacing the CO2 but lengthening the chain by 2 carbons • Reduction: the β-keto group is reduced to a β-hydroxyl group by NADPH • Dehydration: the elements of water are removed across the α-β carbon atoms forming an α-β trans double bond • Reduction: the double bond is reduced using NADPH, produces saturated, fully reduced carbon bonds 9 9 Objective D FA Synthesis: The BIG Picture Note the use of one acetyl CoA to initiate the process, then only malonyl CoA after that. 10 10 Some Really Cool Gobbledegook Timm Maier, Marc Leibundgut, Nenad Ban Science 05 Sep 2008: Vol. 321, Issue 5894, pp. 1315-1322 DOI: 10.1126/science.1161269 By Kosigrim at English Wikipedia - Transferred from en.wikipedia to Commons., Public Domain, https://commons.wikimedia.org/w/index.php?curid=34130940 11 11 The Condensation Step • ACP is site of FA synthesis • Carboxyl group activates methylene carbon for new bond with neighbor acetyl group • Two groups condensed into one. Note presence of β-keto group. • Site of action for reduction and dehydration steps • Note elimination of CO2: respiration à energy consumption for anabolism 12 12 The Reduction Dehydration Reduction Steps • Ketone carbonyl is reduced to an alcohol group. • The elements of water are removed (H and OH) during dehydration. • Electrons needed to form new C-H bond come from NADPH hydride, extra proton forms other C-H bond • Note: energy content of FA is increasing as it becomes more reduced. • Requires energy input: stores the energy for β-oxidation later. 13 13 Objective D The Cycle Repeats… • This cycle is repeated 7 times forming palmitate (16 carbon chain), uses malonyl ACP each time. • The final reaction of FA synthesis is the cleavage of palmitoyl ACP by a thioesterase to release the free fatty acid • Palmitate is rapidly converted to palmitoyl CoA by acyl CoA Synthetase • Fatty acid synthesis is energy efficient using only 7 ATP to form 7 malonyl CoA and 2 ATP equivalents to form the acyl CoA 14 14 Objective E Elongation of Fatty Acids • Cells contain many types of FAs besides palmitate. • Ex: stearate (C18:0) and oleate (18:1). • The elongation and desaturation enzymes are localized to the cytoplasmic face of the endoplasmic reticulum • The elongase uses fatty acyl CoA as substrate and extends the chain two carbons at a time using malonyl CoA condensation followed by reduction, dehydration and reduction using NADPH in analogous fashion to FA synthase. 15 15 Polyunsaturated Fatty Acids (PUFA) Objective F • Unsaturated FA contain one or more double bonds • Single double bond? Monounsaturated FA • Two or more double bonds? Polyunsaturated FA • Enzymes that catalyze double bond formation in single bonded chains are desaturases • require NADH and O2 • involve an ER electron transport chain which transfers electrons and protons 16 16 Objective G FA desaturation in humans • Human desaturases can not introduce double bonds in a saturated fat past carbon 10. AND… only at positions C5, C6 and C9 • Popular examples: palmitic acid to palmitoleic acid (16:1, Δ9) and stearic acid to oleic acid (18:1, Δ9) • Humans can’t place double bonds between carbon 10 and the ω-carbon (terminal methyl group) • Problem - eicosanoid synthesis: • 20 carbon fatty acids (ex: arachadonic acid) with double bonds at C5, C8, C11, C14 • Key signaling molecules in humans (covered later) • Need PUFAs with double bonds 3 carbons from the methyl end (ω3) and 6 carbons from the methyl end (ω6) 17 17 Objective G Essential FAs and PUFA synthesis • Example: arachidonic acid • Start with an essential FA: linoleic acid • FA we can’t make, must consume • Usually plant oils or fish oil • Introduce those double bonds that we can make • Lengthen the chain to move the bonds to the high number carbon positions • Introduce more double bonds at positions where we can 18 18 Objective H Comparison of FA Synthesis and FA Oxidation • Location: Synthesis cytosol Oxidation mitochondria • Thioester carrier: malonyl CoA acetyl CoA • Enzymes: E complex individual Es • electron carriers: NADPH NADH 19 19 Objective H Review: Protect Those Newly Formed Fatty Acids! • Why aren’t new FAs immediately oxidized by the βoxidation pathway? • FAs synthesized in cytosol, FAs oxidized in mitochondria • Malonyl CoA is the key intermediate for FA synthesis • Malonyl CoA inhibits CPTI, the primary player in getting FAs into the mito for oxidation. 20 20 Regulation of FA Synthesis: Acetyl CoA Carboxylase Objective I • Allosteric • Feed forward and feed back inhibition • Phosphorylation • AMP-dependent kinase à inactive • Insulin à active • Inducible • Cells can alter the amount of enzyme present Rate-limiting step of FA synthesis: committed step…Lots of regulation! 21 21 Objective J AMP-dependent Protein Kinase (AMPK) • AMP is a major signal for a low ATP level. • AMP will activate the AMPK, which will phosphorylate target proteins. • Catabolic pathways activated: make ATP • Anabolic pathways inhibited: stop synthesis • Low AMP turns “off” AMPK and turns “on” the opposing protein phosphatase that reverses these metabolic changes 22 22 Sample Question Which enzyme commits acetyl CoA to fatty acid synthesis? a. Pyruvate dehydrogenase b. Acetyl CoA carboxylase c. Citrate lyase d. Malic enzyme 23 And Another One Which enzyme if deficient in a patient would prevent fatty acid synthesis? a. Pyruvate dehydrogenase b. Acetyl CoA carboxylase c. Citrate lyase d. Malic enzyme e. phosphoenolypyruvate carboxy kinase 24 Sample Question A patient has a disorder in his carnitine-palmitoyl tranferase I enzyme such that it does not recognize malonyl CoA. How will this defect affect fatty acid metabolism in his liver? a. His cells will no longer oxidize fatty acids for energy. b. His cells will likely oxidize newly synthesized fatty acids. c. His liver will store excess fatty acids leading to fatty liver syndrome. d. Fatty acyl CoA will not be converted to fatty acyl-carnitine by CPT-1 25 Sample Exam Question A young child presents with a metabolic disorder manifesting as an inability to synthesize fatty acids. Specific clinical tests indicate that the enzymes in the fatty acid synthesis pathway are all present and functional, and cytoplasmic acetyl CoA and malonyl CoA levels are normal. A defect in which enzyme could cause this problem? a. Acetyl CoA carboxylase b. Carnitine palmitoyl transferase I c. Citrase lyase d. Glucose 6-phosphate dehydrogenase 26 Yet Another One A patient refuses to consume linoleic acid. How can this patient meet his body’s need for arachidonic acid? a. His body will simply synthesize it de novo. b. He can consume α-linolenic acid to synthesize linoleic acid. c. He can consume DHA instead. d. He can consume EPA instead. e. He can consume arachidonic acid instead. 27 Thank you! 28 28