TCA cycle Flashcards
What are the steps of Aerobic oxidation ?
- Glycolysis
- PDC reaction
- TCA/Krebs Cycle
- ETC
Fully explain Pyruvate Dehydrogenase Complexes (PDC) ?
- Catalyses the committed step of the conversion of pyruvate to acetyl-CoA.
- Regulates this irreversible step in the TCA and control is by:
a) product inhibition [allosteric control]
b) a reversible phosphorylation [covalent modification]. - The enzyme activity is inhibited by the products, Acetyl-CoA and NADH.
- A multi subunit enzyme complex E1 , E2 , and E3.
- The PDC comprises 5 coenzymes: FAD / CoA-SH / NAD+ / TPP / Lipoate
Enzymatic events catalysed by PDC ?
- E1 has the substrate binding site for pyruvate. Pyruvate undergoes decarboxylation to its hydroxyethyl derivative on the enzyme E1 bound TPP (thiamine pyrophosphate) coenzyme. This reaction gives CO2.
- Oxidation of the hydroxyethyl group from TPP to the acetate group takes place on the E2 enzyme. This acetyl group is first transferred to one of the lipoyl-SH group
- From the -SH group, transesterification of CoA to form acetyl CoA and fully reduced form of lipoyl group
- Transfer of two hydrogen atoms from the reduced lipoyl groups of E2 to the FAD of E3 by dihydrolipoyl dehydrogenase. This results in regeneration of oxidised form of lipoyl-lysyl group of E2
- Transfer of hydride ion (proton) from FADH2 to NAD+ forming NADH2.
What inhibits PDC activity?
The binding of end products, NADH and Acetyl CoA
How does PDC become inactive ?
If there is enough end products
What does the binding of substrates, pyruvate and ADP to the PDC promote?
It promotes the enzyme catalytic activity of PDC
How does PDC become active ?
If there are more substrates available
A serine residue on the pyruvate dehydrogenase enzyme complex is phosphorylated by PDC kinase (PDK). This phosphorylation of serine leads to ?
An inactivation of PDC
PDC has the phosphate group removed by ?
PDC phosphatase. PDC is activated reversibly by this PDC phosphatase.
What are the 8 steps of the TCA cycle ?
- OAA (4C) condenses with acetyl CoA (2C) => Citrate (6C) [via citrate synthase]
- Citrate isomerises ==> isocitrate (6C) [via aconitase]
- Isocitrate oxidative decarboxylation => α-ketoglutarate (5C) [via isocitrate dehydrogenase]
- α - ketoglutarate oxidative decarboxylation ==> succinyl CoA (4C)
- Succinyl CoA ==> succinate (4C) (+ GTP) [via succinate thiokinase]
- Succinate ==> Fumarate(4C) (+ FADH2 [via succinate dehydrogenase]
- Fumarate isomerises by hydration ==> malate (4C) [via fumarase]
- Malate ==> OAA [via malate dehydrogenase]
Now describe the enzyme reactions of the TCA cycle step by step ?
- The TCA cycle starts when an acetyl-CoA condenses with an oxaloacetate catalysed by citrate synthase enzyme to give citrate.
- Citrate is isomerized by aconitase enzyme to give isocitrate.
- Isocitrate is decarboxylated by isocitrate dehydrogenase enzyme to give α-ketoglutarate. This reaction also gives CO2 and NADH.
- α-ketoglutarate is decarboxylated by α-ketoglutarate dehydrogenase complex enzyme to give succinyl Co-A. This reaction also gives CO2 and NADH
Now describe the enzyme reactions of the TCA cycle 2 step by step ?
- Coenzyme A of succinyl-CoA is removed by succinyl CoA synthase (succinate thiokinase) enzyme to give succinate. This reaction also generate a GTP.
- Succinate is oxidized by succinate dehydrogenase enzyme to give fumarate. The hydrogen acceptor in this reaction is FAD+ that is reduced to FADH2.
- Fumarate is then reduced by hydration catalyzed by fumarase enzyme to form malate. of H + in the inter membrane space of the mitochondria.
- Finally, malate is oxidized by malate dehydrogenase enzyme to form oxaloacetate. NAD+ is reduced in this reaction to give NADH.
Explain the process of the electron transport chain ?
- Complex I (NADH-Q oxidoreductase) first transfers electrons (e-) from NADH to ubiquinone (Q)
- Complex II (Succinate-Q reductase) transfers electrons (e-) from FADH2 to ubiquinone (Q)
- Electrons (e-) are then passed down to complex III (Q-cytochrome c oxidoreductase complex).
- Cytochrome c receives and transfers electrons from complex III to complex IV (cytochrome c oxidase).
- Complex IV(Cytochrome c oxidase) uses molecular oxygen as an oxidising agent and reduces it by donating electrons. This results in reduction of molecular oxygen to water (H2O).
What is the purpose of the electron transport chain (ETC) is to ?
Transport protons (H+) from the matrix to the intermembrane space across the inner membrane of mitochondria via complexes I, III, and IV.
What are the three irreversible reactions in the TCA cycle ?
- Citrate synthase
- Isocitrate dehydrogenase
- α - ketoglutarate dehydrogenase
