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Metabolic Control
TCA Cycle & Anabolism
• Supply of cycle components need to be
replenished to keep cycle operating as they are
used for synthesisused for synthesis
– Anaplerotic reaction – reaction that replenishes a
citric acid cycle intermediate
– [Oxaloacetate] must allow acetyl-CoA to enter
cycle
In mammals Pyruvate + CO + ATP + H O →– In mammals, Pyruvate + CO2 + ATP + H2O →
oxaloacetate + ADP + Pi + 2 H+
–](https://image.slidesharecdn.com/17-tcacycle-151206095503-lva1-app6891/85/TCA-Cycle-21-320.jpg)
More Related Content
TCA Cycle
- 1. 1 Berg • Tymoczko• Stryer Biochemistry Sixth Edition Chapter 17: Th Cit i A id C lThe Citric Acid Cycle Copyright © 2007 by W. H. Freeman and Company The Citric Acid Cycle • Glycolysis produces just 2 ATP molecules • Aerobic metabolism of glucose → CO2 gives more ATP • Main part called the citric acid cycle – Tricarboxylic acid (TCA) cyclecycle – Krebs cycle –
- 2. 2 The Citric AcidCycle • Pyruvate must be converted to acetyl-CoA • Pyruvate + coenzyme A + NAD+ → acetyl-CoA + CO + NADHCO2 + NADH • Acetyl-CoA then enters the citric acid cycle, which occurs inside mitochondria The Citric Acid Cycle • Citric acid cycle the “metabolic hub” of the cell 6 carbon tricarboxylic acid cell – Fuels aerobically oxidized – A source of precursors for amino acids nucleotideacids, nucleotide bases, porphyrin –
- 3. 3 The Citric AcidCycle • How does the citric acid cycle connect to other metabolic pathways? The Citric Acid Cycle • Pyruvate + coenzyme A + NAD+ → acetyl-CoA + CO2 + NADH – Pyruvate transported through membrane protein– Pyruvate transported through membrane protein into mitochondria – Pyruvate dehydrogenase complex catalyzes this irreversible reaction • Complex of 3 enzymes • Member of a large family with masses from 4 million toMember of a large family, with masses from 4 million to 10 million daltons •
- 4. 4 The Citric AcidCycle The Citric Acid Cycle
- 5. 5 Mechanism of Pyruvate→ Acetyl CoA • Pyruvate + CoA + NAD+ → Acetyl CoA + CO2 + NADH + H+ – Requires 5 coenzymes • Catalytic cofactors: thiamine pyrophosphate (TPP), lipoic acid, and FADFAD • Stoichiometric cofactors: CoA and NAD+ – Mechanism of Pyruvate → Acetyl CoA
- 6. 6 Mechanism of Pyruvate→ Acetyl CoA • Decarboxylation – Catalyzed by E1 of pyruvate dehydrogenase complexcomplex Mechanism of Pyruvate → Acetyl CoA • Oxidation – Catalyzed by the pyruvate dehydrogenase component of thedehydrogenase component of the complex (E1) –
- 7. 7 Mechanism of Pyruvate→ Acetyl CoA • Transfer to CoA – Catalyzed by dihydrolipoyl transacetylase (E2) Thioester bond remains in product– Thioester bond remains in product – Mechanism of Pyruvate → Acetyl CoA • Step 4: Dihydrolipoamide oxidized to lipoamide – Catalyzed by dihydrolipoyl dehydrogenase (E3) 2 e transferred to FAD then to NAD+– 2 e- transferred to FAD then to NAD+ – –
- 8. 8 Mechanism of Pyruvate→ Acetyl CoA • Complex structure of the complex 12 E3 (αβ) N-terminus 24 E1 (α2β2) 8 E2 (α3) Mechanism of Pyruvate → Acetyl CoA • Advantages of a compact multienzyme complex – Reactions more efficient because reactants and enzymes so close to each other increases overallenzymes so close to each other, increases overall rate and minimizes side reactions • Lipoamide swings to pyruvate dehydrogenase to accept acetyl group • Swings to transacetylase to transfer it to CoA-SH • Swings to dihydrolipoyl dehydrogenase to regenerate sulfhydryl groups –
- 9. 9 Reactions of theCitric Acid Cycle • 1, Formation of citrate, a condensation reaction – Catalyzed by citrate synthase – Reactions of the Citric Acid Cycle • Mechanism of citrate synthase, how does it prevent hydrolysis of acetyl CoA? – Large conformational changes during catalysis– Large conformational changes during catalysis – Oxaloacetat e Acetyl CoA CoA Citrate Enzym e Enzyme Condensation – Reaction
- 10. 10 Reactions of theCitric Acid Cycle Acetyl CoA transformed to enol intermediate Citryl CoA causes conformational changes that close active siteactive site Reactions of the Citric Acid Cycle • 2, Isomerization of citrate to isocitrate – –
- 11. 11 Reactions of theCitric Acid Cycle • Aconitase in a class called iron-sulfur proteins – – 4 Fe atoms complexed to 4 sulfides and 3 cysteine S, one Fe binds to citrate through COO- & OH groups Reactions of the Citric Acid Cycle • Fluoracetatyl-CoA also a substrate for citrate synthase – Fluoracetate found in leaves of some poisonous– Fluoracetate found in leaves of some poisonous plants – Fluorocitrate inhibits aconitase (enzyme in next rxn of citric acid cycle)
- 12. 12 Reactions of theCitric Acid Cycle • 3, 1st oxidation, formation of α-ketoglutarate and CO2 –– – Reactions of the Citric Acid Cycle • 4, 2nd oxidation, formation of succinyl-CoA and CO2 – Another oxidative decarboxylation catalyzed by– Another oxidative decarboxylation, catalyzed by the α-ketoglutarate dehydrogenase complex –
- 13. 13 Reactions of theCitric Acid Cycle • 5, Formation of succinate – Catalyzed by succinyl CoA synthetase – Reactions of the Citric Acid Cycle • 6, Formation of fumarate, an FAD-linked oxidation – Catalyzed by succinate dehydrogenase an– Catalyzed by succinate dehydrogenase, an integral protein of the mitochondrial membrane, also is directly associated with the electron- transport chain – Because FAD covalently bound, transfer e- to Fe- S clusters of the protein, then to electron transportE ES clusters of the protein, then to electron transport chain – E- E-
- 14. 14 Reactions of theCitric Acid Cycle • 7, Formation of L-malate by hydration – Reactions of the Citric Acid Cycle • 8, The final oxidation, regeneration of oxaloacetate – Catalyzed by malate dehydrogenase– Catalyzed by malate dehydrogenase – 2.5 ATP for each NADH – ΔGo’ = + 29.7 kJ/mol
- 15. 15 Reactions of theCitric Acid Cycle • Net of steps 6-8 – Summary of Reactions • Pyruvate dehydrogenase complex in conjunction with the citric acid cycle yields –– – – – • Pyruvate dehydrogenase complex: – Pyruvate + CoA-SH + NAD+ → Acetyl-CoA + NADH + CO2 + H+
- 16. 16 Summary of Reactions •Citric acid cycle – Acetyl-CoA + 3 NAD+ + FAD + GDP + Pi + 2 H2O → 2 CO2 + COA-SH + 3 NADH + 3 H+ + FADH2 +→ 2 CO2 + COA SH + 3 NADH + 3 H + FADH2 + GTP • Overall reaction – Pyruvate + 4 NAD+ + FAD + GDP + Pi + 2 H2O → 3 CO2 + 4 NADH + FADH2 + GTP + 4 H+ E t l ATP d ti t• Eventual ATP production per pyruvate: – 4 NADH → 10 ATP – 1 FADH2 → 1.5 ATP – 1 GTP → 1 ATP – Sum: 12.5 ATP per pyruvate (25 per glucose) Summary of Reactions • Interesting points – Enzymes of the citric acid cycle may be physically associated with each other leading products toassociated with each other, leading products to pass directly from one to the other in a process called “substrate channeling” – Citric acid cycle strictly aerobic because O2 required to regenerate NAD+ and FAD in the mitochondriontoc o d o – Net pyruvate → acetyl CoA has ΔGo’ = -33.4 kJ/mol –
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- 18. 18 Metabolic Control • Entryinto cycle & rate of cycle tightly controlled • Pyruvate → acetyl CoA• Pyruvate → acetyl CoA irreversible in animals – C oxidized to CO2 by TCA cycle – Incorporated into lipids • Pyruvate dehydrogenase (PDH) complex inhibited by products – – Metabolic Control • PDH complex activated by ADP and pyruvate – ADP & pyruvate inhibit the kinase that turns off– ADP & pyruvate inhibit the kinase that turns off PDH – Both the kinase and phosphatase are associated with the PDH complex
- 19. 19 Metabolic Control • Howis the phosphatase activated? – Recall the β-adrenergic receptor is stimulated by epinephrine leads to release of Ca2+ intoepinephrine, leads to release of Ca into cytoplasm and stimulates muscle contraction – Metabolic Control • Citric acid cycle controlled at 3 points, rxns of – Citrate synthase, isocitrate dehydrogenase, & the α-ketoglutarate dehydrogenase complexg y g p • Citrate synthase – Inhibited by – Activated by • Isocitrate dehydrogenase – Activated byActivated by – Inhibited by • The α-ketoglutarate dehydrogenase complex – Inhibited by
- 20. 20 Metabolic Control • Cellsin a resting • Cells in a highly active Relationship between metabolic state of a cell and the ATP/ADP and NADH/NAD+ ratios Cells in a resting metabolic state – Need and use little energy – High ATP, low ADP l l i l hi h Cells in a highly active metabolic state – Need and use more energy than resting cells – Low ATP, high ADP levels imply low ATP/ADP ratiolevels imply high ATP/ADP ratio – High NADH, low NAD+ levels imply high NADH/NAD+ ratio ATP/ADP ratio – Low NADH, high NAD+ levels imply low NADH/NAD+ ratio Metabolic Control • Inhibition of isocitrate dehydrogenase leads to buildup of citrate – Citrate signals glycolysis to stop– Citrate signals glycolysis to stop – Can be a source of acetyl CoA for fatty acid synthesis • Inhibition of α-ketoglutarate dehydrogenase leads to buildup of α-ketoglutarate – Used as precursor for synthesis of many amino acids and purine baes
- 21. 21 Metabolic Control TCA Cycle& Anabolism • Supply of cycle components need to be replenished to keep cycle operating as they are used for synthesisused for synthesis – Anaplerotic reaction – reaction that replenishes a citric acid cycle intermediate – [Oxaloacetate] must allow acetyl-CoA to enter cycle In mammals Pyruvate + CO + ATP + H O →– In mammals, Pyruvate + CO2 + ATP + H2O → oxaloacetate + ADP + Pi + 2 H+ –
- 22. 22 TCA Cycle &Anabolism Beriberi • Beriberi – a disorder caused by a lack of thiamine (vitamin B1) in the diet, results in weight loss, pain, emotional disturbance, weakness,loss, pain, emotional disturbance, weakness, irregular heart rate • Rare except in the Far East where rice is major food – Rice has a low content of thiamine O i ll l h li ill b l i h d– Occasionally alcoholics will be malnourished enough to suffer beriberi • What is the biochemistry of this? – Thiamine is precursor to thiamine pyrophosphate (TPP), a coenzyme of pyruvate dehydrogenase,
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- 24. 24 Glyoxylate Cycle • Plantsand bacteria can synthesize carbohydrates fromcarbohydrates from acetyl-CoA – Similar to TCA cycle, but decarboxylations bypassed & 2 Unique reactions of glyoxylate cycle bypassed & 2 acetyl-CoA molecules enter per cycle – Lets them grow on acetate Carbohydrate s Summary • Pyruvate dehydrogenase complex links glycolysis to the citric acid cycle • TCA cycle starts with condensation of 4C + 2C• TCA cycle starts with condensation of 4C + 2C molecule, 4C molecule regenerated • 12.5 ATP/pyruvate from TCA cycle & PDH reaction • TCA cycle tightly controlled – Control closely tied to energy status of cell • TCA cycle gives provide synthetic precursors • Glyoxylate cycle lets plants & bacteria synthesize carbohydrates from acetyl-CoA