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

Complete oxidation of glucose  stoichiometry: glc + 6 O 2  6 CO H 2 O  G' º = – 686 kcal/mol  ATP yield theoretical: >90theoretical: >90 actual: 30-32actual:  first stage: glycolysis (10-11 steps) location: cytosol of all cells (including microorganisms)location: cytosol of all cells (including microorganisms) 2 parts2 parts glc  2 glyceraldehyde 3-P (GAP)(steps 1-5) 2 GAP  2 pyruvate/lactate(steps 6 -10/11) (686/7.3) 1

Glycolysis 3.phosphoryl transfer phosphofructokinase irreversible committed step 2.isomerization phosphoglucose isomerase 1. glc 1. ( see L2sl9 “Phosphorylation of glc”) ATP ( see L2sl9 “Phosphorylation of glc”) ADP 2

4. aldol cleavage aldolase H 3

5. isomerization triose phosphate isomerase 6. oxidation-driven phosphorylation GAP DHase 7. phosphoryl transfer phosphoglycerate kinase 4

8. phosphoryl shift phosphoglycerate mutase 9. dehydration enolase 10. phosphoryl transfer irreversible pyruvate kinase 5

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: NADH cyt + oxid e – carrier mito → NAD + cyt + red. e – carrier mito 2 e – cyt → 2 e – mito  malate-aspartate shuttle main shuttle in heart & liver cellsmain shuttle in heart & liver cells e – pair eventually transferred to mitochondrial matrix NAD +, so ATP yield is 2.5/ e – paire – pair eventually transferred to mitochondrial matrix NAD +, so ATP yield is 2.5/ e – pair 6

GOP-DHAP shuttle  main shuttle in brain & skeletal muscle  net reaction NADH cyt + H + + E-FAD ↓ NAD + cyt + E-FADH 2  yields 1.5 ATP per e – pair Fig  ‚ 7 e – s from complex II, others

Regeneration of NAD + : 2. reduction of pyruvate  conditions limiting electron shuttles: mitochondria scarce (“fast” muscle) or absent (RBC)mitochondria scarce (“fast” muscle) or absent (RBC) limited O 2 supply (ischemia)limited O 2 supply (ischemia) high demand for ATP causes glycolysis rate > shuttle ratehigh 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

Glycolysis stoichiometries Aerobic glycolysis: ATP yield steps 1-10 glc + 2 NAD + → 2 pyruvate + 2 NADH + 4H + 2 Regen. of NAD + : GOP shuttle + ox phos 2 H NADH + O 2 → 2 NAD H 2 O 3* glc + O 2 → 2 pyruvate + 2 H H 2 O 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

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:microorganisms:  their environment becomes acidic e.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 + ] riseskeletal muscle during exercise: [lactate], [pyruvate] & [H + ] rise 10

Fate of pyruvate/lactate  pyruvate has a number of alternative fates e.g., oxidized further in mitochondria (next lecture)e.g., oxidized further in mitochondria (next lecture) diffusion out of cell (efflux)diffusion out of cell (efflux)  lactate has only 1 metabolic fate: oxidation back to pyruvate if oxidation limited, efflux occursif 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 11 LIVERMUSCLE glucose   glucose 6 ATP   2 ATP out pyruvate blood pyruvate  lactate  lactate gluconeogenesisglycolysis Net effect is transfer of energy from liver to muscle The Cori cycle

stepenzymeinhibitoractivator stepenzymeinhibitoractivator 1 hexokinaseglc 6-P 1 hexokinaseglc 6-P 3phosphofructokinase ATP, AMP, citrate* ADP 3phosphofructokinase ATP, AMP, citrate* ADP mechanism of control: mechanism of control: both kinases have allosteric sites to which activators/inhibitors bind Control of glycolysis * provides coordination with Krebs (citric acid) cycle hexokinase has allosteric site for glc 6P See Lehninger et al. p

Next time: 4. Gluconeogenesis Pyruvate oxidation