LECTURE 9 - PYRUVATE DEHYDROGENASE COMPLEX Flashcards
which reaction does pyruvate dehydrogenase catalyse?
pyruvate+CoA+NAD+ –> acetyl-CoA+CO2+NADH
where does the PDC reaction occur?
in the mitochondrion
how does pyruvate get into the mitochondrion?
through pyruvate translocase (H+ symport)
what is the size of the PDC?
9.5MegaDaltons in eukaryotes
what are the important enzymes in the PDC that we need to remember?
E1, E2, E3
how many coenzymes are required for the PDC?
5
TPP
lipoic acid
CoA
FAD
NAD+
what is the E1 reaction of pyruvate dehydrogenase?
- pyruvate is decarboxylated by a nucleophilic attack by thiamine pyrophosphate
- irreversible step, now you are committed, because CO2 just diffuses out
- the remaining carbons are attached to TPP to make hydroxyethyl TPP
- TPP is always bound to E1, because it is needed
what is the E2 reaction of the PDC?
- E2 removes the two carbons (acetyl) from TPP and puts them onto lipoamide to make acetyl dihydrolipoamide, this creates a high energy ester bond
- lipoamide (always bound to E2) had oxidized disulfide bonds, now those sulfures are reduced, and TPP is regenerated
- CoA is added to the acetyl groups to make acetylCoA (transesterification)
what is the function of CoA?
functions as a carrier of acetyl and other acyl groups
what are some characteristics of acetyl CoA?
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
what are the E3 reactions?
- the aim is to reset E2 and E3 to their active state
- FAD (which also has disulfide bonds) oxidises dihydrolipoamide to restore lipoamide
- FAD is now reduced
- NAD+ picks up those two hydrogen protons to become NADH and restore FAD
- the enzyme can now do another round of reactions
what is the overall reaction pathway?
what are the advantages of multi enzyme complexes?
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
why is the regulation of the PDC critical?
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
what are the two levels of control of the PDC?
product inhibition by acetyl CoA and NADH
covalent modification (phosphorylation of E1)
how is the PDC regulated by product inhibition?
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
how is the PDC regulated by phosphorylation?
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
what controls the utilization of pyruvate?
PDC and lactate dehydrogenase
what happens in cancer cells
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)
