The citric acid cycle and pyruvate dehydrogenase

Question 1

What are the other names for the citric acid cycle?

Answer 1

Kreb’s cycle

Tricarboxylic acid cycle (TCA)

Question 2

How much NADH and FADH2 is generated in each turn of the citric acid cycle?

Answer 2

3 NADH

1 FADH2

Question 3

How much CO2 is produced in the citric acid cycle?

Answer 3

2 molecules per turn for each acetyl group

Question 4

Which enzyme catalyses the conversion of pyruvate to acetyl CoA?

Answer 4

Pyruvate dehydrogenase

Question 5

What does the pyruvate dehydrogenase complex consist of?

Answer 5

3 enzymes and 5 co-enzymes

Question 6

Why does pyruvate dehydrogenase made of multiple enzymes?

Answer 6

By co-localising these enzymes, side reactions are minimised and the overall rate is increased

Question 7

How does PDH convert pyruvate to acetyl CoA?

Answer 7

Each reaction has to be coupled to ensure the free energy released during the loss of CO2 is coupled to the subsequent generation of acetyl-CoA and NADH

To achieve this a flexible arm (lipoamide) tethers the acetyl group to transfer it between two active sites, as well as transferring reducing potential to a third site

Catalysis REQUIRES a chemical cofactor – THIAMINE diphosphate (from vitamin B1)

Question 8

What are the 3 stages of the reaction catalysed by the enzyme PDH?

Answer 8

Decarboxylation

Oxidation

Transfer to CoA

Question 9

Which parts of metabolism have mostly cytosolic enzymes?

Answer 9

Glycolysis, pentose phosphate pathway and fatty acid synthesis

Question 10

Which parts of metabolism have mostly mitochondrial enzymes?

Answer 10

Citric Acid Cycle, β-oxidation and the respiratory chain

Question 11

Which part of the mitochondria contains the enzymes of β-oxidation and most of the Citric Acid Cycle?

What is an important exception to this?

Answer 11

Matrix

Succinate dehydrogenase

Question 12

Why is it necessary to have an anaplerotic pathway alongside the citric acid cycle?

Give an example of an anaplerotic reaction

Answer 12

If the cycle is used to generate new compounds then carbon is lost from the cycle

Pyruvate + CO2 + ATP + H2O → oxaloacetate + ADP + Pi + 2H+

Question 13

How much energy is derived from 1 NADH and 1 FADH2 molecule?

Answer 13

NADH = 2.5 ATP

FADH2 = 1.5 ATP

Question 14

How much ATP is generated by oxidative glycolysis?

Answer 14

5 molecules per glucose

Question 15

Why do we not get 2.5 ATP per NADH generated by glyceraldehyde-3-phosphate?

Answer 15

We ‘lose energy’ getting NADH into the mitochondria (only get 1.5 ATP per NADH)

Question 16

How much ATP is generated by the citric acid cycle from 1 molecule of glucose?

Answer 16

25 (2 cycles per glucose)

Question 17

How much ATP is generated by oxidative glycolysis and the citric acid cycle together?

Answer 17

30

Question 18

Why is acetyl CoA seen as the point of no return for glucose derived carbon?

Answer 18

At this point the carbon source cannot be converted back into glucose for use in the brain.

This is controlled by pyruvate dehydrogenase

Question 19

What is the role of PDH kinase?

Answer 19

Phosphorylates the E1 of the complex and deactivates PDH

Question 20

What is the role of PDH phosphatase?

Answer 20

Dephosphorylates the complex and activates PDH

Question 21

What inhibits PDH kinase?

What is the effect of this?

Answer 21

Pyruvate

Ensures PDH is ‘on’ when pyruvate conc is high

Question 22

What activates PDH phosphatase?

What is the effect of this?

Answer 22

Ca2+ and insulin in adipocytes

Stimulates PDH during exercise (Ca2+) and feeding (for lipid synthesis)

Question 23

How do substrates and products of the citric acid cycle regulate PDH?

Answer 23

Regulated by the ratio of NADH/NAD+ and acetyl CoA/CoA

Turn ‘off’ PDH if lots of NADH and acetyl CoA

(via stimulation of the inhibitory kinase)

Question 24

What are the effects of a PDH phosphatase deficiency?

Answer 24

Pyruvate dehydrogenase is always phosphorylated
(Inactive)

Glucose is processed to lactic acid.
(Unremitting lactic acidosis)

Malfunctioning of many tissues
especially the central nervous system

Question 25

What allosterically inhibits citrate synthase?

Answer 25

ATP

Question 26

When is citric acid cycle regulation via citrate synthase important?

Answer 26

In periods of starvation to promote gluconeogenesis

Question 27

Why does inhibition of citrate synthase by ATP help to fuel the brain in periods of starvation?

Answer 27

Oxaloacetate is diverted to gluconeogenesis

Acetyl CoA is used to make ketone bodies

Question 28

What inhibits iso-citrate dehydrogenase?

Answer 28

High NADH/NAD+ ratio typical of the fed state

ATP

Question 29

What stimulates iso-citrate dehydrogenase?

Answer 29

ADP

Question 30

What inhibits α-ketoglutarate dehydrogenase?

Answer 30

Its products; succinyl CoA and NADH

Question 31

What stimulates α-ketoglutarate dehydrogenase?

Answer 31

Ca2+

Question 32

Which enzymes regulate metabolic control of the citric acid cycle?

Answer 32

1. Pyruvate dehydrogenase
2. Citrate synthase
3. Iso-citrate dehydrogenase
4. α-ketoglutarate dehydrogenase

Question 33

What is the purine nucleotide cycle?

Answer 33

The breakdown of ATP during strenuous exercise to generate fumarate (through several steps!) to prime the cycle with more oxaloacetate

Question 34

What are the symptoms of Patients with deficiencies in the purine nucleotide cycle?

Answer 34

Muscle cramps during exercise

Question 35

How can the rate of the citric acid cycle be measured?

Answer 35

Oxygen consumption

Labelling of the intermediates – ‘chase the label experiments’
-using radioactive carbon isotopes

Question 36

How is oxidative metabolism measured in the brain?

Answer 36

Magnetic resonance imaging (MRI) relies on imaging the brain by detecting hydrogen nuclei largely in water.

Paramagnetic substances will modify this signal.
Deoxyhaemoglobin is paramagnetic, while oxyhaemoglobin is diamagnetic.

We can use MRI as a tool for monitoring blood oxygenation through the paramagnetism of deoxyhaemoglobin.
Increased neuronal activity is indicated by increased blood flow.