Krebs cycle Energy Petr Tůma Eva Samcová

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1 Krebs cycle Energy Petr Tůma Eva Samcová

2 Overview of Citric Acid Cycle Key Concepts The citric acid cycle (Krebs cycle) is a multistep catalytic process that converts acetyl groups derived from carbohydrates, fatty acids, and amino acids to CO 2, and produces NADH, FADH 2, and GTP.

3 Overview: Oxidative Fuel Metabolism

4 Reactions of Citric Acid Cycle

5 Tricarboxylic acid cycle (TCA)= Krebs cycle the hub of the metabolic system It accounts for the major portion of carbohydrate, fatty acid, amino acid oxidation and generates numerous biosynthetic precursors TCA is therefore amphibolic, that is, it operates both catabolically (biosynthesis of ATP) and anabolically (biosynthesis of amino acids, heme, glucose, lipids) Starting compounds Acetyl-coenzyme A (acetyl-coa) Oxaloacetate Cataplerotic reactions which utilize and therefore drain TCA Anaplerotic reactions replenish citric acid cycle intermediates

6 General features of CAC Oxidizes the acetyl-coa not only from pyruvate The net reaction :3NAD + + FAD + HS-CoA + GDP + P i 3NADH + H + + FADH 2 + GTP + CoA + 2CO 2 All enzymes of CAC are in mitochondrion, so all substrates including NAD + and GDP must be generated in the mitochondrion or be transported to mitochondrion from cytosol. Similarly all the products of CAC must be consumed in mitochondrion or transferred to cytosol. The net effect of each round of CAC is oxidation of 1 acetyl group to 2CO 2

7 Synthesis of Acetyl-Coenzyme A Key Concepts Pyruvate dehydrogenase is a multienzyme complex that catalyzes a five-part reaction in which pyruvate releases CO 2 and the remaining acetyl group becomes linked to coenzyme A. The reaction sequence requires the cofactors TPP, lipoamide, coenzyme A, FAD, and NAD +.

8 Cofactors of PDH Complex

10 Enzymes of the Citric Acid Cycle Key Concepts The eight enzymes of the citric acid cycle catalyze condensation, isomerization, oxidation reduction, phosphorylation, and hydration reactions. Two reactions produce CO 2, one reaction produces GTP, and four reactions generate the reduced coenzymes NADH or FADH 2.

11 Reaction 2: Aconitase

12 Reaction 3: Isocitrate Dehydrogenase Releases 1 st CO 2

13 Reaction 4: α-ketoglutarate Dehydrogenase Resembles PDH Complex 2 nd NADH

14 Reaction 6: Succinate Dehydrogenase Produces FADH 2

15 Malonate Competitively Inhibits Succinate Dehydrogenase

16 Reaction 7: Fumarase

17 Reaction 8: Malate Dehydrogenase Produces 3 rd NADH

18 Products of Citric Acid Cycle

19 Energy review acetyl-coa + 3NAD + + Q + GDP + P + H 2 O 2CO 2 + 3NADH+H + + FADH 2 + GTP +CoA Reaction Cofactor ATP isocitrate 2-oxoglutarate NADH + H + 3 (2,5) 2-oxoglutarate succinyl- CoA NADH + H + 3 (2,5) succinyl-coa succinate - 1 succinate fumarate FADH 2 2 (1,5) malate oxaloacetate NADH + H + 3 (2,5) 12 (10)

20 Regulation of Citric Acid Cycle

21 Regulation of TCA cycle activation low rate of ATP/ADP and NADH/NAD + inhibition high rate of ATP/ADP and NADH/NAD + The flux controlling enzymes Citrate synthase Isocitrate dehydrogenase 2-ketoglutarate dehydrogenase specific inhibitors fluoroacetate aconitase Arsenic compounds inactivates lipoamide enzymes (pyruvate dehydrogenase and 2-ketoglutarate dehydrogenase malonate succinate dehydrogenase

22 3 Enzymes Function Far From Equilibrium

23 Amphibolic Functions of CAC

24 Metabolic Sources of Acetyl-CoA Acetyl-CoA (high energy compound) the common product of carbohydrate, fatty acids, and amino acids breakdown Pyruvate Dehydrogenase reaction pyruvate + NAD + + CoA acetyl-coa + NADH/H + + CO 2 Irreversible reaction CoA, thiamine diphosphate, lipoamide, FAD, NAD + β-oxidation of fatty acids Amino acids (ketogenic AA, glucogenic AA)

25 Anabolic reactions citrate fatty acids, steroids 2-oxoglutarate AAs of glutamate family succinyl-coa heme malate, oxaloacetate glucose, AAs of pyruvate family

26 Anaplerotic Reactions Replenish CAC Intermediates

27 Anaplerotic reactions replenishment of intermediates of KC pyruvate + CO 2 + ATP OAA + ADP Pyruvate carboxylase Amino acids Asp, Asn OAA Glu, Gln, His, Pro, Arg 2-oxoglutarate Ile, Val, Met, Trp succinyl-coa Ala, Ser, Thr, Cys, Gly - pyruvate Degradation of fatty acids with uneven (odd) number of carbon atoms: Propionyl-CoA succinyl-coa Acetyl-CoA has no anaplerotic effect

28 The Mitochondrion The site of eukaryotic oxidative metabolism with enzymes that mediate this process Is bounded by a smooth outer membrane and contains invaginated inner membrane The inner mitochondrial membrane is impermeable to most hydrophilic compounds therefore contain specific transport systems (proteins 80%)

29 Mitochondrion - metabolic functions

30 Mitochondrion - structure

31 Transport of substances through inner mitochondrial membrane

32 Shuttles Transfer of NADH + H + from cytoplasm into mitochondrion 1. The malate-aspartate shuttle heart, liver, kidney 2. The glycerolphosphate shutles Glycerol-3-phosphate /Dihydroxyacetone phosphate brain, skeletal muscles

33 The malate-aspartate shuttle

34 The glycerolphosphate shuttle

Citric Acid Cycle: Central Role in Catabolism. Entry of Pyruvate into the TCA cycle

Citric Acid Cycle: Central Role in Catabolism Stage II of catabolism involves the conversion of carbohydrates, fats and aminoacids into acetylcoa In aerobic organisms, citric acid cycle makes up the final