LECTURE 9 - PYRUVATE DEHYDROGENASE COMPLEX

Question 1

which reaction does pyruvate dehydrogenase catalyse?

Answer 1

pyruvate+CoA+NAD+ –> acetyl-CoA+CO2+NADH

Question 2

where does the PDC reaction occur?

Answer 2

in the mitochondrion

Question 3

how does pyruvate get into the mitochondrion?

Answer 3

through pyruvate translocase (H+ symport)

Question 4

what is the size of the PDC?

Answer 4

9.5MegaDaltons in eukaryotes

Question 5

what are the important enzymes in the PDC that we need to remember?

Answer 5

E1, E2, E3

Question 6

how many coenzymes are required for the PDC?

Answer 6

5

TPP
lipoic acid
CoA
FAD
NAD+

Question 7

what is the E1 reaction of pyruvate dehydrogenase?

Answer 7

1. pyruvate is decarboxylated by a nucleophilic attack by thiamine pyrophosphate
2. irreversible step, now you are committed, because CO2 just diffuses out
3. the remaining carbons are attached to TPP to make hydroxyethyl TPP
4. TPP is always bound to E1, because it is needed

Question 8

what is the E2 reaction of the PDC?

Answer 8

1. E2 removes the two carbons (acetyl) from TPP and puts them onto lipoamide to make acetyl dihydrolipoamide, this creates a high energy ester bond
2. lipoamide (always bound to E2) had oxidized disulfide bonds, now those sulfures are reduced, and TPP is regenerated
3. CoA is added to the acetyl groups to make acetylCoA (transesterification)

Question 9

what is the function of CoA?

Answer 9

functions as a carrier of acetyl and other acyl groups

Question 10

what are some characteristics of acetyl CoA?

Answer 10

acetyl thioester bond
high energy compound because of the thioester bond
has a high acyl group transfer potential and an donate the acetyl group to several acceptors
the entry point of the citric acid cycle

Question 11

what are the E3 reactions?

Answer 11

1. the aim is to reset E2 and E3 to their active state
2. FAD (which also has disulfide bonds) oxidises dihydrolipoamide to restore lipoamide
3. FAD is now reduced
4. NAD+ picks up those two hydrogen protons to become NADH and restore FAD
5. the enzyme can now do another round of reactions

Question 12

what is the overall reaction pathway?

Answer 12

Question 13

what are the advantages of multi enzyme complexes?

Answer 13

minimised distance between active sites: intermediates jump from one to the other
increased reaction rate without having to have a lot of intermediates
side reactions are minimized and protection for chemically labile intermediate
coordinated control of reactions: shutting down one enzyme shuts off the whole system

Question 14

why is the regulation of the PDC critical?

Answer 14

mammals do not have any other pathway to make acetyl CoA from pyruvate
animals cannot synthesize glucose from acetyl CoA, so this commits you to the CAC or fatty acid synthesis

Question 15

what are the two levels of control of the PDC?

Answer 15

product inhibition by acetyl CoA and NADH
covalent modification (phosphorylation of E1)

Question 16

how is the PDC regulated by product inhibition?

Answer 16

when acetyl CoA and/or NADH are high, E2 and E3 are not active
CoA does not become AcetylCoA and the disulfide bonds cannot be regenerated
this also shuts down E1, and there is an accumulation of hydroxyethyl TPP
prevents the useless consumption of pyruvate

Question 17

how is the PDC regulated by phosphorylation?

Answer 17

inactivation:
1. pyruvate dehydrogenase kinase (PDK) is allosterically activated by acetyl CoA and NADH
2. PDK phosphorylates E1
3. E1 is shut down, which shuts down the entire PDC
4. PDK is inhibited by pyruvate, ADP and Ca2+ (reverses the inhibition)

reactivation:
1. pyruvate dehydrogenase phosphatase (PDP) is activated by Ca2+ (for when you exercise)
2. PDP dephosphorylates E1
3. E1 is activates, the PDC can now proceed with its reactions

Question 18

what controls the utilization of pyruvate?

Answer 18

PDC and lactate dehydrogenase

Question 19

what happens in cancer cells

Answer 19

metabolites of CAC can be transported out of the cell for biosynthesis
cancer cells make lactate even if there is oxygen, PDC is repressed by PDK (Warburg effect)