Download 15. The Importance of Energy Changes and Electron Transfer in

19. The Citric Acid Cycle
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
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).
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.
19.2 The Overall Pathway of the Citric
Acid Cycle
◈ Mitochondrial structure
- Mitochondrial matrix and intermembrane space.
19.2 The Overall Pathway of the Citric
Acid Cycle
◈ Citric acid cycle
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)
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°’ = -33.4 kJ mol-1)
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
19.3 How Pyruvate is Converted to
Acetyl-CoA
◈ Reaction sequence that converts pyruvate to carbon dioxide and
acetyl-CoA
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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.
19.4 The Individual Reactions of the
Citric Acid Cycle
◈ Reactions of citric acid cycle
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 CoASH
- Condensation: new carbon-carbon bond is formed.
- By hydrolysis of citryl-CoA
- By enzyme citrate synthase (condensing enzyme)
- Exergonic (ΔG°’ = -32.8 kJ mol-1)
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
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
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.
19.4 What Are the Individual Reactions of the
Citric Acid cycle ?
Step 3. Formation of α-Ketoglutarate and CO2 – First Oxidation
◈ Oxidative decarboxylation of isocitrate to α-ketoglutarate and
carbon dioxide
- By isocitrate dehydrogenase
19.4 The Individual Reactions of the
Citric Acid Cycle
Step 4. Formation of Succinyl-CoA and CO2 – Second Oxidation
◈ Carbon dioxide and succinyl-CoA are formed from α-ketoglutarate
and CoA
- By enzyme system, α-ketoglutarate dehydrogenase complex
- Exergonic ( ΔG°’ = -33.4 kJ mol-1)
- Release of CO2 has a profound influence on mammalian physiology.
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
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
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.
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
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-FADH2
- Nonheme iron protein: Succinate dehydrogenase contains iron
atoms but does not contain a heme group.
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.
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
19.4 The Individual Reactions of the
Citric Acid Cycle
◈ Oxidation of pyruvate, production of three molecules of CO2
Pyruvate dehydrogenase complex
Pyruvate + CoA-SH + NAD+
Acetyl-CoA + NADH + CO2 + H+
Citric acid cycle
Acetyl-CoA + 3NAD+ + FAD + GDP + Pi + 2H2O
2CO2 + CoA-SH + 3NADH + 3H+ + FADH2 + GTP
Overall reaction
Pyruvate + 4NAD+ + FAD + GDP + Pi + 2H2O
3CO2 + 4NADH + FADH2 + GTP + 4H+
19.4 The Individual Reactions of the
Citric Acid Cycle
Eventual ATP production per pyruvate:
4NADH
1FADH
1GTP
10ATP (2.5 ATP for each NADH)
1.5ATP (1.5 ATP for each FADH2)
1ATP
Total 12.5 ATP per pyruvate or 25 ATP per glucose
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.
19.5 Energetics and Control of the Citric
Acid Cycle
◈ Control of the citric acid cycle is exercised at three points
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
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.
19.5 Energetics and Control of the Citric
Acid Cycle
Control of Pyruvate Dehydrogenase
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.
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.
19.6 The Glyoxylate Cycle:
A Related Pathway
◈ Glyoxylate cycle
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.
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.
19.7 The Citric Acid Cycle in Catabolism
- Various catabolic pathways
Amino acids
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.
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+
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
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
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.
19.8 The Citric Acid Cycle in Anabolism
Anabolism of Amino Acids and Other Metabolites
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 + 6O2
6H2O + 6CO2
THE END!!