Krebs Cycle

Question 1

Where does the Krebs cycle occur?

Answer 1

Mitochondrial matrix

Question 2

Give some examples of the cofactors involved

Answer 2

• CoA
• NAD+
• FAD+
• Thiamine pyrophosphate
• Lipoic acid

Question 3

What is the net reaction for the conversion of pyruvate to acetyl CoA?

Answer 3

Pyruvate + NAD+ + CoA —-> CO2 + NADH + H+ + acetyl CoA

Question 4

What does the pyruvate dehydrogenase complex consist of?

Answer 4

20-30 copies of pyruvate dehydrogenase
>60 opies of other enzymes
Cofactors

Question 5

How many copies of pyruvate dehydrogenase are in the complex?

Answer 5

20-30

Question 6

How many copies of other enzymes are in the complex?

Answer 6

More than 60

Question 7

Reaction 1: what is it?

What is required?

Answer 7

Acetyl CoA —–> Citrate

Requires citrate synthase

Question 8

Reaction 2: what is it?

What is required?

Answer 8

Citrate —–> Isocitrate

Requires aconitase

Question 9

Reaction 3: what is it?

What is required?

Answer 9

Isocitrate —-> α-ketoglutarate
Requires isocitrate dehydrogenase
CO2 and NADH produced

Question 10

Reaction 4: what is it?

What is required?

Answer 10

α-ketoglutarate —> succinyl-CoA
Requires α-ketoglutarate dehydrogenase
CO2 and NADH produced

Question 11

Reaction 5: what is it?

What is required?

Answer 11

Succinyl-CoA —-> Succinate
Requires succinate thiokinase
GTP produced

Question 12

Reaction 6: what is it?

What is required?

Answer 12

Succinate —-> Fumarate
Requires succinate dehydrogenase
FADH2 produced

Question 13

Reaction 7: what is it?

What is required?

Answer 13

Fumarate —-> L-malate

Requires fumerase and H2O

Question 14

Reaction 8: what is it?

What is required?

Answer 14

L-malate —> Oxaloacetate
Requires malate dehydrogenase
NADH produced

Question 15

What is the net yield of Krebs cycle in one turn?

Answer 15

2xCO2, 3xNADH, GTP and FADH2

Question 16

Per glucose molecule, what is the net yield of the Krebs cycle?

Answer 16

4xCO2, 6xNADH, 2xGTP and 2xFADH2

Question 17

Per glucose molecule, what is the net yield of the pyruvate dehydrogenase complex?

Answer 17

2xAcetyl CoA, 2xCO2 and 2xNADH

Question 18

How is the PDH complex regulated?

Answer 18

When the PDH complex is phosphorylated it is inactive, when dephosphorylated it is active
So PDH kinase deactivates it and PDH dephosphatase activates it

Question 19

Which 2 enzymes regulate the PDH complex?

Answer 19

PDH kinase inhibits it

PDH phosphatase activates it

Question 20

Other than enzyme activity, how is the PDH complex regulated?

Answer 20

If there’s a high [substrates] present the it is stimulated, if high [products] then inhibited

Question 21

At what 3 points is the Krebs cycle regulated? What is the regulation linked to?

Answer 21

• Citrate synthase
• Isocitrate dehydrogenase
• α-ketoglutarate dehydrogenase
  The levels of substrates and products

Question 22

Products from where, also feed into the Krebs cycle?

Answer 22

Products from the breakdown of amino acids and fatty acids

Question 23

Give an example of a poison that affects the Krebs cycle?

Answer 23

Sodium fluroacetate

Question 24

How does sodium fluoroacetate result in poisoning?

Answer 24

Inhibits aconitatase citrate—/-> isocitrate so citric acid accumulates

Question 25

Give an example of a disease associated with abnormal Krebs cycle function

Answer 25

Beri beri, vitamin B1 (thiamine) deficiency

Question 26

How does beri beri affect the Krebs cycle;e?

Answer 26

Thiamine pyrophosphate is needed for the PDH complex and α-ketoglutarate dehydrogenase so deficiency leads to high levels of pyruvate and α-ketoglutarate in blood

Question 27

What is thiamine needed for?

Answer 27

PDH complex and α-ketogluatarate dehydrogenase

Question 28

Symptoms of beri beri

Answer 28

Damage to peripheral nervous system and enlargement of the heart