CITRIC ACID CYCLE ERT106 BIOCHEMISTRY SEM /19 BY: MOHAMAD FAHRURRAZI TOMPANG

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1 CITRIC ACID CYCLE ERT106 BIOCHEMISTRY SEM /19 BY: MOHAMAD FAHRURRAZI TOMPANG

2 Chapter Outline (19-1) The central role of the citric acid cycle in metabolism (19-2) The overall pathway of the citric acid cycle (19-3) How pyruvate is converted to acetyl-coa (19-4) The individual reactions of the citric acid cycle (19-5) Energetics and control of the citric acid cycle (19-6) The glyoxylate cycle: A related pathway (19-7) The citric acid cycle in catabolism (19-8) The citric acid cycle in anabolism (19-9) The link to oxygen

3 The Central Role of the Citric Acid Cycle Processes that play key roles in aerobic metabolism Citric acid cycle: Central metabolic pathway Electron transport: Series of oxidation reduction reactions by which the electrons derived from oxidation of nutrients are passed to oxygen Oxidative phosphorylation Process for generating ATP Depends on the creation of a ph gradient within the mitochondrion as a result of electron transport Metabolism consists of: Catabolism - Oxidative breakdown of nutrients Anabolism - Reductive synthesis of biomolecules

4 Citric Acid Cycle Amphibolic: Able to be part of both catabolism and anabolism Also called Krebs cycle or tricarboxylic acid cycle (TCA cycle) Takes place in mitochondria Most enzymes are present in the mitochondrial matrix

5 Figure Structure of a Mitochondrion

6 Reactions in Citric Acid Cycle Pyruvate produced by glycolysis is oxidized further to one carbon dioxide molecule and to one acetyl group, which becomes linked to an intermediate, coenzyme A (CoA) Acetyl-CoA enters the citric acid cycle Two more molecules of carbon dioxide are produced for each molecule of acetyl-coa that enters the cycle, and electrons are transferred Immediate electron acceptors NAD + - Reduced to NADH FAD - Takes up two electrons and two hydrogen ions to produce FADH 2

7 Reactions in Citric Acid Cycle (continued) Electrons are passed from NADH and FADH 2 through several stages of an electron transport chain with a different redox reaction at each step Final electron acceptor is oxygen, with water as the product Cycle produces guanosine triphosphate (GTP), a high-energy compound, directly Cycle is completed by regeneration of oxaloacetate from succinate in several steps

8 Figure Overview of the Citric Acid Cycle

9 Conversion of Pyruvate to Acetyl-CoA Pyruvate dehydrogenase complex: Multienzyme complex that catalyzes the conversion of pyruvate to acetyl-coa and carbon dioxide Enzymes - Pyruvate dehydrogenase (PDH), dihydrolipoyl transacetylase, dihydrolipoyl dehydrogenase, pyruvate dehydrogenase kinase, and pyruvate dehydrogenase phosphatase

10 Conversion of Pyruvate to Acetyl-CoA (continued) Pyruvate loses CO 2, and hydroxyethyl TPP (HETPP) is formed Active form of lipoic acid is bound to the enzyme, dihydrolipoyl transacetylase, by an amide bond to the e-amino group of a lysine Hydroxyethyl group (HE) is oxidized and transferred to a sulfur atom of the reduced form of lipoamide Lipoamide is reduced to dihydrolipoamide Acetyl group is transferred to the sulfhydryl group of coenzyme A Dihydrolipoamide is oxidized to lipoamide

11 Figure Mechanism of the Pyruvate Dehydrogenase Reaction

12 Table Reactions of the Citric Acid Cycle

13 Individual Reactions of the Citric Acid Cycle Acetyl-CoA reacts with oxaloacetate to form citrate and CoA-SH DG = 32.8 kj mol 1 (exergonic) Catalyzed by citrate synthase, an allosteric enzyme that is inhibited by NADH, ATP, and succinyl-coa

14 Individual Reactions of the Citric Acid Cycle (continued 1) Citrate is isomerized to isocitrate Reaction is catalyzed by aconitase Citrate is achiral and has no stereocenter Isocitrate is chiral and has two stereocenters and four possible stereoisomers, but only one of the four stereoisomers of isocitrate is formed in the cycle

15 Individual Reactions of the Citric Acid Cycle (continued 2) Oxidative decarboxylation of isocitrate to form a- ketoglutarate and CO 2 Reaction is catalyzed by isocitrate dehydrogenase, an allosteric enzyme that is inhibited by ATP and NADH and activated by ADP and NAD +

16 Individual Reactions of the Citric Acid Cycle (continued 3) Oxidative decarboxylation of α-ketoglutarate to form succinyl-coa Reaction is catalyzed by the a-ketoglutarate dehydrogenase complex, a multienzyme complex that requires thiamine pyrophosphate (TPP), lipoic acid, FAD, and Mg 2+

17 Individual Reactions of the Citric Acid Cycle (continued 4) Thioester bond of succinyl-coa is hydrolyzed to produce succinate and CoA-SH Reaction is catalyzed by succinyl-coa synthetase Two CH 2 COO groups of succinate are equivalent First energy-yielding step of the cycle Overall reaction is slightly exergonic

18 Individual Reactions of the Citric Acid Cycle (continued 5) Succinate is oxidized to fumarate Reaction is catalyzed by succinate dehydrogenase

19 Individual Reactions of the Citric Acid Cycle (continued 6) Hydration of fumarate to form L-malate Reaction is catalyzed by fumarase

20 Individual Reactions of the Citric Acid Cycle (continued 7) Malate is oxidized to oxaloacetate, and another molecule of NAD + is reduced to NADH Reaction is catalyzed by malate dehydrogenase

21 Figure Formation of Oxaloacetate from Succinate

22 Oxidation of Pyruvate Forms CO 2 and ATP Pyruvate dehydrogenase complex Pyruvate + CoA -SH + NAD Citric acid cycle Overall reaction Acetyl - CoA + NADH + CO + H Acetyl - CoA + 3NAD FAD + GDP + P i + 2H2O + 2CO + CoA -SH + 3NADH + 3H + FADH + GTP Pyruvate + 4NAD FAD + GDP + P i + 2H2O + 3CO + 4NADH + FADH + GTP + 4H 2 2 +

23 Oxidation of Pyruvate Forms CO 2 and ATP (continued) Eventual ATP production per pyruvate 4NADH 10 ATP (2.5 ATP for each NADH) 1FADH ATP (1.5 ATP for each FADH ) 1GTP 1ATP 2 Total 12.5 ATP per pyruvate or 25 ATP per glucose

24 Table Energetics of Conversion of Pyruvate to CO 2

25 Regulatory Enzymes That Control the Citric Acid Cycle Within the cycle Citrate synthase - Inhibited by ATP, NADH, succinyl CoA, and its own product, citrate Isocitrate dehydrogenase - Activated by ADP and NAD + and inhibited by ATP and NADH α-ketoglutarate dehydrogenase complex - Inhibited by ATP, NADH, and succinyl CoA and activated by ADP and NAD+ Outside the cycle Pyruvate dehydrogenase - Inhibited by ATP, NADH, and acetyl-coa

26 Figure Control Points in the Conversion of Pyruvate to Acetyl-CoA and in the Citric Acid Cycle

27 Table Relationship between the Metabolic State of a Cell and the ATP/ADP and NADH/NAD + Ratios

28 Glyoxylate Cycle Pathway in plants and some bacteria that is an alternative to the citric acid cycle Bypasses the two oxidative decarboxylation steps of the citric acid cycle Routes isocitrate via glyoxylate to malate Key enzymes Isocitrate lyase: Catalyzes the cleavage of isocitrate to glyoxylate and succinate Malate synthase: Catalyzes the reaction of glyoxylate with acetyl-coa to produce malate

29 Figure Reactions in Plants and Bacteria That Produce Carbohydrates from Fats

30 Figure Glyoxylate Cycle

31 Glyoxylate Cycle (continued) Takes place in: Glyoxysomes Glyoxysomes: Specialized organelles in plants Cytoplasm in yeast and algae Involved in the growth of plants in the dark Seeds are rich in lipids, which contain fatty acids During germination, plants use the acetyl-coa produced in fatty acid oxidation to produce oxaloacetate and other intermediates for carbohydrate synthesis Once plants begin photosynthesis and can fix CO 2, glyoxysomes disappear

32 Citric Acid Cycle in Catabolism Catabolism of proteins, carbohydrates, and fatty acids feed into the citric acid cycle at one or more points In catabolic pathways, nutrients are broken down to smaller molecules, such as sugars, fatty acids, and amino acids Small molecules are processed further and enter the citric acid cycle, which plays a key role in metabolism Catabolic reactions occur in the cytosol, and citric acid cycle takes place in mitochondria End products of catabolism cross the mitochondrial membrane and participate in the citric acid cycle

33 Figure Summary of Catabolism, Showing the Central Role of the Citric Acid Cycle

34 Citric Acid Cycle in Anabolism Citric acid cycle is the source of starting materials for the biosynthesis of other compounds If a component of the citric acid cycle is taken out for biosynthesis, it must be replenished Anaplerotic reaction: Reaction that ensures an adequate supply of an important metabolite Example - Oxaloacetate is replaced by the carboxylation of pyruvate

35 Figure How Mammals Keep an Adequate Supply of Metabolic Intermediates

36 Role of Pyruvate and Acetyl-CoA to Animals and Plants Pyruvate cannot be produced from acetyl-coa in mammals because acetyl-coa is the end product of catabolism of fatty acids Carbohydrates are the principal energy and carbon source in animals Plants carry out the conversion of acetyl-coa to pyruvate and oxaloacetate, so they can exist without carbohydrates as a carbon source Conversion of pyruvate to acetyl-coa takes place in both plants and animals

37 Figure Transfer of the Starting Materials of Gluconeogenesis from the Mitochondrion to the Cytosol

38 Figure Transfer of the Starting Materials of Lipid Anabolism from the Mitochondrion to the Cytosol

39 Lipid Anabolism Begins with acetyl-coa and takes place in the cytosol Acetyl-CoA is produced mainly in mitochondria from catabolism of fatty acids and carbohydrates Indirect transfer mechanism involving citrate exists Citrate + CoA -SH + ATP Acetyl - CoA + Oxaloacetate + ADP + P i Oxaloacetate that is formed provides a means for the production of the NADPH needed for biosynthesis

40 Lipid Anabolism (continued) Net effect of these two reactions is replacement of NADH by NADPH While there is some NADPH produced by this means, its principal source is the pentose phosphate pathway

41 Amino Acids and Citric Acid Cycle Anabolic reactions that produce amino acids and many other biomolecules begin with citric acid cycle molecules that are transported into the cytosol Catabolism and anabolism are closely related Operation of any metabolic pathway can be speeded up or slowed down in response to the needs of an organism by control mechanisms

42 Figure Summary of Anabolism, Showing the Central Role of the Citric Acid Cycle

43 Link to Oxygen Citric acid cycle is considered part of the aerobic metabolic process because of its link to the electron transport chain and oxidative phosphorylation Citric acid cycle provides a vital link between the chemical energy of nutrients and the chemical energy of ATP NADH and FADH 2, two important cofactors generated by the citric acid cycle, pass their electrons to oxygen

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