Krebs Cycle

Question 1

What is the PDH Complex?

Answer 1

• Multi-subunit enzyme
• Takes pyruvate and converts it into acetyl Co-A (releases CO2)
• Allosterically regulated
• Many cofactors
• Can be considered the 1st control point of the citric acid cycle
• It catalyses a 3,4,5 reaction in which there are 3 subunits that catalyse a 4 step reaction using 5 cofactors
• Pyruvate dehydrogenase complex (E1, E2 and E3)
• Cofactors are TPP, lipoate, FAD, NAD+ and CoA-SH

Allosterically regulated by:
Negative -> ATP, acetyl-CoA and NADH
Positive -> ADP and pyruvate

Question 2

1st step

Answer 2

1. Conversion of acetyl-CoA to citrate
   - The enzyme is citrate synthase
   - acetyl-CoA reacts with oxaloacetate to produce citrate
   - Highly exergonic process seeing as there is hydrolysis of the thioester
   - H2O is a cofactor
   - CoA-SH is produced

Question 3

2nd step

Answer 3

2. Aconitase
   - Isomerisation of citrate to isocitrate
   - Produced in two steps:
   - In the first step water is removed and cis-Aconitate is formed
   - In the second step water is introduced and isocitrate is formed

Question 4

3rd step

Answer 4

3. Conversion of isocitrate into a-ketoglutarate
   - Isocitrate dehydrogenase
   - NAD+ to NADH (reduction)
   - CO2 is released
   - Reaction is oxidation

• Negative modulator is ATP and NADH
• Positive is ADP
• 2nd control point
• Produces 2.5 ATP

Question 5

4th step

Answer 5

4. a-ketaglutarate to succinyl-CoA
   - a-ketoglutarate dehydrogenase complex (similar to PDH complex)
   - NAD+ to NADH
   - CoA-SH is an input
   - CO2 is produced as a by-product

• Negative modulators are ATP, succinyl-CoA and NADH
• 3rd control point

Question 6

5th step

Answer 6

5. Succinyl CoA to succinate
   - Succinyl CoA synthetase

• GDP + Pi is converted to GTP
• CoA-SH is removed

Question 7

6th step

Answer 7

6. Succinate to fumarate
   - Succinate dehydrogenase

• FAD is reduced to FADH2
• 1.5 ATP

Question 8

7th step

Answer 8

7. Fumarate to L-malate
   - Fumarase

• Carbanion intermediate
• The enzyme is highly stereospecific and so it does not like the cis isomer or D-malate isomer

Question 9

8th step

Answer 9

8. Fumarate to oxaloacetate
   - Malate dehydrogenase

• NAD+ is reduced to NADH
• 2.5 ATP

Question 10

What are the free energy change patterns?

Answer 10

Reactions 2, 6 and 8 are positive in free energy change.