Citric acid cycle Flashcards
What reaction si responsible for transition from pyruvate to Acetyl-CoA?
Pyruvate = product of glycolysis, enters the mitochondria
Acetyl-CoA = enters CAC
Happens in the mitochondria:
Pyruvate + CoA + NAD+ → (PDC) → Acetyl-CoA + CO2 + NADH + H+
PDC = pyruvate dehydrogenase complex
How does pyruvate enter the mitochondrion?
Pyruvate gets into the mitochondrion through the pyruvate translocase (H+ symport)
Symport of H+ is required for maintenance of the electrochemical gradient (pyruvate is negative)
*Also ATP/ADP antiport in mitochondrial membrane
What is the structure/composition of the Pyruvate Dehydrogenase Complex (PDC)?
PDC = Massive mutli-enzyme complex
Size = 9.5 MegaDaltons in eukaryotes (substrate = pyruvate = 88 Daltons)
Composition: (*remember)
- *E1 pyruvate dehydrogenase → 30 heterotetramers/complex
- E2 dihydrolipoyl transacetylase → 60 core monomers/complex
- E3 dihydrolipoyl dehydrogenase → 12 homodimers
- E3-binding protein
- Pyruvate dehydrogenase kinase → 1-3/complex
- Pyruvate dehydrogenase phosphatase → 1-3/complex
- *5 different coenzymes required for catalytic activities
What occurs in the 1st step of Pyruvate Dehydrogenase Reaction?
Mediated by E1 (pyruvate dehydrogenase), decraboxylation step:
Pyruvate + TPP → Hydroxyethyl-TPP + CO2 (from pyruvate, then diffused out of the mitochondria)
*Pyruvate put onto TPP as a committment
Step 1 is irreversible → whole Pyruvate dehydrogenase reaction irreversible (all other steps are reversible)
How does step 5 of the pyruvate dehydrogenase reaction occur?
E3 mediated oxidation of FAD by NADH:
FADH2+ + NAD → FAD + NADH + H+
*In FADH2, H+ are on the cysteines → FAD+ has disulfide bond
Which step of the pyruvate dehydrogenase reaction is responsible for synthesis of the final acetyl-CoA product?
3rd step (out of 5), E2 generates Acetyl-CoA:
Acetyl-dihydrolipoamide + CoA → Acetyl-CoA + dihydrolipoamide
*Acetyl-CoA = entry point of the CAC
What are the mechanistic advantages of multi-enzyme complexes?
- Minimized distances for substrates in between active sites → increases reaction rate without having to maintain large pool of intermediates
- Metabolic intermediates are channeled between successive enzyme sites → minimization of side reaction + protection for chemically labile intermediate
- Coordinated controls of reactions → shutting off one enzyme effectively shuts the system down
How is the Pyruvate Dehydrogenase reaction regulated?
*ALL E1
By allosteric product inhibition
Accumulation of NADH and Acetyl-CoA shut down E1 through product inhibition → prevents useless consumption of pyruvate
By phsphorylation of E1 in the PDC
How does PDC regulation by phosphorylation occur?
Inhibition:
E1-CH2OH + ATP → (PDH kinase) → E1-PO4(2-) + ADP
- Activated by ATP, NADH, Acetyl-CoA (activation of the inhibition)
- Inhibited by pyruvate, ADP
Activation:
E1-PO4(2-) + H2O → (PDH phosphatase) → E1-OH + Pi
- Activated by presence of Mg2+, Ca2+, insulin
*By phosphorylation of 3 specific serine residues
Does the CAC consume O2?
No, it is part of the aerobic metabolism, but does not direclty consume O2
The CAC can run without O2, but the cell would run out of NAD and FAD+
Is the CAC present in prokaryotes and eukaryotes?
No, only in aerobic metabolism of eukaryotes
When and by who was discovered the CAC?
1937
- Albert Szent-Györgyi → respiration
- Hans Krebs → CAC
Summarize the CAC.
The CAC is a series of 8 enzymatic reactions that combine acetyl CoA (2 C) with oxaloacetate (4 C) to generate CO2, NADH, FADH2 and regenerates the starting product oxaloacetate.
What does it mean for the CAC to be amphibolic?
It is a site of anabolism and catabolism
What part of the CAC corresponds to anabolism and catabolism?
Anabolism → CAC intermediates are the starting point of anabolic pathways (ex: gluconeogenesis, fatty acid synthesis, amino acid synthesis)
Catabolism → CAC intermediates are the end point of catabolic pathways
- Aerobic catabolism of carbs, lipids and aa merge into the CAC (ex: oxaloacetate ↔ AA)
*Catabolic = casse
What is a Cataplerotic reaction vs an anaplerotic reaction?
Cataplerotic (cata = emtpying) → depletes the CAC intermediates → decreases cycle chain
Anaplerotic (ana = filing up) → replenish the depleted CAC intemediates
*CAC intermdediates are simple compounds so Cataplerotic reactions are part of anabolism of the cells and Anaplerotic reactions are part of catabolism of the cell