Lecture 2. Glycolysis and Gluconeogenesis PDF

BMOL20110 Biomolecular Sciences Lecture 23. Glycolysis and Gluconeogenesis 6 November 2024 Jens Rauch, PhD School of Biomolecular and Biomedical Science Systems Biology Ireland Email: [email protected] Phone: +353-(0)1-716 6337 @jensrauch...

BMOL20110 Biomolecular Sciences Lecture 23. Glycolysis and Gluconeogenesis 6 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 30th. 2  Today’s Class & Learning Objectives By the end of this lecture, you should be able to: Describe the glycolytic pathway and its role in metabolism Describe how ATP is generated in glycolysis. Explain why the regeneration of NAD+ is crucial to fermentations. Describe how gluconeogenesis is powered in the cell. Stryer, Biochemistry, Chapter 16 Campbell, Biology, Chapter 9 Alberts, Essential Cell Biology, Chapter 13 3  Less work capacity (a) Gravitational motion. Objects (b) Diffusion. Molecules in a drop (c) Chemical reaction. In a cell, a move spontaneously from a of dye diffuse until they are glucose molecule is broken down RECAP - Metabolism higher altitude to a lower one. randomly dispersed. into simpler molecules. ! Figure 8.5 The relationship of free energy to stability, work capacity, and spontaneous change. Unstable systems (top) are rich in free energy, G. They have a tendency to change We see once again how important spontaneously it isstable to a more to state think of organ- (bottom), A key feature in the way cells manage their energy resources and it is possible to " Figure 8.6 Free energy changes (!G ) in exergonic isms as open systems. Sunlight provides a daily source of free harness this “downhill” change to perform work. to do this andwork is energy endergonic reactions.coupling, the use of an exergonic energy for an ecosystem’s plants and other photosynthetic process (a) toExergonic drive an endergonic one. spontaneous reaction: energy released, ATP is responsible for me- Cells Extract Energy from their Environment organisms. Animals and other nonphotosynthetic Free Energy and Metabolism in an ecosystem must have a source of free energy in the organisms diating most energy coupling in cells, and in most cases it acts as the immediate Reactantssource of energy that powers cellular work. and Use the Energy for a Host of Biological We can now apply the free-energy concept more specifically form of the organic products to theof photosynthesis. chemistry Now that we of life’s processes. The Structure and Hydrolysis of Amount ATP of Activities including Biosynthesis have applied the free-energy concept to metabolism, we are Exergonic and Endergonic Reactions in Metabolism ready to see how a cell actually performs the work of life. energy released Free energy (∆G < 0) ATP (adenosine triphosphate) was introduced Based on their free-energy changes, chemical reactions can Energy in Chapter 4 when we discussed the phosphate Products group as a functional The reactions of energy extraction and be classified as either exergonic (“energy outward”) or ender- 8.2 CONCEPT CHECK gonic (“energy inward”). An exergonic reaction proceeds group. ATP contains the sugar ribose, with the nitrogenous with a net release of free energy (Figure 8.6a). Because the energy use are called metabolism or chemical mixture loses free energy (G decreases), ∆G is nega- 1. Cellular respiration uses glucose and oxygen, which base adenine and a chain of three phosphate groups bonded Progress of the reaction tive for an exergonic reaction. Using ∆G as a standard for to it (Figure 8.8a). In addition to its role in energy coupling, intermediary metabolism. have high levels of free energy,exergonic spontaneity, neously. and releases reactionsCO andthat occur sponta- are 2those ATP is also one of reaction: the nucleoside triphosphates nonspontaneousused to make water, which have low levels(Remember, the word Is of free energy. cellularimplies that it is spontaneous (b) Endergonic energy required, energetically favorable, not that it will occur rapidly.) The RNA (see Figure 5.26). respiration spontaneous or not? Is it forexergonic an exergonicor en- represents the Fuels are degraded and large molecules are dergonic? What happens magnitude to the maximum of ∆G energy amount released of work reaction from the reaction can perform.* The The bonds between the phosphate Products groups of ATP can be constructed step by step in a series of linked glucose? greater the decrease in free energy, the greater the amount of work that can be done. broken by hydrolysis. When the terminal phosphate bond is broken by addition of a water molecule, a molecule Amount of energy of inor- 2. MAKE CONNECTIONS AsWeyou can saw inoverall Figure 7.20for oncellular respiration as reactions called metabolic pathways. use the reaction 2– ganic phosphate (HOPO3 , abbreviated P(∆G required Free energy i throughout > 0) this page 137, a key process in metabolism is the trans- an example: Energy book) leaves the ATP, Reactants which becomes adenosine diphosphate, port of hydrogen ions (H ) across ! C6H12aOmembrane 6 ! 6 O2 S 6 CO to2 !cre- 6 H2O An energy currency common ate a concentration gradient. to all Other life forms, "G # $686 kcal/mol ($2,870 kJ/mol) processes can adenosine triphosphate (ATP), links energy- result in an equal concentration of H on each side. *The word maximum qualifies !this statement, because some of the free energy is released as heat and cannot do work. Therefore, ∆G represents a theoretical Progress of the reaction Which situation allows the H to perform work in ! upper limit of available energy. releasing pathways with this system? How energy-requiring is the answer consistent with what Adenine NH2 pathways. 3. isWHAT An Introduction to Metabolism 147 shown in regard to energy in Figure 7.20? CHAPTER 8 N C IF? Some night-time partygoers wear glow- C N in-the-dark necklaces. The necklaces start glowing O O O HC Although there are many metabolic once they are “activated,” which usually involves –O P O P O P O CH2 N C N CH O pathways, a limited number of types of snapping the necklace in a way that allows two O– O– O– H H chemicals to react and emit light in the form of reactions and particular intermediates are chemiluminescence. Is the chemical reaction exer- Phosphate groups H H Ribose common to many pathways. gonic or endergonic? Explain your answer. OH OH For suggested answers, see Appendix A. (a) The structure of ATP. In the cell, most hydroxyl groups of phosphates are ionized (—O – ). CONCEPT 8.3 P P P ATP powers cellular work by  Key Metabolic Pathways L26 L23 L27 L24 L25 5  Glucose is Generated from Dietary Carbohydrates Starch Glycogen Polymer of (1→4) linked D-glucose with (1→6) linked branches every 8-14 residues Glucose storage molecule of animals, easily mobilized Stored in liver and muscle Energy reserve for approx. half a day Glycogen Glycogen does not possess a free reducing end: the “reducing end” is covalently bound to a protein termed glycogenin. 11 October 2019 BMOL20110 - Jana Haase - Lecture 31 1 31 Rings structures of sugars Glycogen breakdown Phosphorylase Transferase (debranching enzyme) -1,6-Glucosidase (debranching enzyme) See lectures S. Nathwani  Glucose is an Important Fuel for Most Organisms Almost all organisms use glucose as a fuel. In mammals, glucose is the only fuel the brain uses under non-starvation conditions and the only fuel red blood cells are able to use at all. Why is glucose such a prominent fuel in all life-forms? Glucose may have been available for primitive biochemical systems because it can form under prebiotic conditions. Glucose is the most stable hexose.  Some fates of Glucose… Glycolysis is anaerobic because it does not require oxygen; it evolved before oxygen was abundant in the atmosphere. Glycolysis produces pyruvate, which has various fates depending on the organism and whether oxygen is abundant. Today’s Lecture Lecture 25  down. Glycolysis involves a sequence of 10 separate reactions, each producing a different sugar intermediate and each catalyzed by a different enzyme. Glycolysis is the Central ATP-producing Pathway These reactions are presented in outline in Figure 13–3 and in detail in gly-colysis—from the Greek STAGE 1: BREAKDOWN proteins polysaccharides fats glykys, “sweet,” and lysis, OF LARGE MACROMOLECULES TO SIMPLE “splitting.” fatty acids SUBUNITS amino acids simple sugars and glycerol CYTOSOL glucose Glycolysis produces ATP STAGE 2: glycolysis ATP without the involvement of O2. BREAKDOWN OF SIMPLE SUBUNITS TO ACETYL CoA; NADH ACCOMPANIED BY PRODUCTION OF pyruvate plasma It occurs in the cytosol of most LIMITED AMOUNTS membrane OF ATP AND NADH of eucaryotic cell cells, including many acetyl CoA anaerobic microorganisms. Glycolysis probably evolved citric acid cycle mitochondrial early in the history of life, matrix Figure 13–2 Three stages STAGE 3: mitochondrial of cellular metabolism before photosynthetic COMPLETE membranes lead from food to waste OXIDATION products in animal cells. This OF ACETYL organisms introduced oxygen series of reactions produces ATP, which is then used to CoA TO H2O AND CO2; ACCOMPANIED reducing power as NADH into the atmosphere. and drive biosynthetic reactions BY PRODUCTION other energy-requiring phosphorylation OF LARGE AMOUNTS ATP processes in the cell. Stage 1 OF ATP IN oxidative mostly occurs outside cells— MITOCHONDRION ATP although special organelles O2 called lysosomes can digest ATP large molecules in the cell interior. Stage 2 occurs mainly in the cytosol, except for the NH3 H2O CO2 final step of conversion of pyruvate to acetyl groups on acetyl CoA, which occurs in mitochondria. Stage 3 occurs waste products entirely in mitochondria. cation of electrons releases energy stored in organic mol- ecules, and this energy ultimately is used to synthesize ATP. REDOX Reactions The Principle of Redox In many How do chemical the catabolic reactions, pathways there is a transfer of onethat or more decompose electrons (e" ) from oneglucose reactant andto other organic another. Thesefuels electron yield energy? transfers are called oxidation-reduction reactions, or redox Cellular respiration does not oxidize glucose reactions for short. In aisredox reaction, the transfer loss of electrons in a single explosive step. The answer based on the of from one substance electrons is calledthe during oxidation, and the addition of chemical reactions. Glucose is broken down in a series of steps, electrons to another substance is known as reduction. (Note each one catalysed by an enzyme. The that adding relocation electrons of electrons is called reduction;releases energy negatively charged stored to electrons added in an organic atommolecules, reduce the and this energy amount At key steps, electrons are stripped from of positive ultimately charge of that atom.)isTo usedtaketoasynthesize ATP. simple, nonbiological exam- the glucose. ple, consider the reaction between the elements sodium (Na) As is often the case in oxidation reactions, and chlorine (Cl) that forms table salt: each electron travels with a proton - thus, becomes oxidized as a hydrogen atom. (loses electron) Na + Cl Na+ + Cl– The hydrogen atoms are not transferred becomes reduced directly to oxygen, but instead are usually (gains electron) passed first to an electron carrier, a is respiration: the oxidation of glucose and other molecules We in could food. generalize Examine again athe redox reaction summary this way: equation for cellular coenzyme called NAD+ (nicotinamide respiration, but this time think of it as a redox process: adenine dinucleotide), a derivative of the becomes oxidized becomes oxidized vitamin niacin). Xe – + Y X + Ye – C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy becomes reduced becomes reduced InAsthe in the combustionreaction, generalized of methanesubstance or gasoline,Xe the " fuel (glu- , the electron cose) is donor, is oxidized and called the oxygen is reduced. reducing agent; itThe electrons reduces lose Y, which ac- potential energy along the way, and energy is released. cepts the donated electron. Substance Y, the electron accep- In general, organic molecules that have an abundance of electrons added to an atom reduce the amount of positive charge of that atom.) To take a simple, nonbiological exam- REDOX ple, consider the reaction between the elements sodium (Na)Reactions Reactants Products is respiration: the oxidation of glucose and other molecules and chlorine (Cl) that forms table salt: in food. Examine again the summary equation for cellular becomes oxidized respiration, but this time think of it as a redox process: becomes oxidized CH4 + 2 O2 electron) CO2 + Energy + 2 H2O (loses becomes oxidized Na + Cl + Na + becomes reduced Cl – C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy H becomes reduced becomes reduced (gains electron) As in the combustion of methane or gasoline, the fuel (glu- H C H O O O C O H O H cose) is oxidized and oxygen is reduced. The electrons lose We could generalize a redox reaction this way: H potential energy along the way, and energy is released. Methane Oxygen oxidized becomes Carbon dioxide Water In general, organic molecules that have an abundance of (reducing – (oxidizing hydrogen are excellent fuels because their bonds are a source agent) Xe + Redox agent) X + Ye – Y (reduction–oxidation, is aoftype of chemical reaction in which becomes reduced “hilltop” electrons, whose energy may be released as these the oxidation states of substrate electrons ! Figure 9.3 Methane combustion as an energy-yielding change. “fall” down an energy gradient when they are trans- In redox the reaction. generalized reaction, The reaction substance releases energy Xe", the electron ferred to oxygen. The summary equation for respiration indi- to the surroundings becauseisthe electrons losereducing agent; potential energy donor, called the unequally, spending Oxidation more iswhen thethey time near electronegative end itloss up being Y,shared which ac- cates ofaselectrons reduces atoms such oxygen. or anthat hydrogen is transferred from glucose to oxygen. But increase in the oxidation cepts the donatedstate electron. the important point, not visible in the summary equation, is of Substance a chemical Y, theor electron atoms accep- within it. tor, is the oxidizing agent; it oxidizes Xe" by removing its that the energy state of the electron changes as hydrogen between methane and oxygen, shown in Figure 9.3, is an ex- (with its electron) is transferred to oxygen. In respiration, the electron. Because an electron transfer requires both a donor Reduction ample. As explained in Chapter 2,isthe the gainelectrons covalent of electrons in or a decrease oxidation in electrons of glucose transfers the oxidation to a lower energy state, and an acceptor, oxidation and reduction methane are shared nearly equally between the bonded atoms always go together. liberating energy that becomes available for ATP synthesis. Not allcarbon redoxand state reactions of a chemical or atoms within it. Wiki because hydrogeninvolve have about thethecomplete transfer same affinity for of The main energy-yielding foods, carbohydrates and fats, electrons from onethey valence electrons; substance are about to equally another; some change electronegative. the de- are reservoirs of electrons associated with hydrogen. Only the But when gree of methane electronreacts with oxygen, sharing forming in covalent carbon The bonds. dioxide, reaction barrier of activation energy holds back the flood of electrons electrons end up shared less equally between the carbon atom to a lower energy state (see Figure 8.12). Without this barrier, and its new covalent partners, the oxygen atoms, which are a food substance like glucose would combine almost instan- very electronegative. In effect, the carbon atom has partially taneously with O2. If we supply the activation energy by ig- “lost”One way its shared to remember electrons; thus, methane has this… been oxidized. niting glucose, it burns in air, releasing 686 kcal (2,870 kJ) of Now let’s examine the fate of the reactant O2. The two heat per mole of glucose (about 180 g). Body temperature is atoms “OILof theRIG. Oxidation oxygen molecule (OIs Loss (of electrons) and 2) share their electrons not highReduction Is Gaining enough to initiate (of burning, of electrons)” course. Instead, if  NAD+ (nicotinamide adenine dinucleotide) as an electron shuttle NAD+ is well suited as an electron carrier because it can cycle easily between oxidized (NAD+) and reduced (NADH) states. As an electron acceptor, NAD+ functions as an oxidizing agent during respiration. 2 e– + 2 H+ 2 e– + H+ NAD+ NADH H+ Dehydrogenase H O O Reduction of NAD+ H H C NH2 + 2[H] C NH2 + H+ (from food) Oxidation of NADH N+ Nicotinamide N Nicotinamide (oxidized form) (reduced form) O CH2 O O P O– O H H O P O– HO OH NH2 O CH2 ! Figure 9.4 NAD! as an electron shuttle. The full name for N N NAD!, nicotinamide adenine dinucleotide, describes its structure: The H molecule consists of two nucleotides joined together at their phosphate N N H O 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 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!. NAD+ NADH/H+ pair of hydrogen atoms (2 electrons and 2 protons) from the gases combine explosively. In fact, combustion of liqu  The stepwise oxidation of sugars begins with glycolysis. 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 a six-carbon sugar into two three-carbon OH investment to be Each of the 10 steps of glycolysis is sugars, so that the number of molecules at every stage after this doubles. As indicated, recouped ATP STEP 1 later catalysed by a different enzyme. 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 During glycolysis, a glucose molecule (6 13–1). Glycolysis is also sometimes referred to as the Embden–Meyerhof pathway, STEP 3 carbon atoms) is cleaved into two named for the chemists who first fructose 1,6- P OH2C O CH2O P molecules of pyruvate (3 carbon atoms) described it. All the steps of glycolysis are reviewed in Movie 13.1. bisphosphate HO OH OH For each molecule of glucose, two STEP 4 cleavage of molecules of ATP are consumed to six-carbon sugar to two provide energy to drive the early steps. three-carbon STEP 5 sugars CHO CHO Four molecules of ATP are produced in two molecules of glyceraldehyde the later steps. CHOH CHOH 3-phosphate CH2O P CH2O P Thus, at the end of glycolysis, there is a NADH STEP 6 NADH net gain of two molecules of ATP for ATP STEP 7 ATP each glucose molecule broken down. STEP 8 STEP 9 energy generation ATP STEP 10 ATP COO– COO– two molecules C O C O of pyruvate CH3 CH3  Glycolysis – steps 1-3 – Investing energy Alberts, Essential Cell Biology, Ch. 13  Glycolysis – steps 4-5. 6-carbon to 3-carbon sugar Alberts, Essential Cell Biology, Ch. 13  Glycolysis – steps 6-10. Energy Generation Alberts, Essential Cell Biology, Ch. 13  Glycolysis – steps 6-10. Energy Generation Alberts, Essential Cell Biology, Ch. 13  pyruvate. The formation of pyruvate from 2-phosphoglycerate is, in essence, an internal oxidation–reduction reaction; carbon 3 takes electrons from carbon 2 in the conversionGlycolysis 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 of a compound with high phosphoryl-transfer potential, phospho- enolpyruvate here and 1,3-bisphosphoglycerate earlier, which allows the synthesis of ATP. Because the molecules of ATP used in forming fructose 1,6-bisphosphate have already been regenerated, the two molecules of ATP generated from phosphoenolpyruvate are “profit.” Two ATP molecules are formed in the conversion of glucose into pyruvate The The netnet reaction reaction in the in the transformation transformation of glucose of glucose intointo pyruvate pyruvate is is Glucose 1 2 Pi 1 2 ADP 1 2 NAD1 y 2 pyruvate 1 2 ATP 1 2 NADH 1 2 H1 1 2 H2O Thus, two molecules of ATP are generated in the conversion of glucose into Two molecules of ATP are generated in the conversion of glucose into two two molecules of pyruvate. The reactions of glycolysis are summarized in molecules of pyruvate. Table 16.1. The energy released The energy in the anaerobic released conversion in the anaerobic of glucose conversion into two into of glucose molecules two ofmolecules pyruvate isofabout -96 kJ pyruvate mol. 296 kJ mol (–23 kcal mol ). We shall see is about -1 21 21 Reaction of Glycolysis  What happens with Pyruvate? The conversion of glucose into pyruvate generates ATP, but for ATP synthesis to continue, NADH must be re-oxidized to NAD+. Cells have limited amounts of NAD+, which is derived from niacin (vitamin B3). NAD+ can be regenerated by further oxidation of pyruvate to CO2 or by the formation of ethanol or lactate from pyruvate. Fermentations Provide Usable Energy in the Absence of Oxygen Obligate anaerobes cannot survive in the presence of O2.  1. Alcoholic Fermentation The first type of fermentation is regeneration of NAD+ by processing pyruvate to ethanol. This is called alcoholic fermentation. BMOL20060. Remember Practical 4 on Alcoholic Fermentation!  2. Lactic Acid Fermentation The second type of fermentation is lactic acid fermentation, in which pyruvate is reduced to lactate to regenerate NAD+. The net reaction starting from glucose:  3. Aerobic Metabolism Through the Citric Acid Cycle and Electron Transport Chain The third possible fate of pyruvate, rather than a type of fermentation, is to L24 completely oxidize the pyruvate through pyruvate processing (to generate acetyl CoA) and the citric acid cycle (to oxidize the acetyl group). The electrons are transferred to the final acceptor in the electron transport chain (i.e., O2) via aerobic metabolism. As the electrons are delivered from NADH to the electron transport chain, NAD+ is restored.  This detour is achieved by substituting a set of alternative, enzyme- catalyzed “bypass reactions” that require an input of chemical energy (reactions A, B, C, and D in Figure 13–20). The reactions that synthesize Gluconeogenesis a molecule of glucose in gluconeogenesis thus require the hydrolysis of four ATP and two GTP molecules, compared with the overall generation of two molecules of ATP for each molecule of glucose consumed during glycolysis. glucose InThe body needs a continuous humans and other mammals, gluconeogenesis occurs mainly in liver ATP supply cells, ofkeepglucose which can to with the blood supplied meet glucose its by using many dif- Step 1 A Pi metabolic ferent molecules as needs. the starting point. One common input is lactate: this ADP H2O molecule, produced by overworked muscle cells, is taken up by the liver glucose 6-phosphate where it gets converted back into glucose to replenish depleted muscles. For example, brain cells depend The balance between glycolysis and gluconeogenesis must be highly reg- ulated, so that glucose is broken down rapidly when energy reserves run almost completely on glucose low, but is synthesized and exported to other tissues when the liver cell fructose 6-phosphate for respiration. has sufficient energy reserves in the form of pyruvate, citrate, or ATP. If ATP Pi both the forward and reverse reactions in Figure 13–20 were allowed to Step 3 B proceed without restraint, they would shuttle metabolites backward and ADP H2O During periods without food and forward in futile cycles that would consume large amounts of energy and fructose 1,6-bisphosphate during hard physical exercise, generate heat for no purpose. One of the key control points in the breakdown of glucose lies in step 3 the of glucose glycolysis, in the the production blood of fructose is used by the enzyme 1,6-bisphosphate GLYCOLYSIS GLUCONEOGENESIS up faster This phosphofructokinase. thanis one ofit is being the reactions that must be bypassed replaced from food. in gluconeogenesis (see step 3 in Figure 13–20 or in Panel 13–1, p. 430). Phosphofructokinase is allosterically activated by AMP, ADP, and inor- ADP ADP ganic phosphate—the byproducts of ATP hydrolysis; it is allosterically ATP ATP One way to replenish blood inhibited by ATP, citrate, and alternative fuels for respiration, such as fatty acids, which can be liberated from stored fat when glucose is not glucose is to synthesize it from phosphoenolpyruvate small non-carbohydrate Figure 13–20 Gluconeogenesis effectivelyorganic“reverses” reactions that occur during glycolysis. A set of four bypass reactions (labeled molecules A through D)such asaroundlactate, is needed to get steps 1, 3, and 10 in glycolysis, C GDP pyruvate, orin gluconeogenesis carried out amino acids in of a which are essentially irreversible. As can be seen, synthetic reactions require an input energy, whereas ADP GTP process called gluconeogenesis of reactions. To glycolysis as a whole is an energetically favorable set keep track of the energy produced or consumed in these processes, Step 10 oxaloacetate ATP recall that during glycolysis, fructose 1, 6-bisphosphate gets cleaved ADP to form two 3-carbon sugars (not shown). Thus, all the reactions that D follow, whether they are part of glycolysis or gluconeogenesis, involve ATP two sugars—and twice the number of energy carriers—for each pyruvate molecule of glucose that is consumed or produced.  Three irreversible steps in glycolysis must be bypassed in gluconeogenesis (1,3, and 10). Gluconeogenesis Is Not a Reversal of Glycolysis!! e- gy ze of Gluconeogenesis-specific enzymatic reactions and enzmes n ng 6-phosphatase, which is bound to the membrane glucose (Figure 16.29). An associated Ca21-binding stabi- SP Cytoplasmic side er ATP lizing protein is essential for phosphatase activity. f- Glucose Step 1 andPPi i are then shuttled back to the cyto- T1 Glucose 6- T2 T3 is ADP plasm by A A a pair of transporters. The glucose trans- phosphatase porter in the H2O endoplasmic reticulum membrane er is6-phosphate glucose like those found in the plasma membrane. It is H2O + glucose Pi + glucose ER lumen s. striking that five proteins are needed to transform 6-phosphate g- cytoplasmic glucose 6-phosphate into glucose. n Figure 16.29 Generation of glucose from glucose 6-phosphate. Several fructose 6-phosphate endoplasmic reticulum (ER) proteins play a role in the generation of glucose ell Six high-transfer-potential phosphoryl groups from glucose 6-phosphate. T1 transports glucose 6-phosphate into the lumen of the If ATP are spent in Psynthesizing glucose from pyruvate ER, whereas T2 and T3 transport Pi and glucose, respectively, back into the cytoplasm. i o Step 3 Glucose 6-phosphatase is stabilized by a Ca21-binding protein (SP). [After A. Buchell nd ADP The formation B getically unfavorable B of glucose from pyruvate is ener- unless it is coupled to reac- and I. D. Waddel. Biochem. Biophys. Acta 1092:129–137, 1991.] H2O nd tions that are favorable. Compare the stoichiometry fructose 1,6-bisphosphate of gluconeogenesis with that of the reverse of gly- 3 colysis. GLYCOLYSIS The stoichiometry of gluconeogenesis is GLUCONEOGENESIS me ed 2 Pyruvate 1 4 ATP 1 2 GTP 1 2 NADH 1 6 H2O S ). ADP ADP glucose 1 4 ADP 1 2 GDP 1 6 Pi 1 2 NAD 1 1 2 H 1 r- ¢G°¿ 5 248 kJ mol 21 (211 kcal mol 21 ) ly ATP ATP as In contrast, the stoichiometry for the reversal of glycolysis is ot 2 Pyruvate 1 2 ATP 1 NADH 1 2 H2O S C phosphoenolpyruvate glucose 1 2 ADP 1 2 Pi 1 2 NAD 1 1 2 H 1 ¢G°¿ 5 184 kJ mol 21 (120 kcal mol 21 ) GDP Note thatC six nucleoside triphosphate molecules are hydrolyzed to synthe- size glucose from GTP pyruvate in gluconeogenesis, whereas only two molecules ADP of ATP are generated in glycolysis in the conversion of glucose into pyru- Step 10 oxaloacetate vate. Thus, the extra cost of gluconeogenesis is four high-phosphoryl- ATP transfer-potential molecules for each molecule of glucose synthesized from ADP pyruvate. The four additional molecules having high phosphoryl-transfer e D D potential are needed to turn an energetically unfavorable process (the rever- ATP sal of glycolysis) into a favorable one (gluconeogenesis). Here we have a pyruvate clear example of the coupling of reactions: NTP hydrolysis is used to power an energetically unfavorable reaction. The reactions of gluconeogenesis are summarized in Table 16.6.  Reactions of Gluconeogenesis * irreversible reactions specific for gluconeogenesis The formation of glucose from pyruvate is ener- endoplasmic and reticulum I. D. Waddel. (ER)Biophys. Biochem. proteinsA Six high-transfer-potential getically unfavorable unless phosphoryl it is coupledgroups to reac- from glucose 6-phosphate. T1 transpor are spent tions that are Stoichiometry infavorable. synthesizing glucose Compare of from the Gluconeogenesis pyruvate stoichiometry ER, whereas T2 and T3 transport Pi and ofThe gluconeogenesis Glucose 6-phosphatase is stabilized by formation of with thatfrom glucose of the reverse is pyruvate of ener- gly- and I. D. Waddel. Biochem. Biophys. colysis. getically unfavorable unless it is coupled to reac- The tions stoichiometry that ofCompare are favorable. gluconeogenesis is the stoichiometry of gluconeogenesis with that of the reverse of gly- Pyruvate 1 4 ATP 1 2 GTP 1 2 NADH 1 6 H2O S 2colysis. 1 1 glucoseof1gluconeogenesis The stoichiometry 4 ADP 1 2 GDP is 1 6 Pi 1 2 NAD 1 2 H ¢G°¿ 5 248 kJ mol 21 (211 kcal mol 21 ) 2 Pyruvate 1 4 ATP 1 2 GTP 1 2 NADH 1 6 H2O S In contrast, the stoichiometry for the reversal of glycolysis is glucose 1 4 ADP 1 2 GDP 1 6 Pi 1 2 NAD 1 1 2 H 1 21 21 2 Pyruvate 1 2 ATP 1 NADH 1 ¢G°¿2 H25O248 S kJ mol (211 kcal mol ) 1 In contrast, the stoichiometry glucose 2 ADP 1 1 reversal for the of 2glycolysis Pi 1 2 NAD is 1 2 H1 ¢G°¿ 5 184 kJ mol 21 (120 kcal mol 21 ) 2 Pyruvate 1 2 ATP 1 NADH 1 2 H2O S Note that six nucleoside triphosphate molecules are hydrolyzed to 1 synthe-1 size glucose from pyruvate inglucose 1 2 ADP 1 gluconeogenesis, 2 Pi 1only whereas 21 2 NAD two 1 2H molecules 21 ¢G°¿ of ATP are generated in glycolysis in the conversion of glucose into pyru-) 5 184 kJ mol (120 kcal mol vate. NoteThus, that sixthe extra cost nucleoside of gluconeogenesis triphosphate moleculesisarefour high-phosphoryl- hydrolyzed to synthe- transfer-potential size glucose from molecules pyruvate in forgluconeogenesis, each molecule ofwhereasglucoseonly synthesized from two molecules pyruvate. of ATP are The four additional generated molecules in glycolysis in thehaving high of conversion phosphoryl-transfer glucose into pyru- potential are needed vate. Thus, to turn the extra costanofenergetically gluconeogenesisunfavorable is fourprocess (the rever- high-phosphoryl-  Glycolysis - Key scientists involved in research The first steps in understanding glycolysis began in the nineteenth century with the wine industry. For economic reasons, the French wine industry sought to investigate why wine sometimes turned distasteful, instead of fermenting into alcohol. 1850: Yeast's glucose consumption decreased 1890: discovers cell- under aerobic conditions free fermentation of fermentation Louis Pasteur Eduard Buchner 1929: discovered the regulatory effects of ATP on glucose consumption during alcohol fermentation. They also shed light on the role of one compound as a glycolysis intermediate: fructose 1,6-bisphosphate Sir Arthur Harden William John Young  Glycolysis - Key scientists involved in research THE EQUILIBRIA OF ISOMERASE AND ALDOLASE, AND THE PROBLEM OF THE PHOSPHORYLATION OF 1920: linked together GLYCERALDEHYDE PHOSPHATE* some of the many BY 0. MEYERHOF AND R. JUNOWICZ-KOCHOLATY individual pieces of (From the Department of Physiological Chemistry, School of Medicine, University 01 Pennsylvania, Philadelphia) glycolysis (Received for publication, February 23, 1943) As was shown some years ago (2-4) hexose diphosphate is split into triose phosphate by the reaction catalyzed by the enzyme zymohexase according Otto Meyerhof t,o the following equation, Downloaded from http://www.jbc.org/ by guest on October 31, 2019 dihydroxyacetone phosphate IL (1) Hexose diphosphate = d-3-glyceraldehyde phosphate + dihydroxyacetone phosphate While in the final state at least 95 per cent of the triose phosphate is di- hydroxyacetone phosphate (2), it can be proved, nevertheless, that the reaction starts with the cleavage into 1 mole of aldotriose and 1 mole of ketotriose, owing to an enzyme which has been called aldolase. Isomeriza- tion is brought about by a second enzyme which has been called isomerase (5). The most direct proof of this sequence of reactions can be supplied by trapping the triose phosphate before isomerization sets in; e.g., by hydrazine. In the presence of hydrazine about equal quantities of glycer- aldehyde phosphate and dihydroxyacetone phosphate are obtained (6). Jakub Karol Parnas Glycolysis is also known as the Embden– The equilibrium constant of the isomerase reaction is of special interest, since in the steady state all sugar breakdown goes by the way of glycer- Meyerhof–Parnas (EMP) pathway aldehyde phosphate (7). Xegelein and Brijmel (8) assumed that the precursor of the 1,3-diphos- phoglyceric acid, which they had isolated, was a 1,3-diphosphoglyceralde- hyde, formed spontaneously by addition of phosphate to 3-glyceraldehyde phosphate. If this is true, then in the presence of phosphat,e the following four equilibrium constants must exist. [3-glyccraldehyde phosphate]. [dihydroxyacetone phosphate] I. zc aldolase = ~~~ ---- [hexose diphosphate] * A preliminary report of part of this paper was given at the Chicago meeting of the Federation of American Societies for Experimental Biology, April, 1941 (1). Aided by grants from Hoffmann-LaRoche, Inc., Nutley, New Jersey, and from the Gustav Embden Penrose Fund of the American Philosophical Society. 71  The next lectures… L26 L23 L27 L24 L25 31 Jens Rauch, PhD School of Biomolecular and Biomedical Science Thanks! Systems Biology Ireland Email: [email protected] Phone: +353-(0)1-716 6337 @jensrauch 32