Lecture 27 - Citric Acid Cycle

Question 1

what is the citric cycle also known as?

Answer 1

The Krebs Cycle or TCA cycle

Question 2

Where does the citric acid cycle occur?

Answer 2

In the mitochondria

Question 3

What does the ‘cycle’ mean in the CAC?

Answer 3

Start and finish with the same molecule

Question 4

What enters the citric acid cycle?

Answer 4

2 carbon molecules as acetyl-CoA

Question 5

Where does the acetyl-CoA used in the CAC come from?

Answer 5

• 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

Question 6

What exits the citric acid cycle?

Answer 6

2x CO2, 3NADH, 3H+, ATP and FADH2

Question 7

What are the two parts of the CAC?

Answer 7

The release of the Carbon molecules and the regeneration of the starting molecule (oxaloacetate)

Question 8

What is the first stage of the CAC?

Answer 8

Condensation of acetyl-CoA with oxaloacetate

Question 9

What is the overview of the condensation of acetyl-CoA with oxaloacetate in words?

Answer 9

2 C entering in acetyl CoA is attached to 4 C oxaloacetate to produce 6 C citrate.

Question 10

Where does the energy come from in the condensation of acetyl-CoA with oxaloacetate?

Answer 10

The hydrolysis of CoA from acetyl-CoA

Question 11

Is the condensation of acetyl-CoA with oxaloacetate energetically favourable or unfavourable?

Answer 11

Energetically favourable (G = -32.8kJ)

Question 12

What does the condensation of acetyl-CoA with oxaloacetate produce?

Answer 12

Citrate

Question 13

What happens to citrate after the condensation reaciton?

Answer 13

Undergoes isomerisation

Question 14

What is the isomerisation of citrate catalysed by?

Answer 14

Aconitase

Question 15

What are the two steps in the Isomerisation of citrate?

Answer 15

A dehyrdation and a rehydration to move the OH group to a different carbon

Question 16

How does 1080 poisoning occur?

Answer 16

Fluroacetate forms fluorocitrate which is converted to a substrate that binds tightly to the aconitase enzyme, inactivating it

Question 17

What type of reaction is the removal of the carbons?

Answer 17

Oxidative decarboxylation reactions

Question 18

How does removal of the first carbon occur?

Answer 18

In two steps; oxidation of the molecule removing 2x H then decarboxylation, removing the CO2

Question 19

How does removal of the second carbon occur?

Answer 19

In one step; oxidation and decarboxylation at the same time

Question 20

What does the removal of the CoA from succinyl to succinate release?

Answer 20

GTP

Question 21

What is GTP?

Answer 21

An energy equivalent of ATP

Question 22

How is ATP generated from GTP?

Answer 22

GTP undergoes a reaction with ADP to form an equilibrium, producing ATP and GDP

Question 23

What is a substrate level phosphorylation?

Answer 23

The direct use of energy from a substrate molecule to drive the synthesis of ATP

Question 24

How does succinate rearrange to form oxaloacetate?

Answer 24

Succinate is oxidised, hydrated and then oxidised again

Question 25

What is reduced in the first oxidation reaction of succinate?

Answer 25

FAD to FADH2

Question 26

What is reduced in the second oxidation reaction of succinate?

Answer 26

NAD+ to NADH + H+

Question 27

What is the overall reaction for the CAC?

Answer 27

Acetyl-CoA + 3NAD+ + FAD + 2H2O + GDP + Pi ——-> 2CO2 + CoASH + 3NADH + 3H+ + FADH2 + GTP

Question 28

Is the CAC energetically favourable or unfavourable?

Answer 28

Favourable -44.3kJ/mol

Question 29

What is the shared enzyme in the CAC?

Answer 29

SDH

Question 30

Where does the shared enzyme sit?

Answer 30

In the inner mitochondrial membrane

Question 31

What occurs with SDH?

Answer 31

Succinate is oxidised to fumarate using SDH. SDH reduces FAD to FADH2 which is restored to FAD in the electron transport chain