Formation of Acetyl Co-A

Question 1

What does pyruvate form if there are anaerobic conditions

Answer 1

1. Lactate- CH3CH(OH)COOH
2. Doesn’t happen in the heart- cramp in heart would cause death
3. NADH + H+ –> NAD+ which can be reused in glycolysis

Question 2

How does yeast regenerate NAD+ without oxygen

Answer 2

1. Pyruvate is decarboxylated to CH3CHO by pyruvate decarboxylase
2. Then NADH + H+ + CH3CHO–> NAD+ + CH3CH2OH (Ethanol)

Question 3

How is NADH reoxidised under aerobic conditions

Answer 3

1. By the mitochondrial electron-transport chain

Question 4

How is acetyl-CoA formed

Answer 4

1. From pyruvate through oxidative decarboxylation

2. By a multienzyme complex- pyruvate dehydrogenase

Question 5

What is the structure of pyruvate dehydrogenase complex

Answer 5

1. Contains 3 enzymes noncovalently bonded
2. E1= Pyruvate decarboxylase
3. E2= Lipoyl-acetyl transferase
4. E3= Dihydrolipoamide dehydrogenase

Question 6

What does isolated E.coli E2 look like

Answer 6

1. Forms a particle with 24 identical sub-units in a cubic symmetry
2. Eukaryotes- 60 copies of E2 - dodecahedron

Question 7

What does the pyruvate dehydrogenase complex look like

Answer 7

1. E1 sub-units form dimers that associate with the E2 cube at the centres of the E2 cube’s 12 edges
2. E3 subunits form dimers that are located at the centres of the cube’s 12 faces
3. Eukaryote- 60:60:12

Question 8

What are the advantages of multi-enzyme complexes

Answer 8

If a series of reactions occurs within a multienzyme complex, the distance that substrates must diffuse between active sites is minimised- rate enhancement

2. Complex formation provides the means for channeling metabolic intermediates between successive enzymes- minimise side reactions
3. Reactions may be coordinately controlled

Question 9

What is the overall equation for the formation of acetyl Co-A

Answer 9

1. Pyruvate + CoA + NAD+ –> acetyl-CoA + CO2 + NADH

Question 10

What are the coenzymes and prosthetic groups required for the formation of acetyl Co-A

Answer 10

1. Thiamine pyrophosphate
2. Flavin adenine dinucleotide (FAD)
3. Nicotinamide adenine dinucleotide (NAD+)
4. Lipoamide

Question 11

What does lipoamide consist of

Answer 11

1. Lysine joined to lipoic acid on the NH3+

Question 12

What is the job of Lipoamide

Answer 12

1. reduction of its cyclic disulfide to dithiol- dihydrolipoamide and its reoxidation

Question 13

What is the first step in the formation of acetyl Co-A

Answer 13

1. Pyruvate decarboxylase decarboxylates pyruvate
2. This forms the intermediate of hydroxyethyl-TPP
3. CH3CH(OH)TPP
4. The hydroxyethyl-TPP is transferred to the next enzyme E2

Question 14

What is the second step in the formation of acetyl Co-A

Answer 14

1. Lipoyl-acetyl-transferase (lipoamide sulfide) is attacked by the Carbanion
2. TPP is eliminated to form acetyl-dihydrolipoamide and regenerate E1
3. The hydroxyethyl carbanion is oxidised on the acetyl group by reduction of lipoamide disulfide bond

Question 15

What is the third step in the formation of acetyl Co-A

Answer 15

1. E2 then catalyses the transfer of the acetyl group to CoA producing acetyl CoA
2. Transesterification

Question 16

What is the fourth step in the formation of acetyl Co-A

Answer 16

1. Dihydrolipoamide dehydrogenase reoxidises E2 completing the catalytic cycle of E2- joins sulfide bond
2. Oxidised E3 contains a reactive disulfide group and tightly bound FAD
3. The lipoamide interchange reaction- lipoamide disulfide bond forms (E2) with the reduction of the two sulfhydryl groups in E3

Question 17

What is the 5th step in the formation of acetyl Co-A

Answer 17

1. Reduced E3 is oxidised by NAD+
2. The sulfhydryl groups are reoxidised by the enzyme bound FAD which is reduced to FADH2
3. FADH2 is then reoxidised to FAD by NAD+ producing NADH

Question 18

What transfers intermediates between enzyme subunits

Answer 18

1. The lipoyllysyl swingin arm- 14A

2. The group between the lipoamide disulfide bond and the E2 polypeptide backbone

Question 19

What are the two regulatory systems used to regulate the entrance of acetyl units into citric acid cycle

Answer 19

1. Product inhibition by NADH and acetyl CoA

2. Covalent modification by phosphorylation/dephosphorylation of the pyruvate decarboxylase E1 subunit

Question 20

Why are regulatory systems needed

Answer 20

1. THe decarboxylation of pyruvate by E1 is irreversible

2. No other pathways for the synthesis of acetyl-CoA from pyruvate

Question 21

Describe how pyruvate dehydrogenase is regulated by product inhibition

Answer 21

1. NADH and acetyl-CoA compete with NAD+ and CoA for binding sites on their respective enzymes
2. They also drive the (E2) and E3 reactions backwards
3. High rations of NADH/NAD+ and acetylCoA/CoA maintain E2 in the acetylated form- can’t accept hydroxyethyl group from TPP

Question 22

Describe how pyruvate dehydrogenase is regulated by phosphorylation/dephosphorylation

Answer 22

1. only in eukaryotic complexes
2. These complexes contain pyruvate dehydrogenase kinase and pyruvate dehydrogenase phosphatase bound to the Lipoyl-acetyl transferase core
3. The kinase inactivates the pyruvate decarboxylase subunit by catalysing the phosphorylation of serine residue by ATP
4. Hydrolysis of this phospho-ser residue by phosphatase reactivates the complex.
5. NADH and acetyl-CoA activate pyruvate dehydrogenase kinase- the phosphorylation then inactivates the complex

Question 23

What is insulin effect on pyruvate dehydrogenase

Answer 23

1. Indirect activation of pyruvate dehydrogenase phosphatase