Lecture 2 - Synthesis of Acetly CoA

Question 1

What are the potential fates of pyruvate?

Answer 1

Pyruvate is at a central point in metabolism
1. Transamination - Ammino acid synthesis (Alanine)
2. Carboxylation into oxaloacetate necessary in gluconeogenesis
3. Reduction into lactate when deficient in oxygen
4. Oxidative decarboxylation into Acetyl CoA
Some microorganisms ferment glucose into ethanol with pyruvate as an intermediate

Question 2

Give an overview of the production of acetyl CoA.

Answer 2

Pyruvate is oxidised to acetyl CoA and CO2 through an oxidative decarboxylation reaction. The reaction occurs in the mitochondria and is an irreversible reaction.

Question 3

Describe the structure of pyruvate dehydrogenase.

Answer 3

Pyruvate dehydrogenase is aa large complex containing 3 enzyme protein subunits termed E1, E2 and E3. The reaction requires 5 different cofactors.

Question 4

What are the associated co-factors of each protein sub-unit?

Answer 4

E1 - Thiamine pyrophosphate
E2 - Co-enzyme A (Free), Lipoamide (Bound)
E3 - FAD (Bound), NAD+ (Free)

Thiamine pyrophosphate has a Carbon between N and S which is very reactive therefore its H+ readily dissociates to leave a negative C ion that can react with the carbonyl group of pyruvate.

Lipoic acid has two sulfur atoms which can be reduced it is also attached to a lysine molecule in the protein which can be extended and moved

Question 5

Describe Step 1 of the Production of acetyl CoA.

Answer 5

Step 1 - Decarboxylation of Pyruvate

TPP Spontaneously forms a carbanion which is then added to the carbonyl group of pyruvate to form Hydroxyethyl TTP

TPP –> Carbanion of TPP + Pyruvate –> Hydroxyethyl-TPP

Question 6

Describe Step 2 of the Production of acetyl CoA.

Answer 6

Step 2 - Transfer of hydroxyethyl group to lipoamide

The hydroxyethyl group is oxidised to form an acetyl group and The carbanion of TPP is reformed to be reused

Hydroxyethyl-TPP + Lipoamide –> Carbanion of TPP + Acetyl lipoamide

Question 7

Describe Step 3 of the Production of acetyl CoA.

Answer 7

Step 3 - Formation of Acetyl CoA

The acetyl group is transferred to CoA to form Acetyl CoA

Coenzyme A + Acetyl lipoamide –> Acetyl CoA + Dihydrolipoamide

Question 8

Describe Step 4 of the Production of acetyl CoA.

Answer 8

Step 4 - Reduction of FAD

The Lipoamide is regenerated and the electrons are transferred to an electron carrier

Dihydrolipoamide + FAD –> Lipoamide + FADH2 —> FAD + NADH + H+

Question 9

How does the structure of the complex maximise efficiency?

Answer 9

The structure of the complex allows coordinated catalysis, reduces side reactions and maximises efficiency. This is because substrates don’t have to diffuse due to the presence of complex channels and metabolic intermediates. The coordinated control also ensures the reactions take place in the correct order.
The flexible arm of the lipoamide of E2 which cam can call on each active site in turn.

Question 10

How is pyruvate dehydrogenase regulated?

Answer 10

High concentrations of reaction products inhibit PDH activity
Conversely NAD, ADP and AMP activate the PDH complex
PDH kinase is where most regulation occurs through phosphorylation