Lipid Metabolism Lecture 5 PDF

BMOL20110 Biomolecular Sciences Lecture 26. Lipid metabolism, Beta Oxidation and Fatty Acid Synthesis 13 November 2024 Jens Rauch, PhD School of Biomolecular and Biomedical Science Systems Biology Ireland Email: [email protected] Phone...

BMOL20110 Biomolecular Sciences Lecture 26. Lipid metabolism, Beta Oxidation and Fatty Acid Synthesis 13 November 2024 Jens Rauch, PhD School of Biomolecular and Biomedical Science Systems Biology Ireland Email: [email protected] Phone: +353-(0)1-716 6337 @jensrauch 1  Stage 2 Biomolecular and Biomedical Science - Academic Year 2024-25 BMOL20110 Biomolecular Sciences Module Co-ordinator: Assist. Prof. Jens Rauch email:[email protected] tel: 6337 Trimester LECTURES MONDAYS 13.00-14.00 B-H221-SCH Timetable 1 WEDNESDAYS 13.00-14.00 B-H221-SCH FRIDAYS 11.00-12.00 A-H2.18-SCH Week Date Day Time Lecturer 09-Sep-24 Mon 13:00 The physical basis of life: biomolecular interactions D. O'Connell 1 11-Sep-24 Wed 13:00 Amino acids, the peptide bond, protein primary structure D. O'Connell 13-Sep-24 Fri 11:00 Secondary structure, fibrous proteins, tertiary structure S. Nathwani 16-Sep-24 Mon 13:00 Diversity of protein functions: e.g. Insulin, haemoglobin D. O'Connell 2 18-Sep-24 Wed 13:00 Antibodies, structure, function and applications D. O'Connell 20-Sep-24 Fri 11:00 Molecular motors, proteins in cell organization and movement S. Nathwani 23-Sep-24 Mon 13:00 Protein dysfunction and disease; clinical analyses S. Nathwani 3 25-Sep-24 Wed 13:00 Protein characterisation, sequencing and structure determination S. Nathwani 27-Sep-24 Fri 11:00 Questions & Answers #1 D O'C and SN 30-Sep-24 Mon 13:00 Midterm Assessment #1 D O'C and SN 4 02-Oct-24 Wed 13:00 Carbohydrate structure (monosaccharides, polysaccharides, glycosidic bond) S. Nathwani 04-Oct-24 Fri 11:00 Carbohydrates in cell-cell interactions, proteoglycans, bacterial cell wall S. Nathwani 07-Oct-24 Mon 13:00 Lipid structure: fatty acids, phospholipids, sphingolipids S. Nathwani 5 09-Oct-24 Wed 13:00 Lipid structure: cholesterol and steroid hormones S. Nathwani 11-Oct-24 Fri 11:00 Membranes and membrane proteins S. Nathwani 14-Oct-24 Mon 13:00 Transporters, ion channels, receptors S. Nathwani 6 16-Oct-24 Wed 13:00 Introduction to Enzymes; coenzymes and isoenzymes M. Worrall 18-Oct-24 Fri 11:00 Enzyme structure and specificity M. Worrall 21-Oct-24 Mon 13:00 Mechanisms of rate enhancement and basic kinetics M. Worrall 7 23-Oct-24 Wed 13:00 Enzymes Inhibition: Enzymes as targets in disease M. Worrall 25-Oct-24 Fri 11:00 Regulation of enzyme activity M. Worrall 28-Oct-24 Mon 13:00 No Lecture: Bank Holiday 8 30-Oct-24 Wed 13:00 Questions & Answers #2 MW and SN 01-Nov-24 Fri 11:00 Midterm Assessment #2 MW and SN 04-Nov-24 Mon 13:00 Introduction to metabolism J. Rauch 9 06-Nov-24 Wed 13:00 Glycolysis and gluconeogenesis J. Rauch ➜ 08-Nov-24 Fri 11:00 TCA cycle J. Rauch 11-Nov-24 Mon 13:00 Electron transport chain and oxidative phosphorylation J. Rauch 10 13-Nov-24 Wed 13:00 Lipid metabolism, beta oxidation J. Rauch 15-Nov-24 Fri 11:00 Amino acid metabolism J. Rauch 18-Nov-24 Mon 13:00 Regulation and integration of metabolism, hormonal regulation of metabolism J. Rauch 11 20-Nov-24 Wed 13:00 Introduction to immune system D. Costello 22-Nov-24 Fri 11:00 Innate immunity: Pathogen recognition D. Costello 25-Nov-24 Mon 13:00 Overview of innate and adaptive immune response D. Costello 12 27-Nov-24 Wed 13:00 Questions & Answers #3 JR and DC 29-Nov-24 Fri 11:00 Midterm Assessment #3 JR and DC Any questions, please contact me after the lectures or by email [email protected] I will collect all questions and address them again in the Q&A session on Nov 27th. 2  Key Metabolic Pathways L26 L23 L27 L24 L25 3  ATP. Although chemiosmotic coupling escaped detection for many years, the RECAP - High-energy electrons, generated during the citric acid cycle, vast majority of living organisms use this mechanism to generate ATP. The source of the electrons that power the proton pumping differs widely power the production of ATP between different organisms and different processes. In aerobic respira- tion in mitochondria and aerobic bacteria, the electrons are ultimately derived from glucose or fatty acids. In photosynthesis, the required outer mitochondrial membrane inner mitochondrial membrane ATP synthase Conversion of Energy + + + + H H H H 460 Chapter 14 Energy Generation in Mitochondr energy in the form of high-energy electrons Figure 1 electron- In oxida transport e– Figure 14–7 High-energy electrons, NADH t chain H+ 2 H2O generatedNADHduring+the ½ O citric 2 + H acid + cycle, NAD+ + H2O in the m OUT power the production of ATP. Pyruvate NADH ATP ATP of ADP NAD + and fatty acids enter the mitochondrion IN (bottom), are converted to acetyl CoA, electron O2 O2 IN ADP + Pi ADP + Pi and are then metabolized by the citric NADH + acid cycle, which reduces NAD + to NADH citric OXIDATIVE PHOSPHORYLATION acid CO2 OUT CO2 (and FAD to FADH2, not shown). In the energy-conversion cycle process of oxidativeprocesses in membrane phosphorylation, high-energy electrons from NADH electro (and FADH2) are then passed along the electron-transport chain in the inner phyll. A acetyl CoA membrane to oxygen (O2). This electron iron, an ADP + Pai proton gradient transport generates ATP pyruvate fatty acids to mak across the inner membrane, which is used to drive the production of ATP by ATP energy in the form of high-energy synthase. In this diagram, the exact phosphate ratios bonds The E ! of “reactants” and “products” have been omitted. For example, we will see shortly Inner pyruvate fatty acids that it requires four electrons from four NADH molecules to convert O2 to The e FOOD MOLECULES FROM CYTOSOL two H2O molecules. ECB3 m14.11/14.08 oxidati chondr which membr (FADH2 not shown in this scheme) of the ECB3 m14.10/14.07  Membrane Is Also Driven by the Electrochemical Proton Gradient The synthesis of ATP is notECB3 m14.15/14.12 the only process driven by the electrochemi- RECAP - Cells have evolved systems for harnessing the energy required cal proton gradient. In mitochondria, many charged molecules, such as for life. pyruvate, ADP, and Pi, are pumped into the matrix from the cytosol, while others, such as ATP, must be moved in the opposite direction. Carrier proteins that bind these molecules can couple their transport to the ener- getically favorable flow of H+ into the mitochondrial matrix. Pyruvate and Electrons are transferred through three ATP Synthase converts the energy of the inorganic phosphate (Pi), for example, are individually co-transported respiratory enzyme complexes in the inner electrochemical proton gradient into inward with H+ as the latter moves down its electrochemical gradient, into the matrix. mitochondrial membrane. chemical bond energy (ATP) Mitochondria and Oxidative Phosphorylation 461 Figure 14– coupling d H+ H+ H+ Figure 14–9 Electrons are transferred energy of H+ INTERMEMBRANE cytochrome c through three respiratory H+ H+ enzyme H + H+ gradient in SPACE complexes in the inner mitochondrial H+ vice versa. H+ H+ + H membrane. The relative H+ size H+ synthesize H+and shape H+ + H+ of each complex are indicated. During the H H+ gradient (A H+ c H+ H+ H + transfer of electrons from NADH to oxygen electrochem Q (red lines), protons derived from water are (B). The dir inner instant dep mitochondrial pumped across the membrane from the membrane matrix into the intermembrane space by change ( G each of the respiratory enzyme complexes the couple 2 e– (Movie 14.2). Ubiquinone (Q) and MATRIX H+ H+ across the MATRIX H + rotor ATP from A ubiquinone H + H2O cytochrome c (c) serve as mobile carriers H+ ATP H+ H+ ATP NADH that ferry electrons from one complex to the H+ electrochem Pi + ADP H+ a certain le 2 H+ + ½ O2 next. stator ATP NAD+ H+ H+ the matrix ATP Pi + ADP drive ATP p NADH cytochrome cytochrome hydrolyzed 10 nm dehydrogenase b-c1 complex oxidase complex complex the gradien (A) ATP SYNTHESIS (B) ATP HYDROLYSIS shown in M Why would ATP Yields* synthase also run in chain results in the pumping of protons across the membrane out of the mitochondrial matrix and into the space between the inner and outer reverse? mitochondrial membranes (see Figure 14–9). ECB3 m14.26/14.09 Later in the chapter we review the detailed molecular mechanisms that QUESTION 14–3 couple electron transport to the movement of protons. For now, we focus ECB3 m14.19/14.13 on the consequences of this nifty biological maneuver. First, the active When the drug dinitrophenol (DNP) pumping of protons generates a gradient of H+ concentration—a pH gra- is added to mitochondria, the inner dient—across the inner mitochondrial membrane, where the pH is about membrane becomes permeable 0.5 unit higher in the matrix (around pH 7.5) than in the intermembrane to protons (H+). In contrast, when space (which is close to 7, the same pH as the cytosol). Second, proton the drug nigericin is added to pumping generates a membrane potential across the inner mitochondrial mitochondria, the inner membrane membrane, with the inside (the matrix side) negative and the outside becomes permeable to K+. (A) How does the electrochemical proton positive as a result of the net outflow of H+. gradient change in response to * Per mol glucose As discussed in Chapter 12, the force driving the passive flow of an ion  Today’s Class & Learning Objectives Triacylglycerols are highly concentrated energy stores The use of fatty acids requires three stages of processing Fatty Acid Catabolism feeds into the TCA cycle Fatty acids are synthesized in four steps by Fatty Acid Synthase Stryer, Biochemistry, Chapter 22 6 Alberts, Molecular Biology of the Cell, Ch. 2 molecules that are produced by the citric acid cycle; NADH, the reduced form of water and form large lipid droplets in the the NAD+/NADH electron carrier system (see Figure 2–36). In addition to three specialized fat cells (adipocytes) in which molecules of NADH, each turn of the cycle also produces one molecule of FADH2 they are stored. (C) The fatty acid oxidation (reduced flavin adenine dinucleotide) from FAD (see Figure 2–39), and one mol- cycle. The cycle is catalyzed by a series of ecule of the ribonucleosideTriglycerides triphosphate GTP Triglycerides (see from GDP.also Lecture The structure 17) of GTP is four enzymes in mitochondria. Each turn of the cycle shortens the fatty acid chain by illustrated in Figure 2–58. GTP is a close relative of ATP, and the transfer of its two carbons (shown in red) and generates terminal phosphate group to ADP produces one ATP molecule in each cycle. As one molecule of acetyl CoA and one we discuss shortly, the energy that is stored in the readily transferred electrons of molecule each of NADH and FADH2. NADH and FADH2 will be utilized subsequently for ATP production throughare the (A, courtesy of Daniel S. Friend.) Fatty acids fuel molecules. (A) (C) They fattyare stored O as activated acyl CoA triacylglycerols fatty acid enters cycle (also R CH2 CH called 2 CH2 C neutral fats or triglycerides), which rest of S–CoA are uncharged esters hydrocarbon tail of fatty acids with glycerol. fat droplet cycle repeats O fatty acyl CoA Triacylglycerols are stored in adipose until fatty acid is completely shortened by R CH2 C two carbons tissue, composed of cells called S–CoA degraded adipocytes. O FAD CH3 C 1 µm S–CoA FADH2 Fatty acid degradation and synthesis mirror each other in O acetyl CoA their chemical reactions O CH2 O C hydrocarbon tail R CH2 and Fatty acid degradation CH consist CHsynthesis C of four steps that are the of each other in their basic chemistry. Degradation S–CoA is an oxidative HS–CoA that converts a fatty acid into a set of activated acetyl units (acetyl C 2O can be processed by the citric acid cycleH(Figure 22.2). An activat O acid is oxidized to introduce a double bond; the double bond is hyd O OH H O O C Ointroduce a hydroxyl group; the alcohol is oxidized to a ketone; and CH hydrocarbon tail the fatty acid isRcleaved CHby coenzyme C AC to yield acetyl CoA and a fa R CH2 C CH2 C chain two carbons shorter. 2 C If the fatty acidShas an even number o –CoA –CoAand is saturated, the process Satoms H His simply repeated until the fatt O completely converted into acetyl CoA units. Fatty acid synthesis is essentially the reverse of this process. T CH2 O C hydrocarbon tail cess starts with the individual units to be assembled—in this case + NADH activatedNAD acyl group (most simply, an acetyl unit) and a malonyl u ester bond + H+ 22.2). The malonyl unit condenses with the acetyl unit to Figure Figure 22.1 Electron micrograph of an four-carbon fragment. To produce the required hydrocarbon ch (B) triacylglycerol adipocyte. A small band of cytoplasm carbonyl group is reduced to a methylene group in three steps: a re surrounds the large deposit of triacylglycerols. [Biophoto Associates/Photo Researchers.]  Degradation and synthesis of fatty acids Fatty acids mobilized from triacylglycerols are oxidized to meet the energy needs of a cell or organism. During rest or moderate exercise, such as walking, fatty acids are our primary source of energy. Fatty acids are building blocks of phospholipids and glycolipids. Many proteins are modified by the Fatty Acid utilisation for energy production covalent attachment of fatty acids Fatty acid derivatives serve as hormones and intracellular messengers 3 Steps 1. Fatty Acid mobilisation (TAG -> FA + Glycerol) Focus in this lecture on 2. Fatty Acid activation degradation and synthesis. 3. Fatty Acid degradation to Acetyl Co-A and entry into TCA cycle  CH2OH CH2OH CH2OH C HO C H HO C H O C H C OH Glycerol Glycerol Triose kinase phosphate 2– phosphate CH2OH CH2OPO32– CH2OPO3 CH2OPO32– 1.Glycerol Fatty Acid mobilisation -Glycerol 3-phosphate I/II L dehydrogenase Dihydroxyacetone phosphate isomerase D-Glyceraldehyde 3-phosphate Hence, glycerol can be converted into pyruvate or glucose in the liver, which contains the appropriate enzymes (Figure 22.7). The reverse process can take place by the reduction of dihydroxyacetone phosphate to glycerol 3-phosphate. Hydrolysis by a phosphatase then gives glycerol. Thus, glycerol and glycolytic intermediates are readily interconvertible. LIVER CELL Glycolysis Pyruvate FAT CELL Gluconeogenesis Glycerol Glucose Triacylglycerol Fatty acids OTHER TISSUES Fatty acid oxidation Acetyl CoA Figure 22.7 Lipolysis generates fatty acids and glycerol. The fatty acids are used CAC Lipolysis - hydrolysis of triacylglycerols as fuel by many tissues. The liver processes glycerol by either the glycolytic or the by lipases (TAG -> FA + glycerol) gluconeogenic pathway, depending on its metabolic circumstances. Abbreviation: CAC, CO2 + H2O citric acid cycle. Hormones such as Epinephrine, glucagon activate an adipose lipase Fatty acids are linked to coenzyme A before they are oxidized Eugene Kennedy and Albert Lehninger showed in 1949 that fatty acids are Induces a G-protein mediated signal oxidized in mitochondria. Subsequent work demonstrated that they are first transduction pathway. activated through the formation of a thioester linkage to coenzyme A before O O they enter – the mitochondrial matrix. Adenosine – triphosphate drives the O R O R1 formation H2O O of R3 the thioester O linkage O between R1 H2O the carboxyl O R1 groupO of a fatty acid O H2C and the sulfhydryl group H2C of coenzyme A. This activation reaction CH2OHtakes place on the outer R2 mitochondrial membrane, where it is R2 catalyzed H acyl C by O R2 OO C H O C H O O O O CoALipase synthetase (also called fatty acid thiokinase). Lipase – P H2C CH2OH CH2OH O O R3 ATP AMP + PPi O Triacylglycerol O Diacylglycerol O Monoacylglycerol HO – of pancreatic lipases. Lipases secreted by the pancreas convert Figure 22.3 Action + HS CoA CoA  hydrocarbon tail fat droplet cycle repeats O until fatty acid fatty acyl CoA is completely 1. Fatty AcidS–mobilisation II/II shortened by R CH2 C two carbons degraded CoA O FAD CH3 C 1 µm Free fatty acids are not S–CoA soluble in blood FADH plasma. 2 O acetyl CoA O CH2 O C hydrocarbon tail R CH2 CH CH C Fatty HS–CoA acids are carried through the Sblood –CoA stream on O serum albumin H O 2 O OH H O O C O CH hydrocarbon tail R CH2 C C C R CH2 C CH2 C Fatty acid enters target S–CoA cell and H H becomes esterified S–CoA O to CoA-SH and enter b-oxidation pathway CH2 O C hydrocarbon tail NADH NAD+ ester bond + H+ (B) triacylglycerol MBoC6 m2.81/2.56 Glycerol generated from fat breakdown is absorbed by the liver it can serve as an intermediate for glycolysis or gluconeogenesis. See step 5 Glycolysis!!  2. Fatty Acid Activation I/II "priming" reaction in fatty acid metabolism catalysed by Acyl CoA synthetase  2. Fatty Acid Activation II/II 646 Acyl carnitine is then shuttled across the inner mitochond CHAPTER 22 Fatty Acid Metabolism a translocase (Figure 22.8). The acyl group is transferred How to transport Acyl-CoA into the mitochondria? A on the matrix side of the membrane. This reaction, wh carnitine acyltransferase II (carnitine palmitoyl transferas Acyl CoA CoA reverse of the reaction that takes place in the cytopla Acyl-CoAs are converted to is thermodynamically acyl-carnitines feasible because by of Acyl the zwitterion Carnitine Acyl carnitine tine. The O-acyl link in carnitine has a high group- Carnitine carnitine transferase acyltransferase I apparently because, being zwitterions, carnitine and its Cytoplasmic side differently from most other alcohols and their esters. Fina returns Acarnitine to the cytoplasmic translocator side in exchange fo (Acyl carnitine translocase) carnitine. imports Acyl Translocase carnitine into the mitochondrial A number of diseases matrix whileto a defic have been traced simultaneously the exporting transferase, or the free translocase. The symptom ciency carnitine range fromto themuscle cytosol Matrix side mild cramping to severe w Carnitine acyltransferase II death. Muscle, kidney, and heart are the tissues primarily Carnitine Acyl carnitine weakness during prolonged exercise is a symptom of a d Acyl-carnitine tine acyltransferases becauseis muscle converted relies on fatty aci Acyl CoA CoA sourceback to Medium-chain of energy. acyl-CoA in(C the matrix 8–C10) fatty acids are in these patients because these fatty acids do not require Figure 22.8 Acyl carnitine translocase. the mitochondria. These diseases illustrate that the impair The entry of acyl carnitine into the olite from one compartment of a cell to another can lead mitochondrial matrix is mediated by a translocase. Carnitine returns to the condition. !! Difference: cytoplasmic sideAcyl vs Acetyl of the inner CoA mitochondrial Carnitine Acetyl CoA, NADH, and FADH are generated in each  3-phosphate phosphate 3-phosphate Hence, glycerol can be converted into pyruvate or glucose in the liver, which contains the appropriate enzymes (Figure 22.7). The reverse process can take place by the reduction of dihydroxyacetone phosphate to glycerol 3. Fatty Acid degradation to Acetyl Co-A and entry into TCA cycle 3-phosphate. Hydrolysis by a phosphatase then gives glycerol. Thus, glycerol and glycolytic intermediates are readily interconvertible. LIVER CELL Glycolysis Pyruvate FAT CELL Gluconeogenesis Glycerol Glucose Triacylglycerol Fatty acids OTHER TISSUES Fatty acid oxidation Acetyl CoA 1 Figure 22.7 Lipolysis generates fatty acids and glycerol. The fatty acids are used as fuel by many tissues. The liver processes CAC glycerol by either the glycolytic or the gluconeogenic pathway, depending on its metabolic circumstances. Abbreviation: CAC, CO2 + H2O citric acid cycle. Fatty acids are linked to coenzyme A before they are oxidized 2 Eugene Kennedy and Albert Lehninger showed in 1949 that fatty acids are Fatty Acid Oxidation / 𝝱-oxidation oxidized in mitochondria. Subsequent work demonstrated that they are first activated through the formation of a thioester linkage to coenzyme A before they enter the mitochondrial matrix. Adenosine triphosphate drives the R formation of the thioester linkage between the carboxyl group of a fatty acid O Beta-oxidation is the catabolic process by and the sulfhydryl group of coenzyme A. This activation reaction takes place on the outer mitochondrial membrane, where it is catalyzed by acyl O O which fatty acid molecules are broken down to CoA synthetase (also called fatty acid thiokinase). 3 – P O O generateO acetyl-CoA in a Orecurring sequence of ATP AMP + PPi O four reactions – in the mitochondrial + HS CoA CoA matrix O R O R S O adenine Paul Berg showed that acyl CoA synthetase accomplishes the activation Acyl adenylate Reaction 1-3 create a carbonyl group on the 𝝱-C of a fatty acid in two steps. First, the fatty acid reacts with ATP to form an Reaction 4 cleaves the "𝝱-keto ester" in a reverse Claisen condensation 4 Products: an acetyl-CoA and a fatty acid two carbons shorter NADH and FADH2 are generated  𝝱-oxidation I Step 1. Oxidation of the C𝜶-C𝝱 bond Step 2. Hydration 𝝱 𝜶 Catalyzed by Acyl-CoA Dehydrogenase Catalyzed by Enoyl-CoA Hydratase Two electrons removed from acyl CoA Adds the elements of water across the are passed to the electron transfer double bond flavoprotein (FAD), and then to the electron transport chain.  𝝱-oxidation II Step 3. Second Oxidation Step 4. Thiolysis Catalyzed by Hydroxyacyl-CoA Catalyzed by Thiolase Dehydrogenase Another molecule of CoA-SH attacks the b- Two electrons are added to NAD+ to carbonyl carbon, which results in cleavage produce NADH of Ca-Cb bond. Formation of an acyl-CoA shortened by two carbon atoms and acetyl-CoA (C1 and C2)  ually oxidized, allowing the energy of this oxidation to be harnessed to produce The complete energy-rich oxidation activated carrier molecules. Theof palmitate chain of eight reactionsyields 106toofmolecules forms a cycle acetyl CoA. (A) Electronof ATP micrograph a lipid droplet in the cytoplasm. (B) The because at the end the oxaloacetate is regenerated and enters a new turn of the structure of fats. Fats are triacylglycerols. We can now calculate the energy yield derived from the oxidation of a fatty cycle, as shown in outline in Figure 2–57. 𝝱-oxidation III The glycerol portion, to which three fatty acids are linked through ester bonds, We have thus far discussed only one of the three types of activated carrier acid. In each reaction cycle, an acyl CoA is shortened by two carbon atoms, molecules that are produced by the citric acid cycle; NADH, the reduced form of is shown in blue. Fats are insoluble in water and form large lipid droplets in the the NAD+/NADH electron carrier system (see Figure 2–36). In addition to three and one molecule each of FADH , NADH, and acetyl CoA are formed. Net reaction molecules of NADH, each turn of the cycle also produces2one molecule of FADH2 specialized fat cells (adipocytes) in which they are stored. (C) The fatty acid oxidation R (reduced flavin adenine dinucleotide) from FAD (see Figure 2–39), and one mol- cycle. The cycle is catalyzed by a series of ecule of the ribonucleoside triphosphate GTP from 1 C -acyl CoA 1 FAD 1 NAD 1 H O 1 CoA ¡ GDP. The structure of GTP is four enzymes in mitochondria. Each turn of n 2 illustrated in Figure 2–58. GTP is a close relative of ATP, and the transfer of its the cycle shortens the fatty acid chain by C -acyl CoA 1 FADH 1 NADH 1 acetyl CoA 1 H1 terminal phosphate group ton22 ADP produces one ATP molecule in each 2cycle. As two carbons (shown in red) and generates one molecule of acetyl CoA and one we discuss shortly, the energy that is stored in the readily transferred electrons of molecule each of NADH and FADH2. The degradation of palmitoyl CoA (C16-acyl CoA) requires seven reaction NADH and FADH2 will be utilized subsequently for ATP production through the (A, courtesy of Daniel S. Friend.) tw cycles. In the seventh cycle, the C4-ketoacyl CoA is thiolyzed (A) (C) O to two moleculesfatty acyl CoA activated fatty acid enters cycle Figu of acetyl CoA. Hence, the stoichiometryR CH of CH the CH oxidation C of palmitoyl CoA is 2 2 2 deg S–CoA rest of degr Palmitoyl CoA 1 7 FAD 1 7 NAD1 1 7 CoA 1 7 H2O ¡ hydrocarbon tail sequ fat droplet 1 cycle repeats 8 acetylR CoA CH C O 1 7 FADH 2 1 7 NADH 1 7 H fatty acyl CoA shortened by 2 until fatty acid is completely oxid two carbons degraded S–CoA Beta oxidation works O as a cycle until the CH3 C FAD 1 µm fatty acid is S–CoA FADH2 O acetyl CoA completely degraded R CH2 CH CH C O CH2 O C hydrocarbon tail S–CoA HS–CoA H2O O O OH H O O C O CH hydrocarbon tail R CH2 C C C R CH2 C CH2 C S–CoA S–CoA H H O CH2 O C hydrocarbon tail NADH NAD+ ester bond + H+ (B) triacylglycerol  The shortened acyl CoA then undergoes another cycle of oxidation, start- ing with the reaction catalyzed by acyl CoA dehydrogenase (Figure 22.10). Fatty acid chainsExample: containing𝝱-oxidation of 18 from 12 to palmitate carbon (C16:0) atoms are oxidized by the long-chain acyl CoA dehydrogenase. The medium-chain acyl CoA dehydro- genase oxidizes fatty acid chains having from 14 to 4 carbons, whereas the short-chainpalmitate acyl CoA dehydrogenase acts only on 4- and 6-carbon fatty acid (C16:0) chains. In contrast, b-ketothiolase, hydroxyacyl dehydrogenase,First and enoyl three CoA hydratase act on fatty acid molecules of almost any length.rounds in the degradation of The degradation of palmitoyl CoA The (C complete 16-acyl oxidation CoA) of palmitate requires seven yields 106 moleculespalmitate. of ATP Two- carbon units are reaction cycles. sequentially We can now calculate the energy yield derived from the oxidation of a fatty removed from acid. InInthe eachseventh reactioncycle, cycle, the an acyl C4- CoA is shortened by two carbon atoms, the carboxyl ketoacyl CoA is thiolyzed to two and molecules one molecule each of FADH2, NADH, and acetyl CoA end of acetyl CoA. are formed. of the fatty 1 acid. Cn-acyl CoA 1 FAD 1 NAD 1 H2O 1 CoA ¡ Cn22-acyl CoA 1 FADH2 1 NADH 1 acetyl CoA 1 H1... The degradation of palmitoyl CoA (C16-acyl CoA) requires seven reaction The stoichiometry of the oxidation of cycles. In theCoA palmitoyl seventh is cycle, the C4-ketoacyl CoA is thiolyzed to two molecules of acetyl CoA. Hence, the stoichiometry of the oxidation of palmitoyl CoA is Palmitoyl CoA 1 7 FAD 1 7 NAD1 1 7 CoA 1 7 H2O ¡ 8 acetyl CoA 1 7 FADH2 1 7 NADH 1 7 H1 cycles. In the seventh cycle, the C4-ketoacyl CoA is thiolyzed to two molecules Fi of acetyl CoA. Hence, the stoichiometry Example: 𝝱-oxidation ofofpalmitate the oxidation of palmitoyl CoA is (C16:0) de de Palmitoyl CoA 1 7 FAD 1 7 NAD1 1 7 CoA 1 7 H2O ¡ se 8 acetyl CoA 1 7 FADH2 1 7 NADH 1 7 H1 ox 1 2 2.5 ATPs per NADH = 17.5 3 1.5 ATPs per FADH2 = 10.5 4 10 ATPs per acetyl-CoA = 80 5 Total = 108 ATPs 6 7 2 ATP equivalents (ATPà AMP + PPi, PPi à 2 Pi) consumed during activation of palmitate to acyl-CoA Net yield = 106 ATPs  In the nineteenth century, biologists noticed that in the absence of air cells pro- duce lactic acid (for example, in muscle) or ethanol (for example, in yeast), while Figure 2–55 Pathwa in its presencefor they consume O2 and produce CO2 and H2O. from Efforts to define the the production of a Pathways the production of acetyl CoA sugars and fats. from sugars and fat pathways of aerobic metabolism eventually focused on the oxidation of pyru- mitochondrion in euk vate and led in 1937 to the discovery of the citric acid cycle, also known as the is where acetyl CoA from both types of m plasma membrane molecules. It is there place where most of oxidation reactions o Sugars and where most of its AT sugars glucose pyruvate pyruvate polysaccharides Amino acids (not sho also enter the mitoch acetyl CoA be converted there in Fats fatty acids fatty acids fatty acids CoA or another interm MITOCHONDRION the citric acid cycle. T and function of mitoc CYTOSOL discussed in detail in The mitochondrion in eukaryotic cells is where acetyl CoA is produced from both types of major food molecules. It is therefore the place where most of the cell’s oxidation reactions occur and where most of its ATP is made. Amino acids (not shown) can also enter the mitochondria, to be converted there into acetyl CoA or another intermediate of the citric acid cycle. MBoC6 m2.80/2.55  the NAD+/NADH electron carrier system (see Figure 2 molecules of NADH, each turn of the cycle also produce (reduced flavin adenine dinucleotide) from FAD (see Fi ecule of the ribonucleoside triphosphate GTP from GD 82 How stored Chapter fats areandmobilized 2: Cell Chemistry Bioenergetics for energy production in animals. illustrated in Figure 2–58. GTP is a close relative of AT terminal phosphate group to ADP produces one ATP m we discuss shortly, the energy that is stored in the readi NADH Figure 2–54and FADH How 2 willfats stored be utilized are subsequently for ATP hydrolysis mobilized (A) for energy production in (C) animals. Low glucose levels in the blood stored fat trigger the hydrolysis of the triacylglycerol fatty acids bloodstream molecules in fat droplets to free fatty acids and glycerol. These fatty acids enter h glycerol the bloodstream, where they bind to the abundant blood protein, serum albumin. fat droplet FAT CELL Special fatty acid transporters in the fatty acyl CoA shortened by plasma membrane of cells that oxidize fatty two carbons acids, such as muscle cells, then pass these fatty acids into the cytosol, from MUSCLE CELL which they are moved into mitochondria for energy production. 1 µm fatty acids oxidation in Electron micrograph O of a lipid mitochondria CO2 droplet CHin 2 theCcytoplasm. O hydrocarbon tail HS O ATP CH O C hydrocarbon tail Low glucose levels in the blood trigger the hydrolysis of the triacylglycerol O molecules rapidly in complex fat decarboxylated by a giant droplets of three enzymes, to called the pyruvate free CH O C fatty hydrocarbon2tail acids and dehydrogenase glycerol. complex. The products of pyruvate decarboxylation are a molecule ester bond of CO2 (a waste product), a molecule of NADH, and acetyl CoA (see Panel 2–9). (B) triacylglycerol The fatty acids These fatty imported acidsfrom the bloodstream enter are moved intowhere the bloodstream, mitochondria, they bind to the abundant where all of their oxidation takes place (Figure 2–55). Each molecule of fatty acid blood (as the protein, activated moleculeserum fatty acylalbumin. CoA) is broken down completely by a cycle of reactions that trims two carbons at a time from its carboxyl end, generating one MBoC6 m2.78/2.54 molecule of acetyl CoA for each turn of the cycle. Ainmolecule Special fatty acid transporters the plasma membrane of cells that oxidize MBoC6 m of NADH and a mol- eculefatty of FADHacids, such 2 are also as muscle produced cells, in this process then (Figure pass these fatty acids into the cytosol, 2–56). Sugars fromand whichfats are the are they majormoved energy sources for most nonphotosynthetic into mitochondria for energy production. organisms, including humans. However, most of the useful energy that can be extracted from the oxidation of both types of foodstuffs remains stored in the ace-  two-carbon units derived from acetyl CoA. The activated donor of two- completely converted into acetyl CoA units. carbon units in the elongation step is malonyl ACP. The elongation reaction Fatty acid synthesis is essentially the reverse of this process. The pro- is driven by the release of CO2. cess starts with the individual units to be assembled—in this case with an activated acyl group (most simply, an acetyl unit) and a malonyl unit (see Figure 22.2). The malonyl unit condenses with the acetyl unit to form a Fatty Acid Synthesis 5. The reductant in fatty acid synthesis is NADPH, whereas the oxidants in fatty acid degradation are NAD1 and FAD. ph of an four-carbon fragment. To produce the required hydrocarbon chain, the 6. Elongation by the fatty acid synthase complex stops on the formation of carbonyl group is reduced to a methylene group in three steps: a reduction, oplasm cylglycerols. Fatty palmitate (C16). acid synthesis Further elongation is essentially and the insertion of double bonds the are carried out by other enzyme systems. archers.] reverse of this process. FATTY ACID DEGRADATION FATTY ACID SYNTHESIS The formation of malonyl CoA is the committed step in O O fatty FA acid Synthesis synthesis starts with 2 monomers Fatty acid synthesis starts with the carboxylation of acetyl CoA to malonyl R H2 C C R! R H2 C C R" CoA. This Activated irreversible acetyl reaction group step in fatty acid is the committed C H2 C H2 S C H2 C H2 S Activated Malonyl units synthesis. Activated acyl group Activated acyl group (lengthened by Oxidation two carbon atoms) O (bicarbonate) – O O CoA + ATP + HCO3– CoA + ADP + Pi + H+ Reduction H3C S O C S O H H2 R C C R! O Acetyl CoA Malonyl CoA C C S H H2 H R C C R" C H2 C H S The The which malonyl synthesis of malonylunit