Class Note for BIOC 460 at UA
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Date Created: 02/06/15
Bioc 460 Dr Miesfeld Fall 2008 Fermeation Lecture 23 Glycolysis Reactions Key Concepts Overview of the Glycolytic Pathway Stage 1 ATP Investment Stage 2 ATP Earnings KEY CONCEPT QUESTION IN GL YCOL YSIS What are the two substrate level phosphoryation reactions in glycolysis OVERVIEW OF THE GL YCOL YTIC PATHWAY Glycolysis is considered one of the core metabolic pathways in nature for three primary reasons 1 glycolytic enzymes are highly conserved amongst all living organisms suggesting it is an ancient pathway 2 glycolysis is the primary pathway for ATP generation under anaerobic conditions and in cells lacking mitochondria such as erythrocytes and 3 metabolites of glycolysis are precursors for a large number of interdependent pathways including mitochondrial ATP synthesis wie i 7 7 The word glycolysis is derived from the Greek gykys meaning sweet and lysis which means to split or break referring to the splitting of one molecule of glucose into two molecules of pyruvate L r l We will focus on two aspects of glycolysis 1 the enzymatic 39 7 7 reactions that convert glucose to pyruvate and 2 the role of Glycolysis glycolysis in providing precursors especially pyruvate for other metabolic pathways in humans As shown in figure 1 glycolysis sits at the top of a set of four interconnected pathways that are chasm responsible for the complete oxidation of glucose to 002 and H20 39 by the following reaction Electron quot i 39 Transport Glucose CGleoe 602 9 6C02 6HZO 82131in AG 2840 kJmol AG 2937 kJmol Let s begin by answering the four questions in our guidebook that pertain to glycolysis 1 What does glycolysis accomplish for the cell Generates a small amount of ATP which is critical under anaerobic conditions Generates pyruvate a precursor to acetyl CoA lactate and ethanol in yeast 2 What is the overall net reaction of glycolysis Glucose 2NAD 2ADP 2 Pi 9 2 pyruvate 2NADH 2H 2ATP 2H20 AG 358 kJmol 3 What are the key enzymes in glycolysis Hexokinase commitment step in glycolysis inhibited by glucose6 P product Phosphofructokinase1 activated by AMP low energy charge and F26 BP inhibited by ATP high energy charge and citrate citrate cycle intermediate Pyruvate kinase activated by AMP and fructose16 bisphosphate feed forward inhibited by ATP and acetyl CoA excess energy source 1 of11 pages Bioc 460 Dr Miesfeld Fall 2008 4 What are examples of glycolysis in real life Glycolysis is the sole source of ATP under anaerobic conditions which can occur in animal muscle tissue during intense exercise Fermentation also relies on glycolysis which is a process that is used to make alcoholic beverages when yeast cells are provided glucose without oxygen As shown in figure 2 glycolysis takes place entirely in the cytosol whereas pyruvate oxidation to C02 and H20 occurs in the mitochondrial matrix and requires the citrate cycle Figure 2 electron transport chain and oxidative G39UEOSE Fatty Acids phosphorylation Acetyl CoA is the primary AnaerOblC ATP metabolite in energy conversion pathways Lactate K Gycoysis inside the mitochondrial matrix and is also I Pyruvate the major breakdown product of fatty acid Ethano COZ 4 l CYTOSOL oxidation Glycolysis does not require oxygen and therefore can function under anaerobic conditions however complete oxidation of glucose to C02 and H20 5 requires oxygen The combined reactions Aerobic of glycolysis the citrate cycle electron 3 L transport chain and oxidative t 1sz Sll liilii39 l i ny39E39Sij ipal yields 2H 0 o a 2 lt 2 ransport only 2 ATP out of the total 32 6 I g5 Pi M Therefore it is the oxidation of pyruvate MITOCHONDRON bxida e V 7 0 and acetyl CoA in the mitochondria that Phosphorylation t H H H 4 generates the majority of ATP 94 for 7 most cells in an organism Figure 3 shows the molecular structures of glucose and pyruvate to illustrate that the six carbons and six oxygens present in glucose are stoichiometrically conserved by glycolysis in the two molecules of pyruvate that are M produced Glucose CoHIZOG The ten enzymatic reactions of glycolysis are summarized in figure and involve primarily bond rearrangements brought about by enzymes that catalyze phosphoryl transfer reactions isomerizations an aldol cleavage an oxidation and a dehydration There is no net loss of carbon or oxygen atoms Because of the F requirement for ATP hydrolysis in the initial reactions followed by ATP synthesis in the later reactions glycolysis is divided into two stages Stage 1 ATP investment reactions 15 and Stage 2 ATP earnings reactions 610 Figure 5 summarizes the gycoytic pathway showing that for every Pyruvate Pyruvate mole of glucose entering glycolysis two moles of glyceraldehyde 3P are metabolized to pyruvate and in the process generating a net 2 ATP and 2 NADH The quotnodesquot in this wiring diagram represent metabolites in the gycoytic pathway Key reaction steps are highlighted along with the corresponding enzymes 2 of11 pages One way to understand how a pathway is regulated is to examine 39ee energy changes AG and AG that take place in each reaction to identify steps that are critical in driving the overall pathway towards product formation The 39ee energy changes for the ten glycolytic reactions are listed in Figure 6 including both the AG of each reaction which is measured in the laboratory under standard conditions and the calculated AG value using AG AG RTlnmass action ratio based on measured metabolite concentrations in erythrocytes under steady state conditions The reactions catalyzed by the enzymes hexokinase phosphofructokinase and pyruvate kinase have large negative AG values and are therefore considered irreversible reactions under physiological conditions Although several ofthe glycolytic reactions are shovm to have positive AG values it is likely that the mass action ratios for these reactions do not represent actual metabolite concentrations under conditions ofhigh metabolic ux A good example of this is reaction 5 catalyzed by the Figure 4 Stage 1 ATP invejtmenr lu39 Hexokinase Glucokinase mumquot inpimpn m Phosphcglucose isomerase rmm A mmmm Phosphofructokinase m hunqu u mpxmnm Aldolase I 1thuauw Human imam Triose phosphate isomerase Bloc 460 7 Dr Mlesfeld Fall 2008 u H n H m n m m u on 39rwrl ll M Ull Mn l in on u no n im w n p U n m x i 1 I an an n H4 1 A wquot Irll H lfrllrl HKA39A llxllt u l Stage 2ATP Earnings l Immimlnm lIrlvwhlmn 2n GIyteradehyderirf JNl dehydrogenase LWHispluu rl hum u Phosphoglycerate kinase mm Phosphaglycerate mutase M nymph i i Enolase 3 m1 m u in Pyruvate kinase mm 2 0 ou MM is a Hit m 0 MM 7 M m 7mm l UH f enzyme triose phosphate isomerase which has an estimated AG of 25 kJmol but is nevertheless pulled to the right due to the rapid depletion of glyceraldehyde3 P by the highly favorable reactions in stage 2 of glycolysis Indeed the net AG value for glycolysis in erythrocytes under steadystate conditions is overwhelmingly negative 765 kJmol con rming that the glycolytic pathway is highly favorable Bot 11 pages STAGE 1 ATP INVESTMENT Stage 1 ofglycolysis includes five enzymatic reactions that accomplish two tasks First using ATP as the phosphate donor they create phosphorylated compounds that are negatively charged and cannot diffuse out of the cell These phosphorylated metabolites are highly specific substrates for glycolytic enzymes and are the precursors to the high energy compounds 13 bisphoglycerate and phosphoenolpyruvate in stage 2 that are used to generate ATP by substrate level phosphorylation Second the aldolase reaction in step 4 splits the six carbon fructose16BP compound into two halves creating glyceraldehyde3P and dihydroxyacetoneP the latter of which is quickly isomerized to form a second molecule ofglyceraldehyde3P Bioc 460 Dr Miesfeld Fall 2008 Figure 5 NADJr NAD Pyruvate kinase ATP OWN Phosphofructokinase l quotDgGyceraldehydeG P dehydrogenase o it kinase 9 a l i mw MW Phosphoglycerate NAD NADH 6 Enolase 3 i 1 L L y A yruvat Figure 6 AG in AG in kcal mol1 keal mol39l Enzyme Reaction type kJ mol kj mol I 1Hexokinase Phosphoryl transfer 740 167 80 335 I 21 hosphoglucose isomerase somerization 04 1 1 7 06 2 5 31 l1osphofructokinase Phosphoryl transfer 3lgt 112 33 222 I 4 Aldolase Aldo cleavage 57 238 03 13 5 Triose phosphate isomerase lsomerization 18 75 l 06 25 6Glyceraldehyde 339ph08phate Phosphorylation coupled 15 63 1 0 6 35 dehydrogenase to oxidation 7 Phogphoglycerate kinase Phosphoryl transfer 45 188 03 13 8 Phosphoglycerate mutase Phosphoryl shift 11 46 02 08 9 Enolase Dehydration 04 17 708 733 3910 Inmate kinase Phosphoryl transfer 73 314 10 167 4 of 11 pages Bloc 460 Dr Miesfeld Fall 2008 Reaction 1 Phosphorylation ofglucose by hexokinase or glucokinase The rst reaction in glycolysis serves to activate glucose for catabolism by attaching a phosphate group to the C6 position to generate glucose6P as illustrated in gure 7 This is the rst oftwo ATP investment steps in stage 1 ofglycolysis and uses the free energy released from ATP hydrolysis to drive the phosphoryl transfer reaction Figure 8 shows the pyranose ring structure of glucose and glucose6P Figure 7 Figure 8 CHZOH CHZOI Oy O o o llmnkindw F1 0H ATP T 0H ADP II 2 HO OH HO OH O 39 O OH OH H Glucose Glucose 6phosphate csv C Glumse GlutoSEGphosphate Two enzymes catalyze this phosphorylation reaction hexokinase which is found in all cells and lucokinase which is present primarily in liver and pancreatic cells Hexokinase has a broad range of substrate speci cities and also phosphorylates mannose and fructose whereas gucokinase is highly speci c for glucose Hexokinase activity is inhibited by the product ofthe reaction glucose6P which accumulates in cells when ux through the glycolytic pathway is restricted As described later gucokinase has a much lower af nity for glucose and is not feedback inhibited by glucose6P These enzymatic properties facilitate the function of gucokinase as a metabolic sensor of high blood glucose levels Hexokinase binds glucose through an induced 59M t mechanism that excludes H20 from the enzyme active site and bringsthe phosphoryl group of ATP into close proximity with the C6 carbon of glucose As shown in figure 9 the molecular structure of yeast hexokinase in the presence and absence of glucose suggests that two domains ofthe enzyme are like jaws that clamp down on the substrate through a large conformational change Glucose Figure 10 illustrates the induced t mechanism of hexokinase and also shows that glucose6P inhibition of hexokinase activity is mediated by binding ofglucose6P to an effector site in the N terminal domain of the protein Reaction 2 lsomerization of glucose6P to fructose6P by phosphoglucose isomerase As shown in gure 11 phosphoglucose isomerase phosphohexose isomerase interconverts an aldose glucose6P and a ketose fructose6P through a multistep pathway that involves opening and closing of the ring structure The reaction is readily reversible AG 17kJmol and controlled by the level of glucose6P and fructose6P in the cell 5 of 11 pages Reaction 3 Phosphorylation of fructose 6P to fructose16BP by phosphofructokinase1 Reaction 3 is the second ATP investment reaction in glycolysis and involves the coupling of ATP hydrolysis to a phosphoryl transfer reaction that is catalyzed by the enzyme phosphofructokinase1 The reaction is essentially irreversible with a large decrease in standard free energy AG 142kJmol and serves as major regulatory site in the pathway through allosteric control of phosphofructokinase activity in response to the energy charge of the cell As shown in figure 12 phosphorylation of fructose6P generates fructose16bisphosphate fructose1 6 BP which will form two different triose phosphates when cleaved in reaction 4 Note that a bisphosphate compound contains two phosphates on different atoms C1 and C6 whereas a diphosphate compound such as ADP contains two phosphates covalently linked to each other Bioc 460 Dr Miesfeld Fall 2008 Figure 10 Active Hexokinase Q ADP ATP Q Glucose Bxquot Glucose 6P 7W Glucose 6P binds to hexokinase and inhibits ATP binding by allosteric regulation Gltgse 39gt O Glucose Inctive Hexokinase Figure 11 H20P032 2 03P0H2C CHZOH O H Phosphoglucose l H H isomerase HHO OH H v H I OH HO OH H OH HO H Glucose 6phosphate Fructose 5P39105Phate GGP F 6P Figure 12 XPOILVC CHDOH F OH2C CPI1054 l V O l Fln39 i ll Ilriii39t39ilxIILNO no N W ADP w OH OH OH on Fructose 6 phosphate Fructose LIBbisphosphate FGP Fl 68 Reaction 4 Cleavage of fructose1 6 BP into glyceraldehyde3P and dihydroxyacetoneP The splitting of fructose1 6BP into the triose phosphates glyceraldehyde 3P GAP and dihydroxyacetoneP DHAP is the reaction that puts the lysis in glycolysis lysis means splitting as shown in figure 13 The enzyme responsible for this cleavage reaction is aldolase fructose bisphosphate aldolase which performs the reverse of an aldol condensation in the context of the glycolytic pathway The aldolase reaction illustrates the important difference between AG and AG 6 of 11 pages Bioc 460 Dr Miesfeld Fall 2008 values Under standard Figure 13 conditions aldol 0 CH 0M condensation is favored 39 since the standard free 0 H 00 r D39hm g gone energy for the cleavage lt 39 Hquot H DHAP reaction is highly positive H AG 239kJmo Hquot H My ll C 390 l Reaction 5 Isomerization l H 0 of dihydroxyacetoneP to H 0H L Glyceraldehyde glyceraldehyde3P by H0570 H c OH 339Phgzghate triose phosphate isomerase Fructose l l The production of IrebiSPl OSPha e CHOW F16BP dihydroxyacetoneP by aldol cleavage of fructose 16 BP in reaction 4 above creates a slight problem because glyceraldehyde3P not dihydroxyacetoneP is the substrate for reaction 6 in the gycoytic pathway This dilemma is solved by the enzyme triose phosphate isomerase which converts the ketose dihydroxyacetoneP to the adose glyceraldehyde3P in an isomerization reaction that is similar to reaction 2 conversion of glucose6P to fructose6P albeit in reverse this time we need an adose formed from a ketose Figure 14 shows the reaction catalyzed by triose phosphate isomerase which completes stage 1 ofglycolysis and at the expense of 2ATPs produces two moles of glyceraldehyde3P for every mole of glucose that is phosphorylated in reaction 1 Figure 14 Figure 15 H ll HYO CfoH nose phosphate isomerase 0C H C OH 2 CHZOPO3 CHZOP032 Dihydroxyacetone Glyceraldehyde phosphate 3phosphate The enzyme triose phosphate isomerase was introduced earlier in the course as an example of an oc protein structure called a TIM barrel which was first identified in the molecular structure of triose phosphate isomerase thus the name TIM As shown in figure 15 the triose substrate sits in the enzyme active site which is formed by the 5 strands and is located in the center of the protein STAGE 2 ATP EARNINGS There are three key features of the stage 2 reactions to keep in mind First two substrate level phosphorylation reactions catalyzed by the enzymes phosphoglycerate kinase and pyruvate kinase generate a total of 4ATPs net yield of 2ATP in stage 2 of glycolysis Second an oxidation reaction catalyzed by glyceraldehyde3P dehydrogenase generates 2 NADH molecules that can be shuttled into the mitochondria to produce more ATP by oxidative phosphorylation Third reaction 10 is an irreversible reaction that must be bypassed in 7 of 1 1 pages E1 lEIE4EEl 7 Dr Miesteid Fall ZEIEIE gluconeogenesis by two separate enzymatic reactions catalyzed by pyruvate carboxylase and phosphoenolpyruvate carboxykinase Reaction 6 Oxidation and phosphorylation of glyceraldehyde3 P to form 13bisphosphoglycerate by glyceraldehyde3P dehydrogenase The glyceraldehyde3P dehydrogenase reaction is a critical step in glycolysis because it uses the energy released from oxidation of glyceradehyde3P to drive a phosphoryl group transfer reaction using inorganic phosphate P to produce 13bisphosphoglycerate gure 16 This coupled reaction requires the coenzyme NAD39 and includes the formation of an acyl thioester intermediate within the enzyme active site to conserve the oxidation energy Since NAD is required for the oxidation step in this reaction NAD39 must be continually replenished within the cylosol to maintain ux through glycolysis This is accomplished aerobically in the mitochondrial matrix by the electron transport chain or anaerobically in the cylosol by the enzyme lactate dehydrogenase which converts pyruvate to lactate Anaerobic fermentation in yeast regenerates NAD39 using the enzyme alcohol dehydrogenase which converts pyruvate to C02 and ethanol Figure 16 kc m UJ U 0 i My 1 tiricioll NAD39 l llrircioll NADH W CHZOPOJ CHIOP031 ctyxevaldetmxe lJrBisphosphaglyzevat 3pzlgls glfate Lsspa lmportantly formation of 1 3 M A5 bisphosphoglycerate by this coupled m oxidationphosphorylation reaction mquot ca quot generates a metabolite with a Phnswvuermiwuvale 519 14 S andard free energy of hydrolysis lfbiiphasnthogly cralc la 3 Phnsr hnglyce39ate s P 1 that is higher than ATP hydrolysis puffjjfjpmf m 732 B 7 This difference in free energies is MPH p 3D 5 7 harnessed by the enzyme MPH AMP pp 7456 710 phosphoglycerate kinase in reaction 7 AMP Hadenosme P 714 2 73 to drive the synthesis ofATP by a Praia 2P 79 if mechanism called substrate level 5 5 1P N W d e f 9 3 phosphorylation Figure 17 shows 39 3 j that another glycolytic intermediate 6mm liphusphm 9 2 2 phosphoenolpyruvate has a standard free energy of phosphate hydrolysis that exceeds both 13bisphosphoglycerate and ATP Reaction 7 Substrate level phosphorylation to generate ATP in the conversion of 1 isphosphoglycerate to 3phosphoglycerate by phosphoglycerate kinase Phosphoglycerate kinase catalyzes the payback reaction in glycolysis because it replaces the 2 ATP that were used in stage 1 to prime the glycolylic pathway As shown in gure 1Bthe high phosphoryl transfer energy present in the substrate is used to phosphorylate ADP to form ATP by the mechanism of substrate level phosphorylation leading to the conversion of 13 Eufii pages Bloc 460 Dr Miesfeld Fall 2008 bisphosphoglycerate to 3 phosphoglycerate Remember that two moles of 13 bisphophoglycerate are formed from every mole of glucose therefore this reaction occurs twice and generates 2ATPglucose Figure18 O 0POquot 0 O C C Himsphnglycmare kinase Hi OH ADP H7C OH ATP CH20P032 CH70P032 13Bisphosphogiycerate 3Phosphoglycerate The molecular structure of phosphoglycerate kinase is similar to Figure 19 hexokinase in that it has two lobes iaws that each bind one of the substrates ADPMg2 or 13 bisphosphoglycerate leading Open COmP39EX to a large conformational change in the enzyme that brings the substrates close together and excludes H20 from the active site The structures of phosphoglycerate kinase in the open and closed conformations are shown in figure 19 The bioenergetics of reaction 7 emphasize two important concepts we have presented in the chapter First reaction 6 H O SubstrateInduced 2 conformational changes glyceraldehyde3P dehydrogenase and reaction 7 phosphoglycerate kinase are coupled reactions in that the large change in standard free energy of reaction 7 AG 189 kJmol pulls the less favorable reaction 6 AG 63 kJmol to the right through the shared intermediate 13 bisphosphoglcerate as shown below Closed complex Rxn 6 Glyceraldehyde3P Pi NAD 69 13 bisphosphoglyoerate NADH H AG 63 kJmol AG 13 kJmol Rxn 7 13 bisphosphoglycerate ADP 69 3 phosphoglycerate ATP AG 189 kJmol AG 01 kJmol Coupled Glyceraldehyde3P Pi ADP NAD 69 3 phosphoglycerate ATP NADH H AG 126 kJmol AG 12 kJmol Second the actual change in free energy for each of these two reactions is very close to zero AG 13 kJmol AG 01 kJmol and therefore both reactions are in fact reversible inside the cell Again this difference in AG and AG is due to the mass action ratio which takes into account the actual concentrations of substrates and products that exist in the cell Why is the reversibility of these two reactions important The answer is that when flux through gluconeogenesis is high these two glycolytic reactions can be reversed and thus quickly respond to changing conditions in the cell 9 of 11 pages Bioc 460 Dr Miesfeld Fall 2008 Reaction 8 Phosphoryl shift in 3phosphyglycerate to form 2 phosphoglycerate by phosphoglycerate mutase The purpose of reaction 8 is to generate a Compound 2phosphoglycerate that 9 Fl re 20 can be converted to O 0 0 C CV phosphoenolpyruvate in the next reaction in preparation for a second 7 7 i 7 2 substrate level phosphorylationthat H C OH H C OPO Phosphoglycerate generates ATP earnIngs In step 10 HC0pO2 mutase HC0H The phosphoglycerate mutase reaction is shown in figure 20 H H 3Phosphoglycerate ZPhosphoglycerate The mechanism of this highly reversible reaction is illustrated in figure 21 where it can be seen to require a phosphoryl transfer from a phosphorylated histidine residue located in the 3PhosphoglycerateEnzyme complex 23eBisphosphoglycerateEnzyme complex enzyme active site In step 1 the substrate 3 phosphoglycerate binds to the enzyme active site and is phosphorylated in the CZ position by a transfer reaction involving the HisP group This type of substrate interaction with the enzyme is noncovalent and referred to as a substrateenzyme complex I39quot Phosphoryl transfer from the whom histidine residue to the CZ atom of the 3phosphoglycerate creates the shortlived intermediate 23 bisphosphoglycerate BPG In the second step of the reaction the C3 phosphate is transferred back to the histidine residue of the enzyme to regenerate HisP leading to the release of 2 phosphoglycerate and binding of a new molecule of 3phosphoglycerate in the third step Note that the BPG formed in step 1 can diffuse out ofthe active site resulting in dephosphorylated enzyme and you may remember that in red blood cells BPG has an important role in regulating oxygen binding to hemoglobin in red blood cells When BPG leaves the active site without rephosphorylating the His group the enzyme can only be activated when trace amounts of BPG diffuse back into the active site Figure 21 3Phosphoglycerate ZvPhosphoglycerate Enzyme complex Reaction 9 Dehydration of 2 phosphoglycerate to form phosphoenolpyruvate by enolase In this penultimate step in glycolysis a dehydration reaction catalyzed by the enzyme enolase converts 2 phosphoglycerate a molecule with only moderate phosphoryl transfer potential to phosphoenolpyruvate figure 22 which we have already seen has extremely high phosphoryl 10 of 11 pages Bloc 460 Dr Miesfeld Fall 2008 transfer potential It is this high phosphoryl transfer potential in phosphoenolpyruvate that is harnessed in the last reaction in O T O Figure 22 glycolysis to form ATP CV H20 04 It is interestin th tth h 39 t d d Hiciopoiz a g a e c ange in sari ar Enolase free energy for this reaction is relatively HiciOH small AG 17 kJmol meaning that the overall metabolic energy available from H Z39Phosphoglycerate and 2Phosphoglycerate Phosphenolpyruvate phosphoenolpyruvate is similar However when enolase converts 2 phosphoglycerate to phosphoenolpyruvate it traps the phosphate group in an unstable enol form resulting in a dramatic increase in the phosphoryl transfer potential of the triose sugar The standard free energy change for phosphate hydrolysis in 2phosphoglycerate is AG 16 kJmol whereas for phosphoenolpyruvate it is an incredible AG 62 kJmol Reaction 10 Substrate level phosphorylation to generate ATP in the conversion of phosphoenolpyruvate to pyruvate by pyruvate kinase In this reaction the high phosphoryl transfer potential of w phosphoenolpyruvate is used by the 0 ADP O enzymte pyrrluvatej kinadse to generate OPOsg H NP O pyruva e e en pro uc o 0quot l l glycolysis and 2 ATP are formed for CM every glucose molecule entering the pyruvate pathway figure 23 This is the kinase CH3 second of two substrate level phosphorylation reactions in QWCOWSiS that couples energy Phosphenolpyruvate Pyruvate released from phosphate hydrolysis AG 62 kJmol to that of ATP synthesis AG 305 kJmol as shown in figure 24 Unlike phosphoenolpyruvate pyruvate is a stable compound in cells that is utilized by many other metabolic pathways as will be described later ANSWER To KEY CONCEPT QUESTION IN GLYCOLYSIS What are the two substrate level phosphorylation reactions in glycolysis Substrate level phosphorylation reactions generate ATP by capturing the high phosphate transfer potential of the substrate in a coupled reaction with ADP The enzymes phosphoglycerate kinase and pyruvate kinase catalyze the two substrate level phosphorylation reactions in stage 2 of glycolysis Since two molecules of glyceraldehyde3P are generated for every molecule of glucose metabolized each of these substrate level phosphorylation reactions yield 2ATPglucose Phosphoglycerate kinase reaction 7 provides the payback of 2ATP invested in stage 1 of glycolysis to generate fructose16BP whereas reaction 10 catalyzed by pyruvate kinase represents the ATP earnings step in the pathway by generating 2 net ATP 11 of11 pages
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