6 CH OPO 2 2 3 5 O H H H 4 H 1 OH OH OH 3 2 H OH glucose-6-phosphateGlycolysis takes place in the cytosol of cells.Glucose enters the Glycolysis pathway by conversionto glucose-6-phosphate.Initially there is energy input corresponding tocleavage of two ~P bonds of ATP.
6 CH2OH 6 CH OPO 2 2 3 ATP ADP 5 O 5 O H H H H H H 4 1 4 H 1 OH H OH Mg2+ OH OH OH OH 3 2 3 2 H OH Hexokinase H OH glucose glucose-6-phosphate1. Hexokinase catalyzes: Glucose + ATP glucose-6-P + ADPThe reaction involves nucleophilic attack of the C6hydroxyl O of glucose on P of the terminal phosphateof ATP.ATP binds to the enzyme as a complex with Mg++.
NH2 ATP N N adenosine triphosphate O O O N N O P O P O P O CH2 adenine O O O O H H H H OH OH riboseMg++ interacts with negatively charged phosphateoxygen atoms, providing charge compensation &promoting a favorable conformation of ATP at theactive site of the Hexokinase enzyme.
6 CH2OH 6 CH OPO 2 2 3 ATP ADP 5 O 5 O H H H H H H 4 1 4 H 1 OH H OH Mg2+ OH OH OH OH 3 2 3 2 H OH Hexokinase H OH glucose glucose-6-phosphateThe reaction catalyzed by Hexokinase is highlyspontaneous.A phosphoanhydride bond of ATP (~P) is cleaved.The phosphate ester formed in glucose-6-phosphatehas a lower DG of hydrolysis.
6 CH2OH 6 CH OPO 2 2 3 ATP ADP 5 O 5 OInduced fit: H H H H H H 4 1 4 H 1 OH H 2+ OHGlucose binding OH OH Mg OH OH 2to Hexokinase 3 2 3 H OH Hexokinase H OHstabilizes a glucose glucose-6-phosphateconformationin which: glucose the C6 hydroxyl of the bound glucose is close to the terminal phosphate of Hexokinase ATP, promoting catalysis. water is excluded from the active site. This prevents the enzyme from catalyzing ATP hydrolysis, rather than transfer of phosphate to glucose.
glucose HexokinaseIt is a common motif for an enzyme active site to belocated at an interface between protein domains that areconnected by a flexible hinge region.The structural flexibility allows access to the active site,while permitting precise positioning of active siteresidues, and in some cases exclusion of water, assubstrate binding promotes a particular conformation.
6 CH OPO 2 2 3 5 6 CH OPO 2 1CH2OH H O H 2 3 O H 4 H 1 5 H HO 2 OH OH OH H 4 3 OH 3 2 OH H H OH Phosphoglucose Isomerase glucose-6-phosphate fructose-6-phosphate2. Phosphoglucose Isomerase catalyzes: glucose-6-P (aldose) fructose-6-P (ketose)The mechanism involves acid/base catalysis, with ringopening, isomerization via an enediolate intermediate,and then ring closure. A similar reaction catalyzed byTriosephosphate Isomerase will be presented in detail.
Phosphofructokinase 6 CH OPO 2 1CH2OH 6 CH OPO 2 1CH2OPO32 2 3 2 3 O ATP ADP O 5 H HO 2 5 H HO 2 H 4 3 OH Mg2+ H 4 3 OH OH H OH H fructose-6-phosphate fructose-1,6-bisphosphate3. Phosphofructokinase catalyzes: fructose-6-P + ATP fructose-1,6-bisP + ADPThis highly spontaneous reaction has a mechanism similarto that of Hexokinase.The Phosphofructokinase reaction is the rate-limiting stepof Glycolysis.The enzyme is highly regulated, as will be discussed later.
2 1CH2OPO3 2C O H O HO 3C H Aldolase 3 CH2OPO32 1C H 4C OH 2C O + H 2C OH 2 H C OH 1CH2OH 3 CH2OPO3 5 6 CH2OPO32 dihydroxyacetone glyceraldehyde-3- phosphate phosphate fructose-1,6- bisphosphate Triosephosphate Isomerase4. Aldolase catalyzes: fructose-1,6-bisphosphate dihydroxyacetone-P + glyceraldehyde-3-PThe reaction is an aldol cleavage, the reverse of an aldolcondensation.Note that C atoms are renumbered in products of Aldolase.
lysine 1CH2OPO3 2 H + 2C NH (CH2)4 Enzyme H 3N C COO + HO 3 CH CH2 H 4 C OH CH2 H 5 C OH CH2 6 CH2OPO32 CH2 Schiff base intermediate of NH3 Aldolase reactionA lysine residue at the active site functions in catalysis.The keto group of fructose-1,6-bisphosphate reacts withthe e-amino group of the active site lysine, to form aprotonated Schiff base intermediate.Cleavage of the bond between C3 & C4 follows.
2 1CH2OPO3 2C O H O HO 3C H Aldolase 3 CH2OPO32 1C H 4C OH 2C O + H 2C OH 2 H C OH 1CH2OH 3 CH2OPO3 5 6 CH2OPO32 dihydroxyacetone glyceraldehyde-3- phosphate phosphate fructose-1,6- bisphosphate Triosephosphate Isomerase5. Triose Phosphate Isomerase (TIM) catalyzes: dihydroxyacetone-P glyceraldehyde-3-PGlycolysis continues from glyceraldehyde-3-P. TIMs Keqfavors dihydroxyacetone-P. Removal of glyceraldehyde-3-Pby a subsequent spontaneous reaction allows throughput.
Triosephosphate Isomerase H H OH H + O + + + H C OH H H C H H C C O C OH H C OH CH2OPO32 CH2OPO32 CH2OPO32 dihydroxyacetone enediol glyceraldehyde- phosphate intermediate 3-phosphateThe ketose/aldose conversion involves acid/base catalysis,and is thought to proceed via an enediol intermediate, aswith Phosphoglucose Isomerase.Active site Glu and His residues are thought to extract anddonate protons during catalysis.
OH HC O O O C C CH2OPO32 CH2OPO32 proposed phosphoglycolate enediolate transition state intermediate analog2-Phosphoglycolate is a transition state analog thatbinds tightly at the active site of Triose PhosphateIsomerase (TIM).This inhibitor of catalysis by TIM is similar in structure tothe proposed enediolate intermediate.TIM is judged a "perfect enzyme." Reaction rate is limitedonly by the rate that substrate collides with the enzyme.
Triosephosphate Isomerasestructure is an ab barrel, orTIM barrel.In an ab barrel there are8 parallel b-strands surroundedby 8 a-helices.Short loops connect alternating TIMb-strands & a-helices.
TIM barrels serve as scaffoldsfor active site residues in adiverse array of enzymes.Residues of the active site arealways at the same end of thebarrel, on C-terminal ends ofb-strands & loops connecting TIMthese to a-helices.There is debate whether the many different enzymes withTIM barrel structures are evolutionarily related.In spite of the structural similarities there is tremendousdiversity in catalytic functions of these enzymes andlittle sequence homology.
OH HC O O O C C CH2OPO32 CH2OPO32 proposed phosphoglycolate enediolate transition stateTIM intermediate analogExplore the structure of the Triosephosphate Isomerase(TIM) homodimer, with the transition state inhibitor2-phosphoglycolate bound to one of the TIM monomers.Note the structure of the TIM barrel, and the loop thatforms a lid that closes over the active site after bindingof the substrate.
Glyceraldehyde-3-phosphate Dehydrogenase H O + H+ O OPO32 1C NAD+ NADH 1C + Pi H C OH H C OH 2 2 3 CH2OPO32 3 CH2OPO32 glyceraldehyde- 1,3-bisphospho- 3-phosphate glycerate6. Glyceraldehyde-3-phosphate Dehydrogenasecatalyzes: glyceraldehyde-3-P + NAD+ + Pi 1,3-bisphosphoglycerate + NADH + H+
Glyceraldehyde-3-phosphate Dehydrogenase H O + H+ O OPO32 1C NAD+ NADH 1C + Pi H C OH H C OH 2 2 3 CH2OPO32 3 CH2OPO32 glyceraldehyde- 1,3-bisphospho- 3-phosphate glycerateExergonic oxidation of the aldehyde in glyceraldehyde-3-phosphate, to a carboxylic acid, drives formation of anacyl phosphate, a "high energy" bond (~P).This is the only step in Glycolysis in which NAD+ isreduced to NADH.
H O H 1C H3N+ C COO H 2 C OH CH2 3 CH2OPO32 SH glyceraldehyde-3- cysteine phosphateA cysteine thiol at the active site of Glyceraldehyde-3-phosphate Dehydrogenase has a role in catalysis.The aldehyde of glyceraldehyde-3-phosphate reactswith the cysteine thiol to form a thiohemiacetalintermediate.
Enz-Cys SH O OH HC CH CH2OPO32 glyceraldehyde-3- OH OH phosphateOxidation to a Enz-Cys S CH CH CH2OPO32carboxylic acid NAD+ thiohemiacetal NADH intermediate(in a ~ thioester) O OHoccurs, as NAD+ Enz-Cys S C CH CH2OPO32is reduced to acyl-thioester PiNADH. intermediate O OH 2 Enz-Cys SH O3PO C CH CH2OPO32 1,3-bisphosphoglycerateThe “high energy” acyl thioester is attacked by Pi toyield the acyl phosphate (~P) product.
H O O H H C C NH2 NH2 + N + N 2e + H R R NAD+ NADHRecall that NAD+ accepts 2 e plus one H+ (a hydride)in going to its reduced form.
Phosphoglycerate Kinase O OPO32 ADP ATP O O 1C 1 C H 2C OH H 2C OH Mg2+ 3 CH2OPO32 3 CH2OPO32 1,3-bisphospho- 3-phosphoglycerate glycerate7. Phosphoglycerate Kinase catalyzes: 1,3-bisphosphoglycerate + ADP 3-phosphoglycerate + ATPThis phosphate transfer is reversible (low DG), sinceone ~P bond is cleaved & another synthesized.The enzyme undergoes substrate-induced conformationalchange similar to that of Hexokinase.
Phosphoglycerate Mutase O O O O C 1 C 1 H 2C OH H 2C OPO32 2 3 CH2OPO3 3 CH2OH 3-phosphoglycerate 2-phosphoglycerate8. Phosphoglycerate Mutase catalyzes: 3-phosphoglycerate 2-phosphoglyceratePhosphate is shifted from the OH on C3 to theOH on C2.
Phosphoglycerate Mutase O O histidine O O H 1 C 1 C H3N+ C COO H 2C OH H 2C OPO32 2 CH2 3 CH2OPO3 3 CH2OH C3-phosphoglycerate 2-phosphoglycerate HN CH HC NH An active site histidine side-chainparticipates in Pi transfer, bydonating & accepting phosphate. O O 1 CThe process involves a H 2C OPO322,3-bisphosphate intermediate. 2 3 CH2OPO3 2,3-bisphosphoglycerateView an animation of thePhosphoglycerate Mutase reaction.
Enolase O H O OH O O O O C C C 1 1 H 2 C OPO32 C OPO32 2C OPO32 3 CH2OH CH2OH 3 CH22-phosphoglycerate enolate intermediate phosphoenolpyruvate9. Enolase catalyzes: 2-phosphoglycerate phosphoenolpyruvate + H2OThis dehydration reaction is Mg++-dependent.2 Mg++ ions interact with oxygen atoms of the substratecarboxyl group at the active site.The Mg++ ions help to stabilize the enolate anionintermediate that forms when a Lys extracts H+ from C #2.
Pyruvate Kinase O O O O ADP ATP 1 C 1 C 2 C OPO32 2 C O 3 CH2 3 CH3 phosphoenolpyruvate pyruvate10. Pyruvate Kinase catalyzes: phosphoenolpyruvate + ADP pyruvate + ATP
Pyruvate Kinase O O O O O O C ADP ATP C C 1 1 1 2 C OPO32 C 2 OH 2 C O 3 CH2 3 CH2 3 CH3 phosphoenolpyruvate enolpyruvate pyruvateThis phosphate transfer from PEP to ADP is spontaneous. PEP has a larger DG of phosphate hydrolysis than ATP. Removal of Pi from PEP yields an unstable enol, which spontaneously converts to the keto form of pyruvate.Required inorganic cations K+ and Mg++ bind to anionicresidues at the active site of Pyruvate Kinase.
glucose Glycolysis ATP Hexokinase ADP glucose-6-phosphate Phosphoglucose Isomerase fructose-6-phosphate ATP Phosphofructokinase ADP fructose-1,6-bisphosphate Aldolaseglyceraldehyde-3-phosphate + dihydroxyacetone-phosphate Triosephosphate Isomerase Glycolysis continued
glyceraldehyde-3-phosphate NAD+ + Pi Glyceraldehyde-3-phosphate NADH + H+ Dehydrogenase 1,3-bisphosphoglycerateGlycolysis ADPcontinued. Phosphoglycerate Kinase ATPRecall that 3-phosphoglyceratethere are 2 Phosphoglycerate MutaseGAP per 2-phosphoglycerateglucose. H2O Enolase phosphoenolpyruvate ADP Pyruvate Kinase ATP pyruvate
GlycolysisBalance sheet for ~P bonds of ATP: How many ATP ~P bonds expended? ________ 2 How many ~P bonds of ATP produced? (Remember 4 there are two 3C fragments from glucose.) ________ Net production of ~P bonds of ATP per glucose: ________ 2
Balance sheet for ~P bonds of ATP: 2 ATP expended 4 ATP produced (2 from each of two 3C fragments from glucose) Net production of 2 ~P bonds of ATP per glucose.Glycolysis - total pathway, omitting H+: glucose + 2 NAD+ + 2 ADP + 2 Pi 2 pyruvate + 2 NADH + 2 ATPIn aerobic organisms: pyruvate produced in Glycolysis is oxidized to CO2 via Krebs Cycle NADH produced in Glycolysis & Krebs Cycle is reoxidized via the respiratory chain, with production of much additional ATP.
Glyceraldehyde-3-phosphate Dehydrogenase H O + H+ O OPO32 1C NAD+ NADH 1CFermentation: + Pi H C OH H C OH 2 2Anaerobic 3 CH2OPO32 3 CH2OPO32organisms lack a glyceraldehyde- 1,3-bisphospho-respiratory chain. 3-phosphate glycerateThey must reoxidize NADH produced in Glycolysisthrough some other reaction, because NAD+ is needed forthe Glyceraldehyde-3-phosphate Dehydrogenase reaction.Usually NADH is reoxidized as pyruvate is converted toa more reduced compound.The complete pathway, including Glycolysis and thereoxidation of NADH, is called fermentation.
Lactate Dehydrogenase O O O O C NADH + H+ NAD+ C C O HC OH CH3 CH3 pyruvate lactateE.g., Lactate Dehydrogenase catalyzes reduction ofthe keto in pyruvate to a hydroxyl, yielding lactate, asNADH is oxidized to NAD+.Lactate, in addition to being an end-product offermentation, serves as a mobile form of nutrient energy,& possibly as a signal molecule in mammalian organisms.Cell membranes contain carrier proteins that facilitatetransport of lactate.
Lactate Dehydrogenase O O O O C NADH + H+ NAD+ C C O HC OH CH3 CH3 pyruvate lactateSkeletal muscles ferment glucose to lactate duringexercise, when the exertion is brief and intense.Lactate released to the blood may be taken up by othertissues, or by skeletal muscle after exercise, and convertedvia Lactate Dehydrogenase back to pyruvate, which maybe oxidized in Krebs Cycle or (in liver) converted to backto glucose via gluconeogenesis
Lactate Dehydrogenase O O O O C NADH + H+ NAD+ C C O HC OH CH3 CH3 pyruvate lactateLactate serves as a fuel source for cardiac muscle aswell as brain neurons.Astrocytes, which surround and protect neurons in thebrain, ferment glucose to lactate and release it.Lactate taken up by adjacent neurons is converted topyruvate that is oxidized via Krebs Cycle.
Pyruvate Alcohol Decarboxylase Dehydrogenase O O CO2 H NADH + H+ NAD+ H C O C O C H C OH CH3 CH3 CH3 pyruvate acetaldehyde ethanolSome anaerobic organisms metabolize pyruvate toethanol, which is excreted as a waste product.NADH is converted to NAD+ in the reactioncatalyzed by Alcohol Dehydrogenase.
Glycolysis, omitting H+: glucose + 2 NAD+ + 2 ADP + 2 Pi 2 pyruvate + 2 NADH + 2 ATPFermentation, from glucose to lactate: glucose + 2 ADP + 2 Pi 2 lactate + 2 ATPAnaerobic catabolism of glucose yields only 2 “highenergy” bonds of ATP.
DGo DG Glycolysis Enzyme/Reaction kJ/mol kJ/mol Hexokinase -20.9 -27.2 Phosphoglucose Isomerase +2.2 -1.4 Phosphofructokinase -17.2 -25.9 Aldolase +22.8 -5.9 Triosephosphate Isomerase +7.9 negative Glyceraldehyde-3-P Dehydrogenase -16.7 -1.1 & Phosphoglycerate Kinase Phosphoglycerate Mutase +4.7 -0.6 Enolase -3.2 -2.4 Pyruvate Kinase -23.0 -13.9*Values in this table from D. Voet & J. G. Voet (2004) Biochemistry, 3rd Edition, JohnWiley & Sons, New York, p. 613.
Flux through the Glycolysis pathway is regulated bycontrol of 3 enzymes that catalyze spontaneous reactions:Hexokinase, Phosphofructokinase & Pyruvate Kinase. Local control of metabolism involves regulatory effects of varied concentrations of pathway substrates or intermediates, to benefit the cell. Global control is for the benefit of the whole organism, & often involves hormone-activated signal cascades. Liver cells have major roles in metabolism, including maintaining blood levels various of nutrients such as glucose. Thus global control especially involves liver. Some aspects of global control by hormone-activated signal cascades will be discussed later.
6 CH2OH 6 CH OPO 2 2 3 ATP ADP 5 O 5 O H H H H H H 4 1 4 H 1 OH H OH Mg2+ OH OH OH OH 3 2 3 2 H OH Hexokinase H OH glucose glucose-6-phosphateHexokinase is inhibited by product glucose-6-phosphate: by competition at the active site by allosteric interaction at a separate enzyme site.Cells trap glucose by phosphorylating it, preventing exiton glucose carriers.Product inhibition of Hexokinase ensures that cells willnot continue to accumulate glucose from the blood, if[glucose-6-phosphate] within the cell is ample.
6 CH2OH 6 CH OPO 2 2 3 ATP ADP 5 O 5 O H H H H H H 4 1 4 1Glucokinase OH H Mg2+ OH His a variant of OH OH OH OH 3 2 3 2Hexokinase H OH Hexokinase H OHfound in liver. glucose glucose-6-phosphate Glucokinase has a high KM for glucose. It is active only at high [glucose]. One effect of insulin, a hormone produced when blood glucose is high, is activation in liver of transcription of the gene that encodes the Glucokinase enzyme. Glucokinase is not subject to product inhibition by glucose-6-phosphate. Liver will take up & phosphorylate glucose even when liver [glucose-6-phosphate] is high.
Glucokinase is subject to inhibition by glucokinase regulatory protein (GKRP). The ratio of Glucokinase to GKRP in liver changes in different metabolic states, providing a mechanism for modulating glucose phosphorylation.
Glycogen GlucoseGlucokinase, Hexokinase or Glucokinasewith high KM Glucose-6-Pasefor glucose, Glucose-1-P Glucose-6-P Glucose + Piallows liver to Glycolysis Pathwaystore glucoseas glycogen in Pyruvatethe fed state Glucose metabolism in liver.when blood [glucose] is high.Glucose-6-phosphatase catalyzes hydrolytic release of Pifrom glucose-6-P. Thus glucose is released from the liverto the blood as needed to maintain blood [glucose].The enzymes Glucokinase & Glucose-6-phosphatase, bothfound in liver but not in most other body cells, allow theliver to control blood [glucose].
Pyruvate KinasePyruvate Kinase, the O O O O ADP ATPlast step Glycolysis, is 1 C 1 Ccontrolled in liver partly 2 C OPO32 2 C Oby modulation of the 3 CH2 3 CH3amount of enzyme. phosphoenolpyruvate pyruvateHigh [glucose] within liver cells causes a transcriptionfactor carbohydrate responsive element binding protein(ChREBP) to be transferred into the nucleus, where itactivates transcription of the gene for Pyruvate Kinase.This facilitates converting excess glucose to pyruvate,which is metabolized to acetyl-CoA, the main precursorfor synthesis of fatty acids, for long term energy storage.
Phosphofructokinase 6 CH OPO 2 1CH2OH 6 CH OPO 2 1CH2OPO32 2 3 2 3 O ATP ADP O 5 H HO 2 5 H HO 2 H 4 3 OH Mg2+ H 4 3 OH OH H OH H fructose-6-phosphate fructose-1,6-bisphosphatePhosphofructokinase is usually the rate-limiting step ofthe Glycolysis pathway.Phosphofructokinase is allosterically inhibited by ATP. At low concentration, the substrate ATP binds only at the active site. At high concentration, ATP binds also at a low-affinity regulatory site, promoting the tense conformation.
60 50 low [ATP] PFK Activity 40 30 20 high [ATP] 10 0 0 0.5 1 1.5 2 [Fructose-6-phosphate] mMThe tense conformation of PFK, at high [ATP], has loweraffinity for the other substrate, fructose-6-P. Sigmoidaldependence of reaction rate on [fructose-6-P] is seen.AMP, present at significant levels only when there isextensive ATP hydrolysis, antagonizes effects of high ATP.
Glycogen Glucose Hexokinase or Glucokinase Glucose-6-Pase Glucose-1-P Glucose-6-P Glucose + Pi Glycolysis Pathway Pyruvate Glucose metabolism in liver.Inhibition of the Glycolysis enzyme Phosphofructokinasewhen [ATP] is high prevents breakdown of glucose in apathway whose main role is to make ATP.It is more useful to the cell to store glucose as glycogenwhen ATP is plentiful.