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Section 6: Carbohydrate Metabolism

Section 6: Carbohydrate Metabolism. 3. Anaerobic & aerobic glycolysis. 10/21/2005. Complete oxidation of glucose. stoichiometry: glc + 6 O 2  6 CO 2 + 6 H 2 O  G' º = – 686 kcal/mol ATP yield theoretical: >90 actual: 30-32 first stage: glycolysis (10-11 steps)

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Section 6: Carbohydrate Metabolism

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  1. Section 6: Carbohydrate Metabolism 3. Anaerobic & aerobic glycolysis 10/21/2005

  2. Complete oxidation of glucose • stoichiometry:glc + 6 O2 6 CO2 + 6 H2O G'º = – 686 kcal/mol • ATP yield • theoretical: >90 • actual: 30-32 • first stage: glycolysis (10-11 steps) • location: cytosol of all cells (including microorganisms) • 2 parts glc  2 glyceraldehyde 3-P (GAP) (steps 1-5) 2 GAP  2 pyruvate/lactate (steps 6-10/11) (686/7.3) 1

  3. Glycolysis glc1. ATP (see L2sl9 “Phosphorylation of glc”)ADP 2. isomerizationphosphoglucose isomerase 3. phosphoryl transferphosphofructokinaseirreversible committed step 2

  4. 4. aldol cleavagealdolase H 3

  5. 5. isomerizationtriose phosphate isomerase 6. oxidation-driven phosphorylation GAP DHase 7. phosphoryl transferphosphoglycerate kinase 4

  6. 8. phosphoryl shiftphosphoglycerate mutase 9. dehydrationenolase 10. phosphoryl transferirreversiblepyruvate kinase 5

  7. Regeneration of NAD+: 1. electron shuttles • stoichiometry of steps 1-10:glc + 2 NAD+→ 2 pyruvate + 2 NADH + 4 H+ • NAD present in cells in only catalytic amounts, so regeneration of NAD+ is necessary • cytosolic NADH cannot enter mitochondria • solution: e–pair carried to mitochondrial e–transport chain via a shuttle (short linking pathway) • net reaction: NADHcyt + oxid e– carriermito → NAD+cyt + red. e– carriermito 2 e–cyt→2 e–mito • malate-aspartate shuttle • main shuttle in heart & liver cells • e–pair eventually transferred to mitochondrial matrix NAD+, so ATP yield is 2.5/e–pair 6

  8. GOP-DHAP shuttle • main shuttle in brain & skeletal muscle • net reaction NADHcyt + H+ + E-FAD ↓ NAD+cyt + E-FADH2 • yields1.5 ATP per e– pair Fig. 18.37  ‚ 7 e–s from complex II, others

  9. Regeneration of NAD+: 2. reduction of pyruvate • conditions limiting electron shuttles: • mitochondria scarce (“fast” muscle) or absent (RBC) • limited O2 supply (ischemia) • high demand for ATP causes glycolysis rate > shuttle rate • e– pair is transferred to pyruvate: • as a result, glycolysis can occur without net oxidation: anaerobically fermentation: any anaerobic process 11. oxidation- reduction 8

  10. Glycolysis stoichiometries Aerobic glycolysis:ATP yieldsteps 1-10 glc + 2 NAD+→ 2 pyruvate + 2 NADH + 4H+ 2 Regen. of NAD+: GOP shuttle + ox phos2 H+ + 2 NADH + O2→ 2 NAD+ + 2 H2O 3* glc + O2 → 2 pyruvate + 2 H+ + 2 H2O 5 Anaerobic glycolysis: steps 1-10 glc + 2 NAD+→ 2 pyruvate + 2 NADH + 4 H+ 2 step 11 2 pyruvate +2 NADH + 2H+→ 2 lactate +2 NAD+ (steps 1-11) glc → 2 lactate + 2 H+2 * 5 if malate-aspartate shuttle used 9

  11. Effect of glycolysis products (pyruvate/lactate):acidification • stoichiometry of both aerobic & anaerobic glycolysis shows production of 2 H+/glc • unlike phosphate-containing metabolites,lactate & pyruvate permeant to most cell membranes(as protonated forms: lactic acid & pyruvic acid) • microorganisms: • their environment becomes acidice.g., plaque bacteria on enamel surface ferment carbs • low pH increases solubility of Ca phosphate minerals • repeated acid attacks produce carious lesion • skeletal muscle during exercise:[lactate], [pyruvate] & [H+] rise 10

  12. Fate of pyruvate/lactate • pyruvate has a number of alternative fates • e.g., oxidized further in mitochondria (next lecture) • diffusion out of cell (efflux) • lactate has only 1 metabolic fate: oxidation back to pyruvate • if oxidation limited, efflux occurs • blood distributes these • liver converts them back to glc by gluconeogenesis (next lecture) • combination of muscle glycolysis & liver gluconeogenesis: Cori cycle The Cori cycle LIVER MUSCLE glucose  glucose6 ATP 2 ATP out pyruvate blood pyruvate  lactate  lactategluconeogenesis glycolysis Net effect is transfer of energy from liver to muscle 11

  13. Control of glycolysis step enzyme inhibitor activator 1 hexokinase glc 6-P 3 phosphofructokinase ATP, AMP, citrate* ADP mechanism of control: both kinases have allosteric sites to which activators/inhibitors bind hexokinase has allosteric site for glc 6PSee Lehningeret al. p. 432 * provides coordination with Krebs (citric acid) cycle 12

  14. Next time:4. GluconeogenesisPyruvate oxidation

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