Lecture 10. The Citric Acid Cycle

Question 1

Why is pyruvate oxidised?

Answer 1

Further oxidation of pyruvate in citric acid cycle releases most of the energy out of the pyruvate molecule

Question 2

What was the interpretation of the sparker effect?

Answer 2

Organic acids act as a catalyst and increases the O₂ consumption

Question 3

How much pyruvate oxidation can be sparked by one molecule of oxaloacetate?

Answer 3

Infinite

Question 4

What is malonate?

Answer 4

A potent inhibitor of respiration in all animal tissues

Question 5

What type of inhibitor is malonate?

Answer 5

Competitive inhibitor - mimics succinate

Question 6

What does the citric acid cycle break down?

Answer 6

Carbohydrates, fats and amino acids

Question 7

Where does the citric acid cycle take place?

Answer 7

The mitochondrial matrix

Question 8

What is pyruvate from glycolysis converted to before being entered into the citric acid cycle?

Answer 8

Acetyl CoA

Question 9

What is used to convert pyruvate into acetyl CoA?

Answer 9

Coenzyme A - (catalysed by pyruvate dehydrogenase multi-enzyme complex)

Question 10

What makes up the pyruvate dehydrogenase complex?

Answer 10

Pyruvate dehydrogenase component (E1)
Dihydrolipoyl transacetylase (E2)
Dihydrolipoyl dehydrogenase (E3)

Question 11

Coenzyme A function

Answer 11

Serves as a carrier of activated acyl groups linked via a thioester bond

Question 12

What is coenzyme A a derivative of?

Answer 12

An adenine nucleotide

Question 13

What is flavin adenine dinucleotide (FAD)?

Answer 13

A coenzyme for redox reactions - it accepts two electrons and two protons

Question 14

What forms the swinging arm of E2?

Answer 14

Lipoamide formed by amide linkage of lipoic acid to a lysine residue of the enzyme

Question 15

Function of E1

Answer 15

Decarboxylates pyruvate
Lipoamide picks up acetyl group

Question 16

Function of E2

Answer 16

Transfers to CoA

Question 17

Function of E3

Answer 17

Regenerates lipoamide

Question 18

Structure of E1E2 complex

Answer 18

E2 forms the core
E1 is the outer layer

Question 19

What is the definition of a multi-enzyme complex?

Answer 19

Groups of two or more non-covalently associated enzymes catalyse two or more sequential steps in a metabolic pathway

Question 20

Advantages of multi-enzyme complexes

Answer 20

Product of the first reaction in the sequence remains attached
Serves directly as a substrate for the second reaction Therefore rate of second reaction not limited by diffusion
Can channel intermediates between successive enzymes, thereby minimising side reactions
The reactions may be coordinately regulated

Question 21

How many times does the citric acid cycle go round per glucose molecule

Answer 21

Twice

Question 22

When are NADH and FADH₂ re-oxidised?

Answer 22

During oxidative phosphorylation

Question 23

How is pyruvate DH switched off?

Answer 23

Phosphorylation - Kinase is stimulated by acetyl CoA, ATP and NADH

Question 24

How is pyruvate DH switched on?

Answer 24

Dephosphorylation - Kinase is inhibited by ADP and NAD⁺

Question 25

What are the principal negative regulators in the citric acid cycle?

Answer 25

ATP and NADH

Question 26

What is the main positive regulator in the citric acid cycle?

Answer 26

The need for energy and for carbon skeletons

Question 27

What will happen to the citric acid cycle is oxaloacetate is depleted?

Answer 27

The cycle will stop

Question 28

What are anaplerotic reactions?

Answer 28

Reactions that replenish cycle intermediates to prevent withdrawl

Question 29

What is the main anaplerotic reaction in humans?

Answer 29

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

Question 30

What catalyses the main anaplerotic reaction in humans?

Answer 30

Pyruvate carboxylase

Question 31

What amino acids can be used in anaplerotic reactions if needed?

Answer 31

Glucogenic amino acids

Question 32

What amino acids can’t be used in anaplerotic reactions?

Answer 32

Ketogenic amino acids