19. The Citric Acid Cycle

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1 19. The Citric Acid Cycle

2 19.1 The Central Role of Citric Acid Cycle Play in Metabolism Evolution of aerobic metabolism - Nutrients are oxidized to carbon dioxide and water. - Organisms can obtain far more energy from nutrient by aerobic metabolism. - Three process: citric acid cycle, electron transport, and oxidative phosphorylation

3 19.1 The Central Role of Citric Acid Cycle Play in Metabolism Catabolism and anabolism - Citric cycle is amphibolic: plays a role in both catabolism and anabolism - Two other common names for citric acid cycle: Krebs cycle, tricarboxylic acid cycle (or TCA cycle).

4 19.2 The Overall Pathway of the Citric Acid Cycle Difference between glycolysis and citric acid cycle - Part in the cell in which pathway occur: glycolysis in cytosol, citric acid cycle in mitochondria - Enzymes of citric acid cycle are present in mitochondria.

5 19.2 The Overall Pathway of the Citric Acid Cycle Mitochondrial structure - Mitochondrial matrix and intermembrane space.

6 19.2 The Overall Pathway of the Citric Acid Cycle Citric acid cycle

7 19.2 The Overall Pathway of the Citric Acid Cycle - Oxidative decarboxylation: Citrate isomerizes, and it both loses carbon dioxide and is oxidized. - Eight steps: oxidation reactions ( steps 3, 4, 6, and 8), a molecule of GDP to GTP ( step 5)

8 19.3 How Pyruvate is Converted to Acetyl-CoA Pyruvate dehydrogenase complex - Enzyme system (5 enzymes) - Responsible for conversion of pyruvate to carbon dioxide and acetyl portion of acetyl-coa - Acetyl-CoA is a thioester. - Overall reaction (ΔG = kj mol -1 )

9 19.3 How Pyruvate is Converted to Acetyl-CoA - Five enzymes in pyruvate dehydrogenase complex in mammals : pyruvate dehydrogenase (PDH), dihydrolipoyl transacetylase, dihydrolipoyl dehydrogenase, pyruvate dehydrogenase kinase, and pyruvate dehydrogenase phosphatase Lipoic acid - Disulfide group in its oxidized form and two sulfhydryl groups in its reduced form - Act as an oxidizing agent

10 19.3 How Pyruvate is Converted to Acetyl-CoA Reaction sequence that converts pyruvate to carbon dioxide and acetyl-coa 10/45

11 19.3 How Pyruvate is Converted to Acetyl-CoA Two great advantages in a compact arrangement 1) Various stages of reaction can take place more efficiently - Reactants and enzymes are so close to each other. 2) Regulatory controls can be applied more efficiently.

12 19.4 The Individual Reactions of the Citric Acid Cycle Reactions of citric acid cycle

13 19.4 The Individual Reactions of the Citric Acid Cycle Step 1. Formation of Citrate Reaction of acetyl-coa and oxaloacetate to form citrate and CoA- SH - Condensation: new carbon-carbon bond is formed. - By hydrolysis of citryl-coa - By enzyme citrate synthase (condensing enzyme) - Exergonic (ΔG = kj mol -1 )

14 19.4 The Individual Reactions of the Citric Acid Cycle Step 2. Isomerization of Citrate to Isocitrate Isomerization - By aconitase: able to select one end of citrate molecule in preference to the other - Symmetrical (achiral) compound to chiral compound

15 19.4 The Individual Reactions of the Citric Acid Cycle Step 2. Isomerization of Citrate to Isocitrate - Type of behavior: enzyme bind a symmetrical substrate in an unsymmetrical binding site

16 19.4 The Individual Reactions of the Citric Acid Cycle Step 2. Isomerization of Citrate to Isocitrate - Intermediate, cis-aconitate, remains bound to enzyme during course of reaction.

17 19.4 What Are the Individual Reactions of the Citric Acid cycle? Step 3. Formation of α-ketoglutarate and CO 2 First Oxidation Oxidative decarboxylation of isocitrate to α-ketoglutarate and carbon dioxide - By isocitrate dehydrogenase

18 19.4 The Individual Reactions of the Citric Acid Cycle Step 4. Formation of Succinyl-CoA and CO 2 Second Oxidation Carbon dioxide and succinyl-coa are formed from α-ketoglutarate and CoA - By enzyme system, α-ketoglutarate dehydrogenase complex - Exergonic ( ΔG = kj mol -1 ) - Release of CO 2 has a profound influence on mammalian physiology.

19 19.4 The Individual Reactions of the Citric Acid Cycle Step 5. Formation of Succinate Thioester bond of succinyl-coa is hydrolyzed to produce succinate and CoA-SH. - By succinyl-coa synthetase - ΔG = -3.3 kj mol -1

20 19.4 The Individual Reactions of the Citric Acid Cycle Step 5. Formation of Succinate Nucleosidediphosphate kinase - Transfer of a phosphate group from GTP to ADP to give GDP and ATP GTP + ADP GDP + ATP

21 19.4 The Individual Reactions of the Citric Acid Cycle Step 5. Formation of Succinate Next three steps to complete the cycle - Four carbon succinate ion is converted to oxaloacetate ion.

22 19.4 The Individual Reactions of the Citric Acid Cycle Step 6. Formation of Fumarate FAD-Linked Oxidation Succinate is oxidized to fumarate. - By succinate dehydrogenase: integral protein of inner mitochondrial membrane

23 19.4 The Individual Reactions of the Citric Acid Cycle Step 6. Formation of Fumarate FAD-Linked Oxidation - Overall reaction is Succinate + E-FAD Fumarate + E-FADH 2 - Nonheme iron protein: Succinate dehydrogenase contains iron atoms but does not contain a heme group.

24 19.4 The Individual Reactions of the Citric Acid Cycle Step 7. Formation of L-Malate Water is added across double bond of fumarate. - By fumarase - Only L-malate is produced.

25 19.4 The Individual Reactions of the Citric Acid Cycle Step 8. Regeneration of Oxaloacetate Final Oxidation step Malate is oxidized to oxaloacetate - By malate dehydrogenase

26 19.4 The Individual Reactions of the Citric Acid Cycle Oxidation of pyruvate, production of three molecules of CO 2 Pyruvate dehydrogenase complex Pyruvate + CoA-SH + NAD + Acetyl-CoA + NADH + CO 2 + H + Citric acid cycle Acetyl-CoA + 3NAD + + FAD + GDP + Pi + 2H 2 O 2CO 2 + CoA-SH + 3NADH + 3H + + FADH 2 + GTP Overall reaction Pyruvate + 4NAD + + FAD + GDP + Pi + 2H 2 O 3CO 2 + 4NADH + FADH 2 + GTP + 4H +

27 19.4 The Individual Reactions of the Citric Acid Cycle Eventual ATP production per pyruvate: 4NADH 10ATP (2.5 ATP for each NADH) 1FADH 1.5ATP (1.5 ATP for each FADH 2 ) 1GTP 1ATP Total 12.5 ATP per pyruvate or 25 ATP per glucose

28 19.5 Energetics and Control of the Citric Acid Cycle Standard free energy changes for individual reactions - Endergonic reaction is coupled to one of the strongly exergonic reactions of cycle. - More released energy to come in electron transport chain.

29 19.5 Energetics and Control of the Citric Acid Cycle Control of the citric acid cycle is exercised at three points

30 19.5 Energetics and Control of the Citric Acid Cycle Control of Pyruvate Dehydrogenase Pyruvate dehydrogenase (PDH) complex is activated by ADP. - In mammals, inhibition by phosphorylation of pyruvate dehydrogenase - Phosphate group is covalently bound to enzyme by pyruvate dehydrogenase kinase. - Pyruvate dehydrogenase activated by phosphoprotein phosphatase

31 19.5 Energetics and Control of the Citric Acid Cycle Control of Pyruvate Dehydrogenase Regulatory control (in citric acid cycle) 1) Citrate synthase is an allosteric enzyme inhibited by ATP, NADH, succinyl-coa, and its product. 2) Isocitrate dehydrogenase reaction: ADP and NAD + are allosteric activators of enzyme. 3) α-ketoglutarate dehydrogenase complex: ATP and NADH are inhibitors.

32 19.5 Energetics and Control of the Citric Acid Cycle Control of Pyruvate Dehydrogenase

33 19.6 The Glyoxylate Cycle: A Related Pathway Acetyl-CoA can serve as starting material for biosynthesis of carbohydrate - In plants and some bacteria, but not in animals - Two enzymes to produce glucose from fatty acids: Isocitrate lyase, malate synthase.

34 19.6 The Glyoxylate Cycle: A Related Pathway Acetyl-CoA can serve as starting material for biosynthesis of carbohydrate - In plants and some bacteria, but not in animals - Two enzymes to produce glucose from fatty acids: Isocitrate lyase, malate synthase.

35 19.6 The Glyoxylate Cycle: A Related Pathway Glyoxylate cycle

36 19.6 The Glyoxylate Cycle: A Related Pathway Glyoxysomes - Specialized organelles in plants - Sites of glyoxylate cycle - Important in germinating seeds - Fatty acid (stored in seeds) broke down for energy during germination. 1 Fatty acids give rise to acetyl-coa: enter citric acid cycle and go on to release energy. 2 Acetyl-CoA serves as a starting point for synthesis of glucose and other compounds.

37 19.7 The Citric Acid Cycle in Catabolism Break down nutrients 1) Degradation of large molecules to smaller ones 2) Glycolytic pathway - Sugars are converted to pyruvate, and then enter citric acid cycle. - Just as All roads lead to Rome, all pathways lead to citric acid cycle.

38 19.7 The Citric Acid Cycle in Catabolism - Various catabolic pathways Amino acids

39 19.8 The Citric Acid Cycle in Anabolism Anaplerotic reaction - Replenishes a citric acid cycle intermediate - In mammals, oxaloacetate is produced from pyruvate by pyruvate carboxylase. - Amphibolic citric acid cycle and anabolism of sugars by gluconeogenesis.

40 19.8 The Citric Acid Cycle in Anabolism Anabolic reactions of gluconeogenesis in cytosol - Oxaloacetate is not transported across mitochondrial membrane. - Phosphoenolpyruvate is transferred to cytosol. - By malate dehydrogenase Malate + NAD + Oxaloacetate + NADH + H +

41 19.8 What Role Does the Citric Acid Cycle Play in Anabolism? Lipid Anabolism Anabolic reactions of lipid metabolism in cytosol - Starting point is acetyl-coa. - By soluble enzymes

42 19.8 The Citric Acid Cycle in Anabolism Lipid Anabolism - Overall reaction is, Citrate + CoA-SH + ATP Acetyl-CoA + Oxaloacetate + ADP + Pi Reduction reaction followed by an oxidation - No net oxidation - Substitution of NADPH for NADH

43 19.8 The Citric Acid Cycle in Anabolism Anabolism of Amino Acids and Other Metabolites Anabolic reactions (amino acids and other metabolites) - Malate to oxaloacetate - Oxaloacetate to produce aspartate - Aspartate forms not only amino acid but also other nitrogencontaining metabolites. - Isocitrate produces α-ketoglutarate. - Glutamate forms more amino acids. - Succinyl-CoA gives rise to amino acids and porphyrin ring of heme group.

44 19.8 The Citric Acid Cycle in Anabolism Anabolism of Amino Acids and Other Metabolites

45 19.9 The Link to Oxygen Citric acid cycle is related to electron transport and oxidative phosphorylation - Citric acid cycle as a vital link between chemical energy of nutrients and chemical energy of ATP - Classic equation of aerobic oxidation of glucose Glucose + 6O 2 6H 2 O + 6CO 2

46 THE END!!

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