Lecture 27 - Citric Acid Cycle Flashcards
what is the citric cycle also known as?
The Krebs Cycle or TCA cycle
Where does the citric acid cycle occur?
In the mitochondria
What does the ‘cycle’ mean in the CAC?
Start and finish with the same molecule
What enters the citric acid cycle?
2 carbon molecules as acetyl-CoA
Where does the acetyl-CoA used in the CAC come from?
- Glycolysis
Pyruvate - Pyruvate oxidation
Remove 1 carbon, add a double bonded oxygen and CoA - Beta-oxidation
Fatty acid cut in half to produce Acetyl-CoA
What exits the citric acid cycle?
2x CO2, 3NADH, 3H+, ATP and FADH2
What are the two parts of the CAC?
The release of the Carbon molecules and the regeneration of the starting molecule (oxaloacetate)
What is the first stage of the CAC?
Condensation of acetyl-CoA with oxaloacetate
What is the overview of the condensation of acetyl-CoA with oxaloacetate in words?
2 C entering in acetyl CoA is attached to 4 C oxaloacetate to produce 6 C citrate.
Where does the energy come from in the condensation of acetyl-CoA with oxaloacetate?
The hydrolysis of CoA from acetyl-CoA
Is the condensation of acetyl-CoA with oxaloacetate energetically favourable or unfavourable?
Energetically favourable (G = -32.8kJ)
What does the condensation of acetyl-CoA with oxaloacetate produce?
Citrate
What happens to citrate after the condensation reaciton?
Undergoes isomerisation
What is the isomerisation of citrate catalysed by?
Aconitase
What are the two steps in the Isomerisation of citrate?
A dehyrdation and a rehydration to move the OH group to a different carbon
How does 1080 poisoning occur?
Fluroacetate forms fluorocitrate which is converted to a substrate that binds tightly to the aconitase enzyme, inactivating it
What type of reaction is the removal of the carbons?
Oxidative decarboxylation reactions
How does removal of the first carbon occur?
In two steps; oxidation of the molecule removing 2x H then decarboxylation, removing the CO2
How does removal of the second carbon occur?
In one step; oxidation and decarboxylation at the same time
What does the removal of the CoA from succinyl to succinate release?
GTP
What is GTP?
An energy equivalent of ATP
How is ATP generated from GTP?
GTP undergoes a reaction with ADP to form an equilibrium, producing ATP and GDP
What is a substrate level phosphorylation?
The direct use of energy from a substrate molecule to drive the synthesis of ATP
How does succinate rearrange to form oxaloacetate?
Succinate is oxidised, hydrated and then oxidised again
What is reduced in the first oxidation reaction of succinate?
FAD to FADH2
What is reduced in the second oxidation reaction of succinate?
NAD+ to NADH + H+
What is the overall reaction for the CAC?
Acetyl-CoA + 3NAD+ + FAD + 2H2O + GDP + Pi ——-> 2CO2 + CoASH + 3NADH + 3H+ + FADH2 + GTP
Is the CAC energetically favourable or unfavourable?
Favourable -44.3kJ/mol
What is the shared enzyme in the CAC?
SDH
Where does the shared enzyme sit?
In the inner mitochondrial membrane
What occurs with SDH?
Succinate is oxidised to fumarate using SDH. SDH reduces FAD to FADH2 which is restored to FAD in the electron transport chain
