10 Cyclic Acid Cycle

Question 1

Why must the citric acid cycle occur under aerobic conditions?

Answer 1

So we have a means of dealing with the reduced cofactors (via ETC)

Question 2

What are four outcomes of the Citric Acid Cycle ?

Answer 2

1. Acetyl-CoA (2C) condenses with oxaloacetate (4C) to make citrate (6C)
2. Two carbons are oxidized to CO2
3. Oxaloacetate is regenerated
4. 3 NADH, 1FADH2/QH2 and 1 GTP are made (high energy products) for each Acetyl-CoA

Question 3

Citric acid cycle occurs in the ______

Answer 3

Citric acid cycle occurs in the mitochondrial matrix

Question 4

Citric acid cycle oxidizes Acetyl-CoA to _____

Answer 4

Citric acid cycle oxidizes Acetyl-CoA to CO2

Question 5

Citric Acid Cycle generates which high energy products?

Answer 5

Reduced cofactors:

• NADH
• FADH2/QH2
• GTP (NTP)

Question 6

What are three sources of Acetyl-CoA?

Answer 6

Carbohydrates (glucose)

Fats (lipids)

Protein (amino acids *alanine*)

Question 7

Citric Acid Cycle is Amphibolic, why?

Answer 7

Anabolic and complete aerobic catabolism

Question 8

Reaction 1:

Acetyl CoA + Oxaloacetate + H2O -> Citrate + HS-CoA + H+

• Reversible?
• Catalyst?
• Regulated?
• Type of reaction?

Answer 8

Reaction 1:

Acetyl CoA + Oxaloacetate + H2O -> Citrate + HS-CoA + H+

• Reversible?
  
  • Irreversible
• Catalyst?
  
  • Citrate Synthase
• Regulated?
  
  • No (not under physiological conditions)
• Type of reaction?
  
  • Condensation with oxaloacetate

Question 9

Reaction 2

Citrate to Isocitrate

• Reversible?
• Type of reaction?

Answer 9

Reaction 2

Citrate to Isocitrate

• Reversible?
  
  • Yes
• Type of reaction?
  
  • Isomerization
    
    • moving position of OH

Question 10

Reaction 3

Isocitrate + NAD+ -> alpha-Ketoglutarate + NADH + CO2

• Reversible?
• Type of reaction?
• Catalyst?
• Regulated?

Answer 10

Reaction 3

Isocitrate + NAD+ -> alpha-Ketoglutarate + NADH + CO2

• Reversible?
  
  • Irreversible
• Type of reaction?
  
  • Oxidative decarboxylation
• Catalyst?
  
  • isocitrate dehydrogenase
• Regulated?
  
  • yes
• ** Energy Capture Step (NADH) **

Question 11

Reaction 4

alpha-ketoglutarate + HS-CoA + NAD+ -> Succinyl CoA + NADH + CO2

• Reversible?
  
  • Irreversible
• Type of reaction?
  
  • Oxidative decarboxylation
• Catalyst?
  
  • alpha-ketoglutarate dehydrogenase complex
• Regulated?
  
  • Yes (similar to PDH)
• * Energy Capture Step *

Question 12

What makes succinyl CoA a High Energy intermediate?

Answer 12

Contains a thioester

Question 13

Reaction 5

Succinyl CoA + GDP (or ADP) + Pi to Succinate + GTP (ATP) + HS-CoA

• Type of Reaction?
• Reversible?
• Catalyst?

Answer 13

Reaction 5

Succinyl CoA + GDP (or ADP) + Pi to Succinate + GTP (ATP) + HS-CoA

• Type of Reaction?
  
  • Oxidation (generating QH2 from oxidation)
• Reversible?
  
  • Yes
• Catalyst?
  
  • Complex II ( succinate dehydrogenase)
• FAD/FADH2 oxidation of C-C single bond

Question 14

Reaction 6

Succinate + Q -> Fumarate + QH2

• Reversible?
• Type of Reaction?

Answer 14

Reaction 6

• Reversible?
  
  • Yes
• Type of Reaction?
  
  • Substrate level phosphorylation
• Energy Capture Step (NTP)

Question 15

Reaction 7

Fumarate + H2O -> L-Malate

• Type of Reaction
• Reversible?

Answer 15

Reaction 7

Fumarate + H2O -> L-Malate

• Type of Reaction
  
  • Hydration (hydratase)
• Reversible?
  
  • Yes

Question 16

Reaction 8

L-Malate + NAD+ -> Oxaloacetate + NADH + H+

• Type of reaction?
• Reversible?

Answer 16

Reaction 8

L-Malate + NAD+ -> Oxaloacetate + NADH + H+

• Type of reaction?
  
  • Oxidation
    
    • Hydroxyl to carbonyl
• Reversible?
  
  • Yes
• *Energy Capture Step*
• Regenerates oxaloacetate

Question 17

How is the Citric Acid Cycle regulated?

Answer 17

• Affected by NAD+/NADH ratio (product/substrate)
• Regulated enzymes (catalyze the oxidative decarboxylation steps)
  
  • Isocitrate dehydrogenase
  • Alpha-ketoglutarate dehydrogenase
• Concentrations of intermediates
  
  • [oxaloacetate]

Question 18

2 Inhibitors and 2 Activators of the Citric Acid Cycle?

Answer 18

Inhibitors:

• NADH, ATP

Activators:

• ADP, Ca++