Control Mechanisms in Carbohydrate Catabolism: Pentose Phosphate Pathway and Krebs Cycle

1. Section 65. Pentose phosphate pathwayKrebs cycleCarbohydrate catabolism:control, dental aspects 10/28/05

2. Pentose phosphate pathway • alternate catabolism of glucose 6-P • energy channeled into reducing potential (high-energy e–s), not ATP • cytosol of most cells, especially adipose tissue, liver • functions: synthesis of pentoses, supply e–s for fat synthesis • coenzyme: NADP • NAD with a phosphoryl group on a 2' ribose • e– carrier used in reductive biosynthesis (e.g., fatty acid synthesis) • stoichiometry varies with specific cell, situation • stoichiometry of version that maximizes making NADPH: glc 6-P + 12 NADP+ + 7 H2O → 6 CO2 + 12 NADPH + 12 H+ + Pi 1

3. Pentose phosphate pathway: first 2 steps, control • Control of the pentose phosphate pathway: glc 6-P DHase rate controlled by [NADP+] 6-P gluconateDHase ¯ribose-P,other sugars 2

4. Overview of catabolism FATS POLYSACCHARIDES PROTEINS Stage 1 glucose,other sugars amino acids fatty acids,glycerol pyruvate Stage 2 acetyl CoA CoA H2O O2 Stage 3 e–CO2 Krebscycle oxidativephosphorylation v v ATP ADP + Pi adapted from Fig. 17-15

5. The Krebs Cycle • aka the citric acid cycle; the tricarboxylic acid (TCA) cycle;the final common pathway for fuel oxidation • location: mitochondrial matrix • function: acetyl group → 2 CO2 ATP production • aerobic O2 not used directly • NADH & FADH2 transfere– pairs to e– transport chain • transfer required to regenerate e– carriers • ATP made by oxidativephosphorylation 3

6. The Krebs Cycle (steps 1-4) step enzyme reaction type 1 citrate synthase* condensation (Claisen) 2,3 aconitase isomerization via dehydration-hydration 4 isocitrate DHase† oxidative decarboxylation *inh by ATP †inh by NADH, ATP; activ by ADP 4

7. The Krebs Cycle (steps 5-9) step enzyme reaction type 5 a-ketoglutarate DHase oxid. decarb. 6 succinyl thiokinase phosphorylation driven by thioester hydrolysis 7 succinate DHase oxid.-reduction (complex II) 8 fumarase hydration 9 malate DHase oxid.-reduction 5

8. Connection to electron transport chain • most e–s enter e– chain via NADH • transferred from NADH via 3 complexes (I, III, IV) to O2 • end up in H2O via transfer to O2 (the final electron acceptor) stoichiometry:NADH + H+ + ½ O2→ NAD+ + H2O (2.5 ATP made) • some e–s enter via FADH2 enzymes (enter e– chain at Q)stoichiometry:FADH2 + ½ O2→ FAD + H2O(1.5 ATP made) NADH →complxI →Q→complxIII→cyt c → complx IV from↑↓ mal-asp shuttle FADH2 enzymes: O2 pyr DHase succinate DHase (complex II) Krebs cycle DHases GOP DHase hydroxyacyl CoA DHase* acyl CoA DHase* others others Lehninger 3edFig 19-8 * fatty acid catabolism (Section 7) 6

9. Stoichiometries ATP Krebs cycle (steps 1-9): yield 2FAD + 6NAD+ + 2acetyl CoA + 6H2O → 2FADH2 + 6NADH + 6H+ + 2CoA + 4CO22 oxidative phosphorylation: 2FADH2 + 6NADH + 6H+ + 4O2→ 2FAD + 6NAD+ + 8H2O 18 stage III: 2acetyl CoA + 4O2 → 4CO2 + 2H2O + 2CoA 20 add in stages I & II:glucose + 2O2 + 2CoA → 2acetyl CoA + 2CO2 + 4H2O complete oxidation of glc: glucose + 6 O2 → 6 CO2 + 6 H2O 10-12 30-32 7

10. Replenishing (anaplerotic) reactions • cycle intermediates used to make other biomolecules • e.g., succinyl CoA →heme oxaloacetate →aspartate • cycle itself results in no net change of [intermediates](slide 7: Stage III) • other reactions needed to ↑[intermediates] • example of a replenishing reaction: CH3COCOO– + HCO3– → –OOCCH2COCOO– pyruvate oxaloacetate • driven by being coupled to ATP hydrolysis • enzyme: pyruvate carboxylase(coenzyme: biotin) • allosterically activated by acetyl CoA • same reaction as gluconeogenesis:step 10'a (S6L4,slide4) 8

11. Krebs cycle: anaerobic conditions • Krebs cycle is the same in microorganisms as in eukaryotes • cycle is aerobic (linked to electron transport chain) • to regenerate e– carriers (e.g., NAD+), e– transfer to O2 must occur • under anaerobic conditions, some microorganisms produce acids from cycle “backup” • examples:acetyl CoA + ADP + Pi → ATP + acetic acid + CoAsuccinate → CO2 + propionic acid (CH3CH2COOH) • these acid products are membrane-permeant • by acidifying local regions, products can damage tissuese.g., in caries & periodontal disease, they are among the numerous substances that cause damage 9

12. Control of metabolic pathways • feedback inhibitionusually an early step (committed step) of a pathway is inhibited by a pathway product • example: a pathway functioning to produce F: A → B → C → D → E → F Fwill often allosterically inhibit step A → B or B → C • in catabolism, main product is ATP, so ATP common allosteric inhibitor • feedforward activation usually a precursor: of the pathway’s product or of a related pathway’s product • example: AMP & ADP as precursors of ATP 10

13. Control of carbohydrate catabolism pentoseP ?pathway GLYCOGEN-OLYSIS × glycogen AMP, ADP  glucose 6-P fructose 6-P AMP, ADP  fructose 1,6 bisP pyruvate AMP   acetylCoA oxaloacetate citrate isocitrate ADP  -ketoglutarate ATP NADH ↑ox phos ADP + Pi ¯ GLYCOLYSIS ¯  KREBSCYCLE 11

14. Dental aspects of carb metabolism: summary • sucrose • source of fermentable monosaccharides • an activated precursor of plaque polysaccharides • plaque polysaccharides (mutans, dextrans, levans) • synthesis catalyzed by bacteria-secreted sucrases • adhesion, fuel, anaerobic conditions for microorganisms • anaerobic conditions (fermentation) • glycolysis lactic acid • Krebs cycle acetic acid, propionic acid, others • low pH • solubilizes enamel hydroxyapatite (caries) • damages supporting tissue proteins, cells (gingivitis, periodontal disease, pulpitis) 12

15. Web links • Stryer site: Chapter 19 (Glycolysis) • see also Chapters 18, 20, 22 at that site • Carbohydrate Structure and Metabolismfrom the University of Kansas Medical Biochemistry Center. This site is essentially an online course in carbohydrates and many biochemical pathways.

16. Next section:7. Lipid MetabolismNext exam (#6):Monday, Nov. 7 at 8 a.m.