Tricarboxylic Acid Cycle

1. Tricarboxylic Acid Cycle SitiAnnisa Devi Trusda Biochemistry Department

3. Overview final pathway where oxidative metabolism of CH, AA, FAcarbon skeleton : CO2 & H2O provides energy (ATP) occurs in mitochondriain close proximity to reactions of electron transport • AerobicO2 required as the final electron acceptor • Participates in synthetic rx/: formation of glucose from carbon skeleton of some AA • Intermediates of the TCA cycle can also be synthesized by the catabolism of some amino acids a traffic circle with compounds entering and leaving as required

6. REACTION OF THE TCA CYCLE • Pyruvate, the product of glycolysis, is converted to acetyl-CoA, the starting material for the citric acid cycle, by the pyruvatedehydrogenasecomplex. • The PDH complex is composed of multiple copies of three enzymes: • pyruvatedehydrogenase, E1 (with its bound cofactor TPP); • dihydrolipoyltransacetylase, E2 (with its covalently bound lipoyl group) • dihydrolipoyldehydrogenase, E3 (with its cofactors FAD and NAD).

7. E1 catalyzes the decarboxylation of pyruvatehydroxyethyl -TPP, then the oxidation of the hydroxyethyl group acetyl group. • E2 catalyzes the transfer of the acetyl group to coenzyme A, forming acetyl-CoA. • E3 catalyzes the regeneration of the disulfide(oxidized) form of lipoate; electrons pass first to FAD, then to NAD.

11. The PyruvateDehydrogenase Complex Requires Five Coenzymes • The combined dehydrogenation and decarboxylationof pyruvateto the acetyl group of acetyl-CoArequires the sequential action of • 3 different enzymes • 5 different coenzymes or prosthetic groups • thiamine pyrophosphate (TPP), • flavin adenine dinucleotide(FAD), • coenzyme A (CoA, sometimes denoted CoA-SH, to emphasize the role of the OSH group), • nicotinamideadenine dinucleotide(NAD) • lipoate. • 4 different vitamins required in human nutrition are vital components of this system: thiamine (inTPP), riboflavin (in FAD), niacin (in NAD), and pantothenate (in CoA).

12. PyruvateDehydrogenaseDeficiency • congenital lactic acidosis • inability to convert pyruvate to acetyl CoA • Pyruvate to be shunted to lactic acid via lactate dehydrogenase • Severe : Neonatal death • Moderate: psychomotor retardation w/ damage of cerebral cortex, basal ganglia, brainstemdeath in infancy • Third form: episodic ataxia induced by Ch rich meal • No treatment available ketogenic diet

13. Mechanism of Arsenic Poisoning • Arsenic inhibits enzymes that require lipoic acid as cofactor : pyruvatedehydrogenase, α-ketoglutaratedehydrogenase. • Arsenite forms a stable complex with the thiol (-SH) group of lipoicacidunavailable to serve as coenzyme • If it binds to lipoic acid in the pyruvatedehydrogenase complex, pyruvate (and lactate) accumulate ≈ pyruvatedehydrogenase complex deficiency neurologic distrubance - death

14. The Citric Acid Cycle Has 8 Steps1. Formation of Citrate

15. 2. Formation of Isocitrate via cis-Aconitate

16. 3. Oxidation of isocitrate to α-ketoglutarate +CO2

17. 4. Oxidation of α-ketoglutarate to succinylcoA+ CO2

18. 5. Conversion of succinylcoA to succinate

19. 6. Oxidation of succinate to fumarate

20. 7. Hydration of fumarate to malate

21. 8.Oxidation of malate to oxaloacetate

23. Products of one turn of the citric acid cycle. At each turn of the cycle(each molecule of acetyl coA), 3 NADH (9 ATP), 1 FADH2(2 ATP), 1GTP (1 ATP), and two CO2 are released in oxidative decarboxylationreactions 12 ATP

25. Citric Acid Cycle Components Are ImportantBiosynthetic Intermediates • The citric acid cycle is an amphibolicpathwaybothcatabolic and anabolic processes. • Besides its role in the oxidative catabolism of carbohydrates, fatty acids, and amino acids, the cycle provides precursors for many biosynthetic pathways through reactions that served the same purpose in anaerobic ancestors. • α–Ketoglutarate and oxaloacetatecan serve as precursors of the amino acids aspartate and glutamate by simple transamination

26. Through aspartate and glutamate, the carbons of oxaloacetate and α–ketoglutarate are then used to build other amino acids, as well as purine and pyrimidine nucleotides. • Oxaloacetate is converted to glucose in gluconeogenesis • Succinyl-CoA is a central intermediate in the synthesis of the porphyrin ring of heme groups, which serve as • oxygen carriers (in hemoglobin and myoglobin) • electron carriers (in cytochromes) • Citrate produced in some organisms is used commercially for a variety of purposes

27. Regulation of the Citric Acid Cycle • The overall rate of the citric acid cycle is controlled by • rate of conversion of pyruvateacetyl-CoA • flux through citrate synthase, isocitratedehydrogenase, & -ketoglutaratedehydrogenase. • These fluxes are largely determined by the concentrations of substrates and products: • the end products ATP and NADH are inhibitory • the substrates NAD and ADP are stimulatory.

28. The production of acetyl-CoA for the citric acid cycle by the PDH complex is : • inhibited allosterically by metabolites that signal a sufficiency of metabolic energy (ATP, acetyl-CoA, NADH, and fatty acids) • stimulated by metabolites that indicate a reduced energy supply (AMP, NAD, CoA).

29. HATURNUHUN…. Have a Nice ‘Cycling’ Time

30. LIKE ALWAYS, A SIMPLE QUIZ ;p • Reaksipertama yang terjadiuntukmasukkedalamsiklus Krebs adalahperubahan…….. menjadi…….. • Produkdarisatuputaransiklus Krebs adalah….., ………, dan……….. denganjumlahenergi yang dihasilkansebanyak…….. • Defisiensienzimpiruvatdehidrogenasemengakibatkanpenyakit…….. • Berikansatucontohkomponendalamsiklus Krebs yang menjadizatantarabiosintesiszat lain!