TCA Cycle

Question 1

Substrates for TCA Cycle

Answer 1

• main substrate = acetyl CoA
• carbon skeletons of sugars, fatty acids and several amino acids enter the citric acid cycle as acetyl CoA
• acetyl CoA is not the only site where amino acids can enter the cycle

Question 2

Key intermediates of TCA cycle

Answer 2

• Citrate
• α keto-glutarate
• Succinyl CoA
• Fumarate
• Oxaloacetate (and to a lesser extent malate)

Question 3

Importance of citrate intermediate in TCA cycle

Answer 3

this is the point @ which fatty acid synthesis takes off

Question 4

Importance of alpha keto-glutarate intermediate in TCA cycle

Answer 4

α keto-glutarate: because this is an important entrance point for a number of amino acids that contribute to gluconeogenesis.

Question 5

Importance of succinyl CoA intermediate in TCA cycle

Answer 5

Succinyl CoA: because this is an important entrance point for a number of amino acids and for products of the breakdown of fatty acids with an odd number of carbons that contribute to gluconeogenesis.

Question 6

Importance of fumarate intermediate in TCA cycle

Answer 6

Fumarate: because this is an entrance point for amino acids and because it is a byproduct of the urea cycle.

Question 7

Importance of oxaloacetate intermediate in TCA cycle

Answer 7

Oxaloacetate (and to a lesser extent malate): because these are involved in the gluconeogenic pathway from pyruvate.

Question 8

Entry points into TCA for amino acids

Answer 8

• AA ==> pyrubate ==> Oxaloacetate
• AA ==> oxaloacetate
• AA ==> fumarate
• AA ==> Succinyl CoA
• AA ==> alpha-ketoglutarate

Question 9

Main products of TCA cycle

Answer 9

• The TCA cycle generates NADH, FADH2, GTP.
• The GTP produced can be used to generate ATP
• NADH and FADH2 enter the electron transport pathway to also generate ATP

Question 10

Metabolic role of TCA

Answer 10

• Oxidation of glucose to pyruvate has not harnessed the energy stored in glucose completely.
  
  • TCAC creates more energy from glucose.
• It also makes biosynthetic precursors (AA’s, nucleotides).
  
  • intermediates are continuously channeled into the pathways of amino acid synthesis, fatty acid synthesis, etc.
  • involved in gluconeogenesis @ liver

Question 11

TCA ==> gluconeogenesis @ liver

Answer 11

• In the liver, during fasting, gluconeogenic precursors are converted to malate
• malate leaves the mitochondria to enter the pathway of gluconeogenesis in the cytosol.

Question 12

Key TCA steps for production of NADH, FADH₂, & CO₂

Answer 12

• Isocitrate + NAD==> alpha-ketoglutarate + CO₂ + NADH
• alpha-ketoglutarate + NAD⁺ ==> Succinyl CoA + CO₂ + NADH
• Succinate + FAD²⁺ ==> Fumarate + FADH₂
• Malate + NAD⁺ ==> Oxaloacetate + NADH