Fatty Acid Degradation Lecture Note

OUTLINE: Introduction and review Transport Glycogenolysis Glycolysis Other sugars Pasteur: Anaerobic vs Aerobic Exam-1 material Fermentations Pyruvate Krebs’ Cycle Oxidative Phosphorylation Exam-2 material Electron transport Chemiosmotic theory: Phosphorylation Fat Catabolism Exam-3 material diet storage Fatty acid Catabolism FOUR stages in the catabolism of lipids: Mobilization from tissues (mostly adipose) hormone regulated specific lipases glycerol Activation of fatty acids Fatty-acyl CoA Synthetase Transport carnitine Oxidation Rationale Saturated FA b-oxidation 4 steps: dehydrogenation, hydration, oxidation, thiolase energetics Unsaturated FA Odd-chain FA Ketone Bodies energetics Fatty Acid Degradation b-Oxidation: Enoyl-CoA hydratase • Catalyzed by two isoforms of enoyl-CoA hydratase: – soluble short-chain hydratase (crotonase) – membrane-bound long-chain hydratase, part of trifunctional protein (TFP) • Water adds across the double bond yielding alcohol on b carbon. • Specific for single trans-D2 double bond, no conjugation • Sometimes called crotonase • Analogous to fumarase reaction in the citric acid cycle crotonate – same stereo-specificity – Same enolic intermediate 1 Fatty Acid Degradation b-Oxidation: b-hydroxyacyl-CoA dehydrogenase b-Hydroxyacyl-CoA dehydrogenase • Catalyzed by b-hydroxyacyl-CoA dehydrogenase • The enzyme uses NAD cofactor as the hydride acceptor. • Only L-isomers of hydroxyacyl CoA act as substrates. • Analogous to malate dehydrogenase reaction in the citric acid cycle and the lactate dehydrogenase in fermentation Fatty Acid Degradation b-Oxidation: b-hydroxyacyl-CoA dehydrogenase b-Hydroxyacyl-CoA dehydrogenase • Catalyzed by b-hydroxyacyl-CoA dehydrogenase • The enzyme uses NAD cofactor as the hydride acceptor. • Only L-isomers of hydroxyacyl CoA act as substrates. • Analogous to malate dehydrogenase reaction in the citric acid cycle and the lactate dehydrogenase in fermentation 2 Fatty Acid Degradation b-Oxidation: Thiolase • Catalyzed by acyl-CoA acetyltransferase (thiolase) via covalent mechanism – The carbonyl carbon in b-ketoacyl-CoA is electrophilic. – Active-site thiolate acts as a nucleophile and releases acetyl-CoA. – Terminal sulfur in CoA-SH acts as a nucleophile and picks up the fatty acid chain from the enzyme. • The net reaction is thio-lysis of the carbon-carbon bond. Fatty Acid Degradation Mechanism b-Oxidation: Thiolase • The carbonyl carbon in b-ketoacyl-CoA is electrophilic. • Active-site thiolate acts as a nucleophile and releases acetyl-CoA. • Terminal sulfur in CoA-SH acts as a nucleophile and picks up the fatty acid chain from the enzyme. 3 Fatty Acid Degradation The b-Oxidation Pathway How much energy from palmitate? palmitic acid (C16 à C14) • For palmitic acid (C16) – Repeating the previous four-step process six more times (seven total) results in eight molecules of acetylCoA. • FADH2 is formed in each cycle (seven total). • NADH is formed in each cycle (seven total). • The 8 Acetyl-CoA molecules enters citric acid cycle and further oxidizes into CO2. – This makes more GTP, NADH, and FADH2 (8, 24, 8, respectively) • Electrons from all FADH2 (15) and NADH (31) enter the respiratory chain. Fatty Acid Degradation Energy from Fatty Acid Catabolism TABLE 17-1 Yield of ATP during Oxidation of One Molecule of Palmitoyl-CoA to CO2 and H2O Number of NADH or FADH 2 formed Number of ATP ultimately formed a Acyl-CoA dehydrogenase 7 FADH 2 10.5 β-Hydroxyacyl-CoA dehydrogenase 7 NADH Enzyme catalyzing the oxidation step β Oxidation Isocitrate dehydrogenase α-Ketoglutarate dehydrogenase Succinyl-CoA synthetase Succinate dehydrogenase Malate dehydrogenase Total 17.5 à77.5 ATP 31 Citric acid cycle 15 à22.5 ATP 8 NADH 20 8 NADH 20 8b 8 FADH 2 12 8 NADH 20 108 – 2 = 106* aThese calculations assume that mitochondrial oxidative phosphorylation produces 1.5 ATP per FADH2 oxidized and 2.5 ATP per NADH oxidized. bGTP produced directly in this step yields ATP in the reaction catalyzed by nucleoside diphosphate kinase (p. 516). *These 2 ”ATP” equivalents were expended in the activation by Fatty acyl–CoA synthetase. 4 Fatty Acid Degradation Each pass removes one acetyl moiety in the form of acetyl-CoA. Summary of the b-Oxidation Pathway Degradation of Unsaturated Fatty Acids 5 Degradation of Unsaturated Fatty Acids 9 oleic (C18:1 D9) linoleic (C18:2 D9,D12) TWO problems: 1. cis 2. often at odd carbon g-linolenic (C18:3 D6,D9,D12) 4 Both linoleic and g-linolenic (w6), as well as a-linolenic (w3; C18:3 D9,D12,D15) are essential fatty acids (EFA) 3 2 1 crotonate Fatty Acid Degradation Oleoyl-CoA (C18:1 D9) • Naturally occurring unsaturated fatty acids contain cis double bonds. – are NOT a substrate for enoyl-CoA hydratase 4 3 2 1 – Bond is cis and D3 • One additional enzyme is required for this: – D2,D3-enoyl-CoA isomerase: converts cis double bonds starting at carbon 3 to trans double bonds at carbon 2 • Mono-unsaturated fatty acids require only the isomerase. • You lose an FADH2 equivalent from first step of boxidation do to double bond……….. 4 3 2 1 crotonate 6 Fatty Acid Degradation cis H Monounsaturated Fatty Acids Forms an enolic intemediate B: trans ⦚ ⦚ ⦚ ⦚ ⦚ During first of five remaining cycles, acylCoA dehydrogenase step is skipped, resulting in 1 fewer FADH2. Fatty Acid Degradation Energy from Fatty Acid Catabolism TABLE 17-1a Yield of ATP during Oxidation of One Molecule of Palmitoyl-CoA Oleoyl-CoA to CO2 and H2O Number of NADH or FADH 2 formed Enzyme catalyzing the oxidation step Number of ATP ultimately formed a β Oxidation Acyl-CoA dehydrogenase (8-1=7; 1 lost due to isomerase) 7 FADH 2 β-Hydroxyacyl-CoA dehydrogenase Isocitrate dehydrogenase α-Ketoglutarate dehydrogenase Succinate dehydrogenase Malate dehydrogenase Total 20 à87.5 ATP 35 Citric acid cycle Succinyl-CoA synthetase 10.5 8 NADH 16 à24 ATP 9 NADH 22.5 9 NADH 22.5 9b 9 FADH 2 13.5 9 NADH 22.5 120.5 – 2 = 118.5* aThese calculations assume that mitochondrial oxidative phosphorylation produces 1.5 ATP per FADH2 oxidized and 2.5 ATP per NADH oxidized. bGTP produced directly in this step yields ATP in the reaction catalyzed by nucleoside diphosphate kinase (p. 516). *These 2 ”ATP” equivalents were expended in the activation by Fatty acyl–CoA synthetase. 7 Degradation of Unsaturated Fatty Acids 9 oleic (C18:1 D9) linoleic (C18:2 D9,D12) TWO problems: 1. cis 2. often at odd carbon g-linolenic (C18:3 D6,D9,D12) 4 Both linoleic and g-linolenic (w6), as well as a-linolenic (w3; C18:3 D9,D12,D15) are essential fatty acids (EFA) 3 2 1 crotonate Fatty Acid Degradation Isomerase moved dbl bond Linoleoyl-CoA (C18:2 D9,D12) S-CoA • Another additional enzyme is required for cis double bonds not at carbon 3: – reductase: reduces cis double bonds at D4 This round of b-oxidation did not use the acyl-CoA dehydrogenase (1 less FADH2 ) S-CoA OH O acyl-CoA dehydrogenase 4 2 1 3 • Polyunsaturated fatty acids require both enzymes. • Remember you lose 2 FADH2 equivalents from first step of b-oxidation & use an NADH for the reductase O S-CoA OH 2,4-dienoyl-CoA Reductase removes conjugation & one dbl bond O Not a substrate for crotonase (hydratase) 4 3 2 1 crotonate S-CoA OH Isomerase moved dbl bond O 4 2 1 3 • S-CoA OH OH S-CoA This IS a substrate for crotonase O 8 Fatty Acid Degradation Oxidation of Polyunsaturated Fatty Acids Results in 1 fewer FADH2 after isomerization, as the acyl-CoA dehydrogenase step is skipped and goes right to the hydratase. NADPH reduces the 2 conjugated unsaturated bonds to one transdouble bond between them; but its still at an odd carbon This bond already made with FADH2 already generated before reductase and isomerization. D3, D2- Fatty Acid Degradation Energy from Fatty Acid Catabolism TABLE 17-1a Yield of ATP during Oxidation of One Molecule of Linoleoyl-CoA Palmitoyl-CoA to CO2 and H2O Number of NADH or FADH 2 formed Enzyme catalyzing the oxidation step Number of ATP ultimately formed a β Oxidation Acyl-CoA dehydrogenase β-Hydroxyacyl-CoA dehydrogenase (8-1=7; 1 lost due to isomerase) 7 FADH 2 Isocitrate dehydrogenase α-Ketoglutarate dehydrogenase Succinate dehydrogenase 17.5 à85 ATP 34 Citric acid cycle Succinyl-CoA synthetase 10.5 (8-1=7; 1 used for reductase) 7 NADH 16 à24 ATP Malate dehydrogenase Total 9 NADH 22.5 9 NADH 22.5 9b 9 FADH 2 13.5 9 NADH 22.5 118 – 2 = 116* aThese calculations assume that mitochondrial oxidative phosphorylation produces 1.5 ATP per FADH2 oxidized and 2.5 ATP per NADH oxidized. b GTP produced directly in this step yields ATP in the reaction catalyzed by nucleoside diphosphate kinase (p. 516). *These 2 ”ATP” equivalents were expended in the activation by Fatty acyl–CoA synthetase. What would it be for g-linolenic (C18:D6,D9,D12)? 9 Degradation of Unsaturated Fatty Acids 9 oleic (C18:1 D9) linoleic (C18:2 D9,D12) TWO problems: 1. cis 2. often at odd carbon g-linolenic (C18:3 D6,D9,D12) Both linoleic and g-linolenic (w6), as well as a-linolenic (w3; C18:3 D9,D12,D15) are essential fatty acids (EFA) 4 3 2 1 crotonate Fatty Acid Degradation crotonate 4 3 2 1 g-linolenoyl-CoA (C18:3 D6,D9,D12) S-CoA O acyl-CoA dehydrogenase O FADH2 S-CoA S-CoA 2,4-dienoyl-CoA Reductase removes conjugation & one dbl bond D6 à lose NADH D9 à lose FADH2 D12 àlose NADH Not a substrate for crotonase (hydratase) OH S-CoA O O Isomerase moved dbl bond S-CoA OH S-CoA acyl-CoA dehydrogenase FADH2 O O S-CoA O S-CoA 2,4-dienoyl-CoA Reductase removes conjugation & one dbl bond O S-CoA Isomerase moved dbl bond O Not a substrate for crotonase (hydratase) S-CoA OH Not a substrate for crotonase (hydratase) S-CoA Isomerase moved dbl bond O S-CoA O 10 Fatty Acid Degradation Energy from Fatty Acid Catabolism TABLE 17-1a -Linolenoyl-CoA to 2CO2 Yield of ATP during Oxidation of One Moleculeof ofgPalmitoyl-CoA to CO and H2O Enzyme catalyzing the oxidation step Number of NADH or FADH 2 formed Number of ATP ultimately formed a 7 FADH 2 10.5 β Oxidation Acyl-CoA dehydrogenase β-Hydroxyacyl-CoA dehydrogenase (8-1=7; 1 lost due to isomerase) (8-2=6; 2 used for reductase) 6 NADH Isocitrate dehydrogenase α-Ketoglutarate dehydrogenase Succinyl-CoA synthetase Succinate dehydrogenase Malate dehydrogenase Total 17.5 à82.5 ATP 33 Citric acid cycle 16 à24 ATP 9 NADH 22.5 9 NADH 22.5 9b 9 FADH 2 13.5 9 NADH 22.5 115.5 – 2 = 113.5* aThese calculations assume that mitochondrial oxidative phosphorylation produces 1.5 ATP per FADH2 oxidized and 2.5 ATP per NADH oxidized. b GTP produced directly in this step yields ATP in the reaction catalyzed by nucleoside diphosphate kinase (p. 516). *These 2 ”ATP” equivalents were expended in the activation by Fatty acyl–CoA synthetase. Degradation of OddChain Fatty Acids 11 Fatty Acid Degradation Oxidation of Odd-Numbered Fatty Acids • Most dietary fatty acids are even-numbered. • Many plants and some marine organisms also synthesize odd-numbered fatty acids. • The metabolism of oxidation of odd-chain fatty acids CONVERGES with that of some amino acids • Details will be discussed with amino acids Ketone Bodies 12 Ketone Body Degradation What are Ketone Bodies? “Fat burns in the flame of sugar” Fatty Acids NADH Acetyl-CoA NADH FADH2 Pyruvate CO2 This is ANABOLISM, which we’ll discuss in a few weeks Ketone Body Degradation What are Ketone Bodies? “Fat burns in the flame of sugar” Fatty Acids NADH NADH FADH2 Pyruvate CO2 Acetyl-CoA Ketone Bodies This is ANABOLISM, which we’ll discuss in a few weeks 13 Ketone Body Degradation • Entry of acetyl-CoA into citric acid cycle requires oxaloacetate. • When oxaloacetate is depleted, acetyl-CoA is converted into ketone bodies in the LIVER. Ketone Bodies – frees coenzyme A for continued b oxidation • Three forms of ketone bodies can leave the liver: acetone, acetoacetate, and bhydroxybutyrate. • Therefore, the anabolism of Ketone Bodies is connect to catabolism of fatty acids and sugars • For now, we will discuss Ketone Body catabolism 14