The Krebs Tricarboxylic Acid Cycle

1. The Krebs Tricarboxylic Acid Cycle The Final Common Pathway of Oxidative Metabolism 9/24/07

2. ⑤ ⑥ ① ④ ② ③  Liver Gluconeogenesis; 1Liver, Kidney e- Ox phos

3. Pyruvate GTP  ATP (substrate level phosphorylation) Citric Acid Cycle (CAC) “Kreb Cycle” Tricarboxylic Acid Cycle 2/3 of O2 consumption needed for oxidation of Acetyl CoA  CO2 • Occurs exclusively in the • mitochondrion (matrix) • OAA acts as carrier or acceptor • of acetyl CoA units – is regenerated • “Burns” acetyl CoA to CO2 – during • this oxidation eˉs from acetyl CoA • are trapped in the form of: NADH Pyruvate Dehydrogenase Complex “links” glycolysis to CAC FADH + 2eˉ + 2eˉ + 2eˉ + 2eˉ

4. The Three Stages of Metabolism

5. The Krebs Cycle Citric Acid Cycle; The TCA Cycle • Pyruvate (actually the acetyl group) from glycolysis is degraded to CO2 • The acetyl group is formed in stage II of metabolism from carbohydrate and amino acid metabolism • 1GTP (ATP in bacteria) and 1 FADH2 is produced during one turn of the cycle • 3 NADH are produced during one turn of the cycle • NADH and FADH2 energize electron transport and oxidative phosphorylation • Eight reactions make up the Krebs cycle

6. The Chemical Logic of the Krebs Cycle • After condensing acetate with oxaloacetate to form citrate – oxidation yields CO2, oxaloacetate is regenerated, and the energy is captured as NADH, FADH2, and GTP (ATP) • Acetyl-CoA is called the stoichiometric substrate; it is consumed in large amounts • Oxaloacetate is called the regenerating substrate; it is continuously regenerated (it is not consumed) • The cycle is catalytic; oxaloacetate is consumed and then regenerated.

7. Overview of the Krebs Cycle: A Mitochondrial Process

8. Anatomy of the Mitochondrion • Which membrane is impermeable to protons and other ions? • Which membrane will allow for the transport of molecules up to a molecular weight of about 1000?

9. * Pyruvate transporter * Pyruvate mito matrix Coenzymes Thiamine Pyrophosphate (TTP) B1 PyruvateDehydrogenaseComplex Cytoplasm Pyruvate Multimolecularaggregate 3 Enzymes 5 Coenzymes 5 Reactions Irreversible CoA contains the vitamin Pantothenic acid Links glycolysis to CAC Product Inhibition Mitochondrial matrix NAD+ FAD+ CoA Lipoic acid

10. CAC PDHDeficiency – results in CongenitalLacticAcidosis Pyruvate cannot enter the CAC and results in ↑ Lactic Acid Primarily affects the brain – neonatal death 3 Forms – psychomotor retardation √ Possible treatment is ketogenic diet: Low in CHO High in fats Produces ketone bodies as an alternate form of energy for the brain Arsenic Poisoning – Pyruvate Dehydrogenase –a-Ketoglutarate Dehydrogenase Both require lipoic acid as a cofactor Arsenite – Trivalent form of arsenic I° – Forms a stable complex with the thiol (-SH) group of Lipoic Acid II° – Glyceraldehyde 3-PO4 step forms complex with inorganic Pi thus prevents ATP formation in glycolysis Affects the brain – Death, neurologic problems

11. Skeletal muscle Contraction cAMP independent √ Allosteric Regulation √ Allosteric Regulation

12. Carrier Fluorocitrate OAA Fluoroacetate Fluroacetyl CoA ( ¯ ) Rat poison ADP (+) NADH ( - ) Aldo condensation The entrance of acetyl CoA does not ↑ or ↓ intermediates in the CAC Isomerization Oxidative decarboxylation e¯ Irreversible (1) One of the rate limiting Rxs of the CAC ATP

13. (¯) ATP. GTP Succinyl CoA NADH From oxidation of odd number FAs ADP GDP ATP Oxidative decarboxylation Very similar to the Pyruvate Dehydrogenase complex Irreversible (2) e¯ “Substrate Level Phosphorylation” Nucleoside Biphosphate Kinase e¯ Oxidation reaction Hydration reaction

14. e¯ ( 4 ) Reversible oxidation reaction

16. Main Points of the Krebs Cycle • Occurs in mitochondrion • All enzymes are hydrophilic and occur in the matrix except for succinate dehydrogenase, which occurs in the inner mitochondrial membrane • Citrate synthase, Isocitrate dehydrogenase and a-ketoglutarate dehydrogenase are the three irreversible reactions • ICD is the main regulatory enzyme, and it is activated by ADP • Succinate dehydrogenase is inhibited by malonate and oxaloacetate

17. Regulation of the CAC Dependent on the energy state of the cell which is reflected by ratio [ADP] [Pi] [ATP] Summary This ratio determines the rate of oxidative phosphorylation Named “Respiratory Control” of energy production because oxidation and phosphorylation of ADP must occur simultaneously

18. Oxidized Reduced Electrochemical gradient

19. 1 2 3 6 4 7 5 8 2 Shuttle systems to bring cytosolic NADH into mitochondria for oxidative phosphorylation 1) Glycerophosphate shuttle = 36 ATP 2) Malate-aspartate shuttle = 38 ATP Count ATPs: Anerobic glycolysis = 2 Glycolysis + CAC + oxidative phosphorylation = 38