Krebs Cycle Flashcards
1
Q
What is the PDH Complex?
A
- 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
2
Q
1st step
A
- 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
3
Q
2nd step
A
- 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
4
Q
3rd step
A
- 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
5
Q
4th step
A
- 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
6
Q
5th step
A
- Succinyl CoA to succinate
- Succinyl CoA synthetase
- GDP + Pi is converted to GTP
- CoA-SH is removed
7
Q
6th step
A
- Succinate to fumarate
- Succinate dehydrogenase
- FAD is reduced to FADH2
- 1.5 ATP
8
Q
7th step
A
- 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
9
Q
8th step
A
- Fumarate to oxaloacetate
- Malate dehydrogenase
- NAD+ is reduced to NADH
- 2.5 ATP
10
Q
What are the free energy change patterns?
A
Reactions 2, 6 and 8 are positive in free energy change.