The TCA Cycle

Question 1

How many reactions in total?

Answer 1

8

Question 2

Steps of the cycle

Answer 2

1. Pyruvate is transported into the mitochondrial matrix.
2. Pyruvate is oxidised to acetate, forming NADH and releasing CO2.
3. Acetate is combined with CoA yielding acetyl CoA.
4. The two-carbon acetyl and four-carbon oxaloacetate combine, forming six-carbon citrate.
5. Citrate is rearranged to form its isomer, isocitrate.
6. Isocitrate is oxidised to a-ketoglutarate, yielding NADH and CO2.
7. a-ketoglutarate is oxidised to acetyl CoA with the formation of NADH and CO2: this step is almost identical to pyruvate oxidation.
8. Succinyl CoA releases CoA, becomming succinate; the energy, thus released converts GDP to GTP, whhich in turn converts ADP to ATP.
9. Succinate is oxidised to fumarate with the formation of FADH2.
10. Fumarate and water react, forming malate.
11. Malate is oxidised to oxaloacetate with the formation of NADH. Oxaloacetate can now react with acetyl CoA to reenter the cycle.

Question 3

The enzymes

Answer 3

• acetyl CoA
• citrate (synthase)
• isocitrate (dehydrogenase)
• a-ketoglutarate (dehydrogenase)
• succinyl CoA
• succinate (dehydrogenase)
• fumarate (hydratase)
• malate (dehydrogenase)
• oxaloacetate

Question 4

All enzymes except which is located in the mitochondrial matrix?

Answer 4

• All enzymes of the TCA cycle are located in the mitochondrial matrix, apart from:
• Succinate dehydrogenase (number 6)…
  succinate + FAD → fumarate + FADH2
  …which is integrated in the inner mitochondrial membrane
• part of complex II of the electron transport chain
  most of these reactions are reversible

Question 5

Control of the TCA cycle

Answer 5

• High ATP, NADH and acetyl-CoA means plenty of energy
• High ADP and NAD+ means lack of energy
• High succinyl-CoA and acetyl-CoA means plenty of precursor molecules for biosynthetic reactions
• Use feedback regulations
• A lot of precursors to do anabolic reactions with
• A lot of citrate = a lot of energy à converted into fatty acids as they can be stored and used later
• a lot of NAD promoting isocitrate into ketoglutarate

Question 6

Products of the TCA cycle

Answer 6

• 3 x NADH + H+
• 1 x FADH2
• 1 x GTP (GTP + ADP → GDP + ATP)
• 2 x CO2