Lecture 3. TCA Cycle_JR - Biomolecular Sciences Lecture Notes PDF

BMOL20110 Biomolecular Sciences Lecture 24. TCA Cycle 8 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  Key Meta...

BMOL20110 Biomolecular Sciences Lecture 24. TCA Cycle 8 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  Key Metabolic Pathways L26 L23 L27 L24 L25 2  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. 3  Today’s Class & Learning Objectives By the end of this lecture, you should be able to: Explain why the reaction catalyzed by the pyruvate dehydrogenase complex is a crucial juncture in metabolism. Identify the primary catabolic purpose of the citric acid cycle. Explain the efficiency of using the citric acid cycle to oxidize acetyl CoA. Describe how the citric acid cycle is regulated. Stryer, Biochemistry, Chapter 17 Campbell, Biology, Chapter 9 Alberts, Essential Cell Biology, Chapter 13 4  for the formation of phosphoenolpyruvate? The answer to this question becomes clear when we compare the structures of 2-phosphoglycerate and pyruvate. The formation of pyruvate from 2-phosphoglycerate is, in RECAP essence, – Glycolysis an internal I oxidation–reduction reaction; carbon 3 takes electrons from carbon 2 in the conversion of 2-phosphoglycerate into pyruvate. Compared with 2-phosphoglycerate, C-3 is more reduced in pyruvate, whereas C-2 is more oxidized. Once again, carbon oxidation powers the synthesis The Breakdown of a compound and Utilization of Sugars and Fatswith high 429 phosphoryl-transfer potential, phospho- CH2OH enolpyruvate here and 1,3-bisphosphoglycerate earlier, which allows the Figure 13–3 The stepwise oxidation of O synthesis sugars of ATP. begins with glycolysis. Each of the 10 steps of glycolysis is catalyzed by a one molecule of glucose OH Because the molecules different enzyme. Note that step 4of ATP used in forming fructose 1,6-bisphosphate cleaves OH a six-carbon sugar into two three-carbon HO energy investment have already sugars, so thatbeen regenerated, the number of molecules at the two molecules of ATP generated from OH phosphoenolpyruvate are “profit.” to be every stage after this doubles. As indicated, ATP STEP 1 recouped later step 6 begins the energy-generation phase of glycolysis, which results in the net STEP 2 synthesis of ATP and NADH (see also Panel ATP Two ATP molecules are formed in the conversion of 13–1). Glycolysis is also sometimes referred STEP 3 to as the Embden–Meyerhof pathway, glucose into pyruvate named for the chemists who first fructose 1,6- P OH2C O CH2O P The The net reaction in in thethe transformation of glucose described it. All the steps of glycolysis are net reaction transformation reviewed in Movie 13.1. of glucose into pyruvate into pyruvate is is bisphosphate HO OH OH cleavage of Glucose 1 2 Pi 1 2 ADP 1 2 NAD1 y STEP 4 six-carbon sugar to two 2 pyruvate 1 2 ATP 1 2 NADH 1 2 H1 1 2 H2O three-carbon sugars STEP 5 420 CHAPTER 13 How Cells Obtain Energy From Food two molecules of CHO CHO Thus, two molecules of ATP are generated in the conversion of glucose into glyceraldehyde 3-phosphate CHOH CHOH two molecules of SYSTEM IN NONLIVING pyruvate. The reactions (A) DIRECT BURNING OF SUGAR of glycolysis are summarized in (B) STEPWISE OXIDATION OF SUGAR IN CELLS CH2O P CH2O P Table 16.1. large activation NADH STEP 6 NADH The energy released in the anaerobic conversion energy overcome by the heat from of glucose into two small activation energies overcome by enzymes that 21 21 ATP STEP 7 ATP molecules of pyruvatea fireis about 296 kJ mol (–23 kcal mol work at body temperature ). We shall see SUGAR + O2 SUGAR + O2 STEP 8 STEP 9 energy free energy generation some free STEP 10 all free energy is ATP ATP energy stored in released as heat; – – activated carrier COO COO none is stored molecules two molecules C O C O of pyruvate CH3 CH3 CO2 + H2O CO2 + H2O Panel 13–1 (pp. 430–431). Like most enzymes (discussed in Chapter 4), ectrons and 2 protons) from the gases combine explosively. In fact, combustion of liquid H2 mple), thereby oxidizing it. The and O2 is harnessed to power the main engines of the space ons along with 1 proton to its shuttle after it is launched, boosting it into orbit. The explo- 4). The other proton is released he surrounding solution: RECAP – Glycolysis II sion represents a release of energy as the electrons of hydrogen “fall” closer to the electronegative oxygen atoms. Cellular res- piration also brings hydrogen and oxygen together to form rogenase C O + NADH + H+ water, but there are two important differences. First, in cellular Stepwise Energy Harvest via NAD and the + respiration, the hydrogen that reacts with oxygen is derived Electron Transport Chain arged electrons but only 1 posi- has its charge neutralized when from organic molecules rather than H2. Second, instead of occurring in one explosive reaction, respiration uses an Glucose is broken down in a series of steps, me NADH s been re- each one catalysed by an enzyme. H2 + 1/2 O2 2H + /2 O2 1 D! is the (from food via NADH) or in cellu- Controlled At key steps, electrons are stripped from the in several the break- 2 H+ + 2 e– release of energy for glucose. synthesis of ATP ATP Elec chain f their po- As is often the case in oxidation reactions, each Free energy, G Free energy, G tron transferred Explosive ATP ch NADH release of electron travels with a proton - thus, as a tran heat and light ATP respiration energy hydrogen atom. spor at can be t 2 e– e electrons an energy 2 H+ 12 O2 The hydrogen atoms are not transferred en. e extracted directly to oxygen, but instead are usually H2O H 2O tential en- passed first to an electron carrier, a coenzyme oxygen? It dox chem- (a) Uncontrolled reaction (b) Cellular respiration called NAD+ (nicotinamide adenine o a much " Figure 9.5 An introduction to electron transport chains. (a) The one-step exergonic dinucleotide), a derivative of the vitamin n between reaction of hydrogen with oxygen to form water releases –a large+ amount of energy in the form of orm water electron transport chain breaks the “fall” of electrons 2e +2H heat and light: an explosion. (b) In cellular respiration, the same reaction in occurs 2 e– + H + in stages: An this reaction into a series of smaller steps niacin). NAD + NADH H+ , provide a and stores some of the released energy in a form thatDehydrogenase can be used to make ATP. (The rest of the H O O , and the Each NADH molecule formed energy is released as heat.) Reduction of NAD+ H H C NH2 + 2[H] C NH2 + H+ (from food) Oxidation of NADH N+ Nicotinamide N Nicotinamide (reduced form) during respiration represents stored (oxidized form) energy that can be tapped to O CH2 O O P O– O O P O– H HO OH H NH2 make ATP when the electrons O CH2 H N N ! Figure 9.4 NAD! as an electron shuttle. The full name for NAD!, nicotinamide adenine dinucleotide, describes its structure: The molecule consists of two nucleotides joined together at their phosphate complete their “fall” down an O N N H groups (shown in yellow). (Nicotinamide is a nitrogenous base, although not one that is present in DNA or RNA; see Figure 5.26.) The enzymatic energy gradient from NADH to oxygen. transfer of 2 electrons and 1 proton (H!) from an organic molecule in food H H to NAD! reduces the NAD! to NADH; the second proton (H!) is released. HO OH Most of the electrons removed from food are transferred initially to NAD!.  Glycolysis in my own research – Malignant Melanoma / Skin Cancer The Breakdown and Utilization of Sugars and Fats 429 CH2OH Figure 13–3 The stepwise oxidation of O sugars begins with glycolysis. Each of the 10 steps of glycolysis is catalyzed by a one molecule of glucose OH different enzyme. Note that step 4 cleaves HO OH energy RTK a six-carbon sugar into two three-carbon investment sugars, so that the number of molecules at OH to be every stage after this doubles. As indicated, ATP STEP 1 recouped later step 6 begins the energy-generation phase of glycolysis, which results in the net Ras STEP 2 synthesis of ATP and NADH (see also Panel 13–1). Glycolysis is also sometimes referred >50% of malignant ATP STEP 3 to as the Embden–Meyerhof pathway, melanomas are caused named for the chemists who NF1 first P OH2C O CH2O P described it. AllPthe steps of glycolysis are by oncogenic fructose 1,6- BRAF reviewed in Movie 13.1. bisphosphate HO V600E mutations in the BRAF OH OH gene ? STEP 4 cleavage of six-carbon P P sugar to two three-carbon MEK BRAF is one component STEP 5 sugars of the RAS-RAF-ERK two molecules of CHO CHO signalling pathway glyceraldehyde CHOH CHOH P P 3-phosphate ERK CH2O P CH2O P BRAF seems to control NADH STEP 6 NADH a number of glycolytic ATP STEP 7 ATP steps in melanomas STEP 8 Glycolysis influences STEP 9 energy generation Proliferation signalling in melanoma ATP STEP 10 ATP Survival COO– COO– Cell Cycle Cytoskeleton two molecules of pyruvate C O C O Differentiation Adhesion CH3 CH3 Transformation Motiliy etc.  Glycolysis - Key scientists involved in research 1931, research into cellular respiration showed that cancer thrives in anaerobic (without oxygen) or acidic conditions Otto Heinrich Warburg Glucose to Pyruvate – What then? Glycolysis releases less than a quarter of the chemical energy in glucose that can be released by cells Most of the energy remains stockpiled in the two molecules of pyruvate. If molecular oxygen is present, the pyruvate enters a mitochondrion (in eukaryotic cells), where the oxidation of glucose is completed. (In prokaryotic cells, this process occurs in the cytosol.) kJ/mol). Instead of this energy being released and wasted in a stages 2 and 3. We include glycolysis, however, because most single explosive step, electrons cascade down the chain from respiring cells deriving energy from glucose use glycolysis to one carrier molecule to the next in a series of redox reactions, produce the starting material for the citric acid cycle. The Link Between Glycolysis and the Citric Acid Cycle losing a small amount of energy with each step until they fi- As diagrammed in Figure 9.6, glycolysis and pyruvate oxida- nally reach oxygen, the terminal electron acceptor, which has tion followed by the citric acid cycle are the catabolic pathways a very great affinity for electrons. Each “downhill” carrier is that break down glucose and other organic fuels. Glycolysis, Upon entering the mitochondrion more electronegative than, and thus capable of oxidizing, its which occurs in the cytosol, begins the degradation process by “uphill” neighbor, with oxygen at the bottom of the chain. breaking glucose into two molecules of a compound called via active transport, pyruvate is first Therefore, the electrons removed from glucose by NAD! fall down an energy gradient in the electron transport chain to a pyruvate. In eukaryotes, pyruvate enters the mitochondrion and is oxidized to a compound called acetyl CoA, which enters converted to a compound called far more stable location in the electronegative oxygen atom. Put another way, oxygen pulls electrons down the chain in an the citric acid cycle. There, the breakdown of glucose to car- bon dioxide is completed. (In prokaryotes, these processes take acetyl coenzyme A, or acetyl CoA. energy-yielding tumble analogous to gravity pulling objects downhill. place in the cytosol.) Thus, the carbon dioxide produced by res- piration represents fragments of oxidized organic molecules. In summary, during cellular respiration, most electrons Some of the steps of glycolysis and the citric acid cycle are travel the following “downhill” route: glucose → NADH → redox reactions in which dehydrogenases transfer electrons electron transport chain → oxygen. Later in this chapter, you from substrates to NAD!, forming NADH. In the third stage of Pyruvate produced by glycolysis is will learn more about how the cell uses the energy released respiration, the electron transport chain accepts electrons from from this exergonic electron fall to regenerate its supply of the breakdown products of the first two stages (most often via converted into acetyl CoA, the fuel ATP. For now, having covered the basic redox mechanisms of cellular respiration, let’s look at the entire process by which NADH) and passes these electrons from one molecule to an- other. At the end of the chain, the electrons are combined with energy is harvested from organic fuels. of the citric acid cycle. molecular oxygen and hydrogen ions (H!), forming water (see ! Figure 9.6 An overview of cellular respiration. During glycolysis, each glucose molecule is broken down into two molecules of Electrons carried Electrons the compound pyruvate. In eukaryotic cells, as via NADH and carried shown here, the pyruvate enters the via NADH FADH2 mitochondrion. There it is oxidized to acetyl CoA, which is further oxidized to CO2 in the citric acid cycle. NADH and a similar electron carrier, a coenzyme called FADH2, transfer Pyruvate Oxidative Glycolysis oxidation Citric phosphorylation: electrons derived from glucose to electron acid electron transport transport chains, which are built into the inner Glucose Pyruvate Acetyl CoA cycle and mitochondrial membrane. (In prokaryotes, the chemiosmosis electron transport chains are located in the plasma membrane.) During oxidative phosphorylation, electron transport chains convert the chemical energy to a form used for CYTOSOL MITOCHONDRION ATP synthesis in the process called chemiosmosis. ANIMATION Visit the Study Area at ATP ATP ATP www.masteringbiology.com for the BioFlix® 3-D Animation on Substrate-level Substrate-level Oxidative Cellular Respiration. phosphorylation phosphorylation phosphorylation  converted into lactate or ethanol, depending on the organism. Under aero- bic conditions, the pyruvate is transported into mitochondria by a specific The Link Between Glycolysis carrier protein embedded and the in the Citric Acid mitochondrial Cycle membrane. In the mito- chondrial matrix, pyruvate is oxidatively decarboxylated by the pyruvate ATP dehydrogenase complex to form acetyl CoA. Upon Pyruvate entering 1 CoA 1 NAD the1 ¡ mitochondrion acetyl CoA 1 CO2 1via NADH active 1 H1 transport, pyruvate This irreversible reaction is the linkisbetween first glycolysis converted to acid and the citric a cycle Figure 17.4 Th (Figure 17.4). Note that the pyruvate dehydrogenase complex produces compound called acetylelectrons CO2 and captures high-transfer-potential coenzyme in the formA, or of NADH. and the citric a by glycolysis is c acetyl CoA.dehydrogenase reaction has many of the key features of Thus, the pyruvate fuel of the citric the reactions of the citric acid cycle itself. The Thesynthesis of acetyl pyruvate dehydrogenase complexcoenzyme is a large, highlyA from integrated plex of three distinct enzymes (Table 17.1). Pyruvate dehydrogenase com- com- pyruvate plex is a member requires of a family three enzymes of homologous complexesand five the that include citric acid cycle enzyme a-ketoglutarate dehydrogenase complex (p. 507). coenzymes These complexes are giant, larger than ribosomes, with molecular masses ranging from 4 million to 10 million daltons (Figure 17.5). As we will see, Table 17.1 Pyruvate dehydrogenase complex of E. coli Number of Prosthetic Enzyme Abbreviation chains group Reaction catalyzed Pyruvate E1 24 TPP Oxidative decarboxylation dehydrogenase of pyruvate component Dihydrolipoyl E2 24 Lipoamide Transfer of acetyl group transacetylase to CoA Figure 17.5 Ele of the pyruvate Dihydrolipoyl E3 12 FAD Regeneration of the dehydrogenase oxidized form of lipoamide complex from Dr. Lester Reed.]  of CO2. (This is three the firstenzymes step in whichandCOfive coenzymes 2 is released during (from glycolysis, respiration.) 2 The remaining two-carbon fragment is 2 molecules per glucose) The mechanism 110 of Chapter the pyruvate 3 Energy, oxidized, forming acetate (CH3COO , the ionized form of ! Catalysis,dehydrogenase and Biosynthesis reaction is wonderfully ATP ATP ATP acetic acid). Thecomplex, Pyruvate more Figure extracted electrons soare 3–36than Acetyl is Dehydrogenase suggested coenzyme transferred NADby A (CoA) to Complex The reaction " its simple stoichiometry. , requires theismolecule. participation of the three enzymes of the NAD another important activated carrier pyruvate CO2 dehydroge- A space-filling model is shown + nase complex aboveand acetyl CoA. The The coenzymes thiamine pyrophosphate five ofcoenzymes. the structure CoA sulfur atom (yellow) forms a thioester bond (TPP), lipoic acid, to acetate. andthe FAD Because serve thioester bond is as catalytic cofactors, NADH and CoA and 1 a high-energy linkage, it releases a large NAD areamount Citric stoichiometric of free energy whencofactors, cofactors that function it is hydrolyzed; +H + as substrates. Acetyl CoA Pyruvate Oxidative Glycolysis acid thus, the acetyl group carried by CoA can phosphorylation oxidation cycle be readily transferred to other molecules. acetyl CoA group NH2 H ATP ATP ATP + Acetyl CoA CoA N N MITOCHONDRION S S nucleotide CYTOSOL CO2 Coenzyme A O O O S ADENINE OH– 3C N H3C P P OH 1 3 S-CoA 2– H H H HO H H O H CH3 H O O 3C C S C C N C C C N C C C C O P O P O CH2 C O O O O O Citric –C O O H H H H H H OH CH 3 HO acid O– O– C O high-energy 2 CO2 Lipoic acid cycle 2 RIBOSE Thiamine pyrophosphate (TPP) CH3 bond CH3 O NAD + NADH + H + Acetyl CoA FADH2 NAD+ Pyruvate –O P O3 O– The conversion of pyruvate into acetyl acetyl group CoA FAD consists of acetyl three steps: coenzyme A (CoA) 3 NADH Transport protein decarboxylation, oxidation, and transfer of the resultant acetyl group to + 3 H+ ! Figure 9.10 CoA. of pyruvate to acetyl CoA, the Oxidation ADP + P i step before the citric acid cycle. Pyruvate is a charged molecule, so in eukaryotic cells it must enter the mitochondrion via active acetyl CoA, this handle portion ATP very often contains a nucleotide. This curi- transport,Owith the help of a transport protein. Next, O a complex–of O O stage of cell evolution. It is thought ous fact may be a relic from an early CO several enzymes (the pyruvate dehydrogenase complex) catalyzes 2 2 e the CoA – that + the main catalystsECB3 for early life forms on Earth were RNA molecules m2.62/3.36 C three numbered O which are described in the C steps, text. The acetyl group C(or!their Figure 9.11 An overview C of pyruvate close relatives) and that proteins oxidation were a later and addi- evolutionary the ofHacetyl 3C CoA C will enter the citric acid H cycle. C The 3 CO 2 molecule will H3C citric acid cycle. H The C inputs and S CoA outputs per pyruvate molecule tion (discussed in Chapter3 7). It is therefore tempting to speculate that are – By diffuse out of the cell. Decarboxylation OxidationS-CoA convention, coenzyme A is abbreviated Transfer shown. To to CoA on a per-glucose basis, multiply by 2, because each calculate many of the carrier molecules that we find today originated in an earlier O to a molecule, emphasizing the sulfur atom (S). when it is attached glucose RNA world,molecule is split where their during glycolysis nucleotide portionsinto twohave would pyruvate been molecules. useful for Pyruvate Acetyl binding these carriers to RNA enzymes. CoA In addition to the transfer reactions catalyzed by the activated carrier 170 UNIT TWO The Cell molecules ATP (transfer of phosphate) and NADPH (transfer of electrons These steps must be coupled to preserve the free energy derived from the and hydrogen), other important reactions involve the transfers of methyl, carboxyl, and glucose groups from activated carrier molecules 12 for the decarboxylation step to drive the formation of NADH and acetyl CoA. purpose of biosynthesis. The activated carriers are usually generated in reactions coupled to ATP hydrolysis (Figure 3–37). Therefore, the energy  beings, greater than Pyruvate Dehydrogenase 90%. It is highly efficient becauseComplex the oxidation of a Glucose limited number of citric acid cycle molecules can generate large amounts of NADH and FADH2. Note in Figure 17.2 that the four-carbon molecule, Glycolysis oxaloacetate, that initiates the first step in the citric acid cycle is regenerated at the end of oneInpassage thethrough mitochondrial the cycle. Thus, matrix, one molecule of oxaloac- etate is capable pyruvate of participating in theis oxidation oxidatively of many acetyl molecules. Pyruvate decarboxylated by the CO2 pyruvate 17.1 Pyruvate Dehydrogenasedehydrogenase Links Glycolysis to 2 e− the Citric Acid Cycle complex to form acetyl CoA. Acetyl CoA Carbohydrates, most notably glucose, are processed by glycolysis into pyruvate (Chapter 16). Under anaerobic conditions, the pyruvate is converted into lactate or ethanol, depending on the organism. Under aero- bic conditions, the pyruvate is transported into mitochondria by a specific Citric carrier protein embedded in the mitochondrial membrane. In the mito- acid cycle chondrial matrix, pyruvate is oxidatively decarboxylated by the pyruvate Netdehydrogenase reactioncomplex of the pyruvate to form dehydrogenase complex acetyl CoA. ATP Pyruvate 1 CoA 1 NAD1 ¡ acetyl CoA 1 CO2 1 NADH 1 H1 8 e− This irreversible reaction is the link between glycolysis and the citric acid cycle (Figure 17.4). Note that the pyruvate dehydrogenase complex produces Figure 17.4 The link between ThisCOirreversible reaction is the link between glycolysis andand the citric acid cycle. Pyruv 2 and captures high-transfer-potential electrons in the form of NADH. by glycolysis is converted into ace theThus, citricthe acid pyruvatecycle dehydrogenase reaction has many of the key features of fuel of the citric acid cycle. the reactions of the citric acid cycle itself. The pyruvate dehydrogenase complex is a large, highly integrated com- plex of three distinct enzymes (Table 17.1). Pyruvate dehydrogenase com- plex is a member of a family of homologous complexes that include the 13 citric acid cycle enzyme a-ketoglutarate dehydrogenase complex (p. 507).  a metabolic furnace that oxidizes organic fuel derived from pyruvate. Figure 9.11 summarizes the inputs and outputs as to Acetyl CoA pyruvate is broken down to three CO2 molecules, including rion via active transport, pyru- An overview of pyruvate oxidation and the citric acid cycle. the molecule of CO2 released during the conversion of pyru- mpound called acetyl coenzyme vate to acetyl CoA. The cycle generates 1 ATP per turn by 0). This step, linking glycolysis ied out by a multienzyme com- tions: 1 Pyruvate’s carboxyl ady fully oxidized and thus has Pyruvate Citric Oxidative Glycolysis acid phosphorylation oxidation ved and given off as a molecule cycle Pyruvate n which CO2 is released during (from glycolysis, ning two-carbon fragment is 2 molecules per glucose) ATP ATP ATP H3COO!, the ionized form of trons are transferred to NAD", The citric acid cycle is CO2 NAD+ CoA NADH also called the on + H+ Acetyl CoA tricarboxylic acid cycle (TCA) or the Krebs cycle CoA CoA MITOCHONDRION Coenzyme A 3 S-CoA Citric C O 2 CH3 acid cycle 2 CO2 Oxidation NADH + H + Acetyl CoA FADH2 3 NAD+ Electron Transfer FAD 3 NADH + 3 H+ Electron Transfer yruvate to acetyl CoA, the ADP + P i e. Pyruvate is a charged molecule, he mitochondrion via active ATP Energy ort protein. Next, a complex of drogenase complex) catalyzes the 14 scribed in the text. The acetyl group ! Figure 9.11 An overview of pyruvate oxidation and the d cycle. The CO2 molecule will citric acid cycle. The inputs and outputs per pyruvate molecule are  Flavin adenine dinucleotide (FAD) as a redox co-factor Flavin adenine dinucleotide (FAD) Nicotinamide Adenine Dinucleotide (NAD) The Breakdown and Utilization of Sugars and Fats 439 (B) + – 2H 2e FAD FADH2 2 e– + 2 H+ O H 2 e– + H+ H2 NAD+ NADH H+ H Dehydrogenase C N C N H O H H O Reduction of NAD+ H3C C C C NH C C NH2 + 2[H] C NH2 + H+ (from food) Oxidation of NADH H3C C C C C C N+ Nicotinamide N Nicotinamide (reduced form) C N N O N O CH2 O (oxidized form) H O P O– CH2 H O H H O P O– HO OH NH2 H C OH O CH2 N ! Figure 9.4 NAD! as an electron shuttle. The full name for N NAD!, nicotinamide adenine dinucleotide, describes its structure: The H molecule consists of two nucleotides joined together at their phosphate H C OH O N N H groups (shown in yellow). (Nicotinamide is a nitrogenous base, although not one that is present in DNA or RNA; see Figure 5.26.) The enzymatic he citric acid cycle H C OH H H transfer of 2 electrons and 1 proton (H!) from an organic molecule in food to NAD! reduces the NAD! to NADH; the second proton (H!) is released. e of FADH2, whose HO OH Most of the electrons removed from food are transferred initially to NAD!. e close relatives of H2C O P P O CH2 ADENINE ing the substitution pair of hydrogen atoms (2 electrons and 2 protons) from the gases combine explosively. In fact, combustion of liquid H ery different structure, substrate (glucose, in this example), thereby oxidizing it. The and O2 is harnessed to power the main engines of the spac RIBOSE enzyme delivers the 2 electrons along with 1 proton to its shuttle after it is launched, boosting it into orbit. The explo drogens and high- FAD coenzyme, NAD! (Figure 9.4). The other proton is released sion represents a release of energy as the electrons of hydrogen orm (FAD) with the as a hydrogen ion (H!) into the surrounding solution: “fall” closer to the electronegative oxygen atoms. Cellular res hese same atoms are piration also brings hydrogen and oxygen together to form Dehydrogenase H C OH + NAD+ C O + NADH + H+ water, but there are two important differences. First, in cellula FAD is another electron shuttle By receiving 2 negatively charged electrons but only 1 posi- respiration, the hydrogen that reacts with oxygen is derived from organic molecules rather than H2. Second, instead o tively charged proton, NAD! has its charge neutralized when occurring in one explosive reaction, respiration uses an it is reduced to NADH. The name NADH shows the hydrogen that has been re- H2 + 1/2 O2 + /2 O2 1 2H ceived in the reaction. NAD! is the (from food via NADH) ier of high-energy most versatile electron acceptor in cellu- Controlled lar respiration and functions in several 15 release of nergy that is stored of the redox steps during the break- 2 H+ + 2e – energy for synthesis of DH and FADH2 are down of glucose. ATP ATP Ele the coenzyme FAD (flavin adenine dinucleotide, derived You can see in Figure 9.12 that for each turn of the citric acid from riboflavin, a B vitamin), during the redox reactions. The cycle, two carbons (red) enter in the relatively reduced form reduced coenzymes, NADH and FADH2, shuttle their cargo of of an acetyl group (step 1), and two different carbons (blue) high-energy electrons into the electron transport chain. The Citric Acid Cycleleave in the completely oxidized form of CO2 molecules Citric Oxidative Glycolysis Pyruvate acid oxidation phosphorylation cycle S-CoA 1 Acetyl CoA (from oxidation of pyruvate) 2 Citrate is C O ATP ATP ATP adds its two-carbon acetyl converted to CH3 group to oxaloacetate, its isomer, producing citrate. isocitrate, by Acetyl CoA removal of 8 The substrate is oxidized, CoA-SH one water molecule and Why is it called addition of reducing NAD+ to O C COO– another. NADH NADH and + H+ CH2 1 COO– H2O tricarboxylic regenerating oxaloacetate. NAD + COO – CH2 COO– Oxaloacetate – acid (TCA) 8 HO C COO CH2 2 CH2 HC COO– COO– – cycle? COO HO CH HO CH Malate Citrate COO– CH2 3 Isocitrate Isocitrate is oxidized, COO– 7 Addition of NAD + reducing a water NAD+ to Citric NADH NADH. Then molecule 3 acid + H+ the resulting rearranges 7 cycle bonds in the H2O compound CO2 loses a CO2 substrate. COO– COO – molecule. CH Fumarate CH2 CoA-SH HC CH2 α-Ketoglutarate COO– C O 4 6 CoA-SH COO– COO– COO– CH2 5 CH2 4 Another CO2 FADH 2 CH2 CH2 CO2 is lost, and the citrate NAD + resulting FAD COO– C O 6 Two compound is Succinate S-CoA NADH oxidized, hydrogens are Pi transferred to reducing NAD+ GTP GDP Succinyl + H+ FAD, forming to NADH. CoA The remain- FADH2 and ing molecule is oxidizing ADP then attached succinate. 5 CoA is displaced by a phosphate group, to coenzyme A which is transferred to GDP, forming GTP, a ATP molecule with functions similar to ATP. GTP by an unstable 16 bond. can also be used, as shown, to generate ATP.  442 PANEL 13–2 The complete citric acid cycle The Citric Acid Cycle + NAD+ NADH + H HS CoA O The complete citric acid cycle. The two CH3 C carbons from acetyl CoA that enter this COO– turn of the cycle (shadowed in red ) will pyruvate CO2 acetyl CoA (2C) be converted to CO2 in subsequent turns O of the cycle: it is the two carbons CH3 C S CoA shadowed in blue that are converted to CO2 in this cycle. HS CoA COO– H2O next cycle – C O COO + CH2 CH2 NADH + H COO– Step 1 HO C COO– Step 2 NAD+ – oxaloacetate (4C) COO CH2 C O COO– COO– CH2 isocitrate (6C) Step 8 CH2 citrate (6C) COO–