CAC - Pyruvate Dehydrogenase

Question 1

What reaction does pyruvate dehydrogenase speed up?

Answer 1

the oxidative decarboxylation of pyruvate –> acetyl CoA + CO2
IRREVERSIBLE

Question 2

Which 5 routes can occur from pyruvate?

Answer 2

1. Reduction: pyruvate –> lactate
2. Oxidative decarboxylation: pyruvate –> acetyl CoA
3. Carboxylation: pyruvate –> oxaloacetate
4. Transamination: Pyruvate –> alanine
5. micro-organism: glucose –> pyruvate –> ethanol

Question 3

How many different enzyme proteins does pyruvate dehydrogenase have? How many cofactors does it have?

Answer 3

3 (E1, E2 & E3)

5 cofactors

Question 4

Describe the cofactors for E1.

Answer 4

Thiamine pyrophosphate
Association type: BOUND
Vit. source: Thiamine (Vit. B1)

Question 5

Describe the cofactors for E2.

Answer 5

Coenzyme A
Association type: FREE
Vit. source: Pantothenic acid (Vit. B5)

Lipoamide
Association type: BOUND
Vit. source: None - able to synthese

Question 6

Describe the cofactors for E3.

Answer 6

FAD
Association type: BOUND
Vit. source: Riboflavin (Vit. B2)

NAD+
Association type: FREE
Vit. source: Niacin (Vit. B3)

Question 7

Lipoamide

Answer 7

a lipoid acid attached to a long extendable lysine residue arm

Question 8

How many subunits do each of PDH’s enzyme proteins have & what happens at each protein?

Answer 8

E1 = 12 & e- transferred here
E2 = 8 & contains a strong oxidising agent 
E3 = 24 & pyruvate interacts here

Question 9

Which of PDH’s enzyme proteins has a slightly different structure?

Answer 9

E2
~ its a trimer
~ transacetylase core (contains lipoamide)

Question 10

Describe the decarboxylation of pyruvate & where it occurs.

Answer 10

E1

1. spontaneous formation of TPP carbanion
2. TPP carbanion + pyruvate + H+ –> Hydroxyethy-TPP + CO2

Question 11

Describe the transacetylation & where it occurs.

Answer 11

E1
Hydroxyehtyl-TPP + lipoamide –> TPP carbanion + acetyllipomide
~ lipoamide is from E2

Question 12

Describe the formation of acetyl CoA & where it occurs.

Answer 12

E2

Coenzyme A + acetyllipoamide –> Acetyle CoA + Dihydrolipoamide

Question 13

Describe the reduction of FAD & where it occurs.

Answer 13

E3
1. Dihydrolipoamide + FAD –> Lipoamide + FADH2
2. FADH2 + NAD+ –> FAD + NADH + H+
(step 2 is an ‘uphill transference’ - occurs due to local environment chaining FAD’s redox pot = more +ve)

Question 14

What is the significance of this complex structure?

Answer 14

~ enables co-ordinated catalysis of a complex possible
~ reduces size reactions & maximised efficiency
~ flexible arm allows movement of tightly-bound intermediates

Question 15

Why are multienzyme complexes better?

Answer 15

~ limited by collision frequency –> distance between active sites is minimised
~ ‘channel’ intermediates between successive enzymes –> fewer side reactions
~ coordinately controlled e.g. happens in correct order

Question 16

How do the products/reactants affect PDH activity?

Answer 16

~ high product conc.s (NADH, acetyl CoA & ATP) = inhibit activity
~ high reactant conc.s (NAD, ADP & AMP) = activate PDH
~ opposite for PDH kinase

Question 17

Which enzymes regulate PDH?

Answer 17

~ PDH KINASE - 3 serine residues on E1 subunit that can be phosphorylated = deactivation of PDH
~ PDH PHOSPHATASE - stimulated by Ca2+ & activates PDH (important in muscles)

Question 18

Describe Beriberi.

Answer 18

~ ‘lameness’
~ thiamine deficiency/alcoholics (unable to absorb thiamine)
~ caused high lvls of pyruvate + lactic acid in blood
~ neurological disorder - glucose only fuel for brain

Question 19

Mercury & arsenite poisoning

Answer 19

Both inhibit pyruvate dehydrogenase by chelating to dihydrolipoamide

Question 20

Describe BAL.

Answer 20

British anti-lewisite

~ antidote to arsenic-based poisons as it binds/removes arsenic from enzyme