Chapter 14 Glycolysis, Gluconeogenesis, and the Pentose Phosphate Pathway

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Text of Chapter 14 Glycolysis, Gluconeogenesis, and the Pentose Phosphate Pathway

  • Chapter 14

    Glycolysis, Gluconeogenesis, and the Pentose Phosphate Pathway

  • GlucoseRoles of glucoseFuel (Glucose CO2 + H2O ; G = ~ -2,840 kJ/mol)Precursor for other moleculesUtilization of glucose in animals and plantSynthesis of structural polymersStorageGlycogen, starch, or sucroseOxidation via glycolysis Pyruvate for ATP and metabolic intermediate generationsOxidation via pentose phosphate pathway Ribose 5-P for nucleic acid synthesisNADPH for reductive biosynthesis

    Generation of glucosePhotosynthesis : from CO2Gluconeogenesis (reversing glycolysis) : from 3-C or 4-C precursors

  • 14.1 GlycolysisGlycolysis Glucose 2 x Pyruvate 2 ATP & 2 NADH

    Fermentation

    the anaerobic degradation of glucose ATP production

  • An Overview: Glycolysis Two phases of glycolysis (10 steps)Preparatory phase : 5 stepsFrom Glc to 2 glyceraldehyde 3-PConsumption of 2 ATP molecules

    Payoff phase : 5 stepsGeneration of pyruvateGeneration of 4 ATP from high-energy phosphate compounds1,3-bisphosphoglycerate, phosphoenylpyruvateGeneration of 2 NADH

  • Preparatory Phase

  • Payoff Phase

  • Fates of PyruvateAerobic conditionsOxidative decarboxylation of pyruvateGeneration of acetyl-CoACitric acid cycleComplete oxidation of acetyl-CoA CO2Electron-transfer reactions in mitochondriae- transfer to O2 to generate H2OGeneration of ATP

    Fermentation : anaerobic conditions (hypoxia)Lactic acid fermentationReduction of pyruvate to lactate NAD+ regeneration for glycolysisVigorously contracting muscleEthanol (alcohol) fermentationConversion of pyruvate to EtOH and CO2 Microorganisms (yeast)

  • Fate of PyruvateAnabolic fates of pyruvateSource of C skeleton (Ala or FA synthesis)

  • ATP & NADH formation coupled to glycolysisOverall equation for glycolysisGlc + 2 NAD+ 2 pyruvate + 2NADH + 2H+DG1o = -146 kJ/mol2ADP + 2Pi 2ATP + 2H2ODG2o = 2(30.5) = 61.0 kJ/molGlc + 2NAD+ + 2ADP + 2Pi 2 pyruvate + 2NADH + 2H+ + 2ATP + 2H2ODGso = DG1o + DG2o = -85 kJ/mol60% efficiency in conversion of the released energy into ATP

    Importance of phosphorylated intermediatesNo export of phosphorylated compounds Conservation of metabolic energy in phosphate estersBinding energy of phosphate groupLower DG & increase reaction specificityMany glycolytic enzymes are specific for Mg2+ complexed with phosphate groups

  • Glycolysis : Step 11. Phosphorylation of Glc

    HexokinaseSubstrates; D-glc & MgATP2-(ease nucleophilc attack by OH of glc)Induced fitSoluble & cytosolic protein

  • Glycolysis : Step 22. Glc 6-P Fru 6-P (isomerization)Phosphohexose isomerase (phosphoglucose isomerase)Reversible reaction (small DGo)

  • Glycolysis : Step 3 3. Phosphorylation of Fru 6-P to Fru 1,6-bisPPhosphofructokinase-1 (PFK-1)Irreversible, committed step in glycolysisActivation under low [ATP] or high [ADP and AMP]Phosphoryl group donorATPPPi : some bacteria and protist, all plants

  • Glycolysi : Step 4 4. Cleavage of Fru 1,6-bisPDihydroxyacetone P & glyceraldehyde 3-PAldolase (fructose 1,6-bisphosphate aldolase)Class I : animals and plantClass II : fungi and bacteria, Zn2+ at the active siteReversible in cells because of lower concentrations of reactant

  • Class I Aldolase Reaction

  • Glycolysis : Step 5 5. Interconversion of the triose phosphatesDihydroxyacetone P glyceraldehyde 3-PTriose phosphate isomerase

  • Glycolysis : Step 6 6. Oxidation of glyceraldehyde 3-P to 1,3-bisphosphoglycerateGlyceraldehyde 3-P dehydrogenaseNAD+ is the acceptor for hydride ion released from the aldehyde groupFormation of acyl phosphateCarboxylic acid anhydride with phosphoric acidHigh DGo of hydrolysis

  • Glyceraldehyde 3-P dehydrogenase

  • Glycolysis : Step 7 7. Phosphoryl transfer from 1,3-bisphosphoglycerate to ADP3-phosphoglycerase kinaseSubstrate-level phosphorylation of ADP to generate ATPc.f. Respiration-linked phosphorylation

    Coupling of step 6 (endergonic) and step 7 (exergonic)Glyceraldehyde 3-P + ADP + Pi + NAD+ 3-phosphoglycerate + ATP + NADH + H+DGo = -12.5 kJ/molCoupling through 1,3-bisphophoglycerate (common intermediate) Removal of 1,3-bisphosphoglycerate in step 7 strong negative DG of step 6

  • Glycolysis : Step 88. 3-phosphoglycerate to 2-phosphoglyceratePhosphoglycerate mutaseMg2+Two step reaction with 2,3-BPG intermediate

  • Glycolysis : Step 9Dehydration of 2-phosphoglycerate to phosphoenolpyruvate (PEP)EnolaseFree energy for hydrolysis2-phosphoglycerate : -17.6 kJ/molPEP : -61.9 kJ/mol

  • Glycolysis : Step 10Transfer of phosphoryl group from PEP to ADPPyruvate kinaseSubstrate-level phosphorylationTautomerization from enol to keto forms of pyruvateIrreversibleImportant site for regulation

  • Overall Balance in Glycolysis Glucose + 2ATP + 2NAD+ + 4ADP + Pi 2Pyruvate + 2ADP + 2NADH + 2H+ + 4ATP + 2H2O

    Multienzyme complex Substrate channeling Tight regulation Rate of glycolysis: anaerobic condition (2ATP)aerobic condition (30-32) ATP consumption NADH regeneration Allosteric regulation of enzymes; Hexokinase, PFK-1, pyruvate kinase Hormone regulations; glucagon, insulin, epinephrine Changes in gene expression for the enzymes

  • 14.2 Feeder Pathways for Glycolysis

  • Entry of Carbohydrates into Glycolysis

  • Degradation of Glycogen and Starch by PhosphorolysisGlycogen phosphorylase(Glc)n + Pi Glc 1-P + (Glc)n-1Debranching enzymeBreakdown of (a16) branchPhosphoglucomutaseGlc 1-P Glc 6-PBisphosphate intermediate

  • Digestion of Dietary Polysaccharides and DisaccharidesDigestion of starch and glycogena-amylase in salivaHydrolysis of starch to oligosaccharidesPancreatic a-amylase maltose and maltotriose, limit dextrinHydrolysis of intestinal dextrins and disaccharidesDextrinaseMaltaseLactaseSucraseTrehalaseTransport of monosaccharide into the epithelial cellsc.f. lactase intoleranceLacking lactase activity in the intestineConverted to toxic product by bacteriaIncrease in osmolarity increase in water retention in the intestine

  • Entry of Other monosaccharides into Glycolytic PathwayFructoseIn muscle and kidney HexokinaseFru + ATP Fru 6-P + ADPIn liverFructokinaseFru + ATP Fru 1-P + ADPFructose 1-P aldolase

    Glyceraldehyde 3-PTriose phosphate isomeraseTriose kinase

  • GalactoseGlactokinase; Gal Glc 1-PGalatosemiaDefects in the enzymatic pathway

    MannoseHexokinaseMan + ATP Man 6-P + ADPPhosphomannose isomeraseMan 6-P Fru 6-PEntry of Other monosaccharides into Glycolytic Pathway

  • 14.3 Fates of Pyruvate under Anaerobic Conditions: Fermentation

  • Pyruvate fatesHypoxic conditions Rigorously contracting muscle Submerged plant tissues Solid tumors Lactic acid bacteriaFailure to regenerate NAD+Fermentation is the way of NAD+ regeneration

  • Lactic Acid FermentationLactate dehydrogenaseRegeneration of NAD+Reduction of pyruvate to lactate

    FermentationNo oxygen consumptionNo net change in NAD+ or NADH concentrationsExtraction of 2 ATP

  • Ethanol FermentationTwo step process

    Pyruvate decarboxylaseIrreversible decarboxylation of pyruvateBrewers and bakers yeast & organisms doing ethanol fermentationCO2 for brewing or bakingMg2+ & thiamine pyrophosphate (TPP)

    Alcohol dehydrogenaseAcetaldehyde + NADH + H+ EtOH + NAD+Human alcohol dehydrogenaseUsed for ethanol metabolism in liver

  • Thiamine Phyrophosphate (TPP) as Active Aldehyde Group CarrierTPPVitamin B1 derivativeCleavage of bonds adjacent to a carbonyl groupDecarboxylation of a-keto acidRearrangement of an activated acetaldehyde group

  • Role of Thiamine Pyrophosphate (TPP) in pyruvate decarboxylationTPPNucleophilic carbanion of C-2 in thiazolium ringThiazolium ring acts as e- sink

  • Fermentation in IndustryFoodYogurtFermentation of carbohydrate in milk by Lactobacillus bulgaricusLactate low pH & precipitation of milk proteinsSwiss cheeseFermentation of milk by Propionibacterium freudenreichiiPropionic acid & CO2 milk protein precipitation & holesOther fermented foodKimchi, soy sauceLow pH prevents growth of microorganisms

    Industrial fermentationFermentation of readily available carbohydrate (e.g. corn starch) to make more valuable productsEthanol, isopropanol, butanol, butanediolFormic, acetic, propionic, butyric, succinic acids

  • 14.4 Gluconeogenesis

  • GluconeogenesisPyruvate & related 3-/ 4-C compounds glucoseNet reaction2 pyruvate + 4ATP + 2GTP + 2NADH + 2H+ + 4H2O Glc + 4ADP + 2GDP + 6Pi +2NAD+In animalsGlc generation from lactate, pyruvate, glycerol, and amino acidsMostly in liverCori cycle ;Lactate produced in muscle converted to glc in liver glycogen storage or back to muscle

    In plant seedlingsStored fats & proteins disaccharide sucroseIn microorganismsGlc generation from acetate, lactate, and propionate in the medium

  • Gluconeogenesis

  • Glycolysis vs. Gluconeogenesis7 shared enzymatic reactions3 bypass reactions; irreversible steps requiring unique enzymesLarge negative DG in glycolysisHexokinase vs. glc 6-phosphatasePhosphofructokinase-1 vs. fructose 1,6-bisphosphatasePyruvate kinase vs. pyruvate carboxylase + PEP carboxykinase

  • From Pyruvate to PEPPyruvate carboxylaseMitochondrial enzyme with biotin coenzymeActivation of pyruvate by CO2 transfer oxaloacetate Pyruvate + HCO3- + ATP oxaloacetate + ADP + Pi

  • From Pyruvate to PEPOxaloacetate + GTP PEP + CO2 + GDPPEP carboxykinaseCytosolic and mitochondria enzymeOverall reaction equationPyruvate + ATP + GTP + HCO3- PEP + ADP + GDP + Pi + CO2, DGo