Lecture 18: TCA Cycle

Question 1

Fate of pyruvate

Answer 1

• Pyruvate turned into lactate in anaerobic glycolysis using lactate dehydrogenase
• Turned into acetyl coA to be used in TCA cycle

Question 2

Fate of Lactate

Answer 2

• Used in cori cycle: convert lactate to glucose in liver -> send glucose to rbc -> glycolysis to form 2 ATP and more lactate

Question 3

Pyruvate Dehydrogenase Complex(PDC)

Answer 3

• Links production of pyruvate in the cytoplasm with TCA cycle in the mitochondrial matrix
• Large complex of multiple copies of 3 different subunits
  - E1, E2, E3
• Requires five cofactors for catalysis(Thiamine pyrophosphate(TPP), Lipoamide, FAD, Coenzyme A, NAD+)

Question 4

PDC Summary

Answer 4

• Complex of 3 enzymatic subunits and 5 different cofactors found in mitochondrial matrix
• Pyruvate is oxidized and activated forming acetyl-coA
• Reaction is a redox decarboxylation reaction, producing CO2 and NADH
• Large, negative standard free energy indicates it is favourable and irreversible

Question 5

Pathway Integration and Regulation

Answer 5

• Increase pyruvate inhibit glycolysis
• Increase NADH, Acetyl-CoA, and ATP inhibit PDC

Question 6

Control by feedback inhibition

Answer 6

• Increase Acetyl-CoA inhibit E2
• Increase NADH inhibit E3

Question 7

PDC Covalent Modification

Answer 7

• PD Kinase phosphorylate PDC, inactive
• PD Phosphatase dephosphorylate PDC, active

Question 8

Muscle at rest

Answer 8

• High energy state: high NADH, Acetyl-CoA activate PD Kinase -> inhibit PDC

Question 9

Muscle when running

Answer 9

• Low energy state: Calcium activates PD phosphatase -> activate PDC

Question 10

Regulation of PDC

Answer 10

• Product Inhibition: Feedback inhibition by Acetyl-CoA and NADH on E2 and E3
• Covalent modification mediated by PD Kinase and PD phosphatase
  - phosphorylation of E1 by PD kinase leads to inactivation
  - dephosphorylation by PD phosphatase restores activity
• Allosteric control: Allosteric control of PD kinase by acetyl-coA, NADH, pyruvate, and ADP
  
  • Ca stimulates PD phosphatase

Question 11

TCA cycle: oxidizing fuel

Answer 11

• links breakdown of fuel molecules to ATP production in oxidative phosphorylation

Question 12

Overview of TCA cycle

Answer 12

• Stage 1: Oxidative decarboxylation
  - Acetyl-CoA combines with oxaloacetate to produce citrate(starts TCA cycle)
  - 2Cs are oxidized to CO2, regenerating oxaloacetate
  Stage 2: Regenerate oxaloacetate
• GTP made by substrate level phosphorylation
• 2 H2O molecules needed by reactions
• 4 pairs of electrons used to reduce and form 3 NADH and 1 FADH2 for oxidative phosphorylation
• Succinate dehydrogenase is a membrane bound protein linking TCA to oxidative phosphorylation

Question 13

Regulating TCA cycle

Answer 13

• Low energy and calcium stimulate cycle
• High energy, reduced coenzymes or products inhibit the cycle

Question 14

Summary of TCA cycle

Answer 14

• TCA cycle uses acetyl-CoA supplied by breakdown of glucose, fatty acids, and amino acids
• Overall pathway favorable despite positive free energy of Malate Dehydrogenase(MDH)
• Regulation occurs at favorable reactions to control cycle
• TCA is considered amphibolic, linking anabolic and catabolic pathways via Acetyl CoA and TCA intermediates