Citric acid cycle 2

Question 1

why cant acetyl just be degraded into 2 single carbon groups/why is the cycle needed

Answer 1

direct oxidation could result in toxic compounds like methane

Question 2

step 1: 2C + 4C = 6C

Answer 2

acetyl CoA+oxaloacetate–>citric acid (+CoA + H2O)
condensation reaction
catalysed by citrate synthase

Question 3

what makes step 1 feasible (-G)

Answer 3

Binding of oxaloacetate to the citrate synthase causes a large conformational change so acetyl-CoA is able to bind. This prevents acetyl-CoA from being hydrolysed without oxaloacetate being present. It is the hydrolysis of the thioester bond between CoA and the acetyl group that releases energy, giving the overall reaction a negative deltaG despite the formation of a carbon-carbon bond having a positive deltaG.

Question 4

why does citrate need to be rearranged

Answer 4

citrate is hard to oxidise further due to lack of hydrogens
rearrangement to form an isomer where the hydroxyl is moved to a different carbon

Question 5

how is citrate rearranged

Answer 5

aconitase (isomerase) catalyses the dehydration of citrate to cis-aconitate. aconitase then catalyses rehydration to form isocitrate which is more susceptible to oxidation

Question 6

step 2: 6C to 5C

Answer 6

isocitrate converted to 2-oxoglutarate
decarboxylation
NAD+—>NADH+H+
catalysed by isocitrate dehydrogenase

Question 7

what causes step 2 to have overall negative delta G?

Answer 7

decarboxylation increases entropy

Question 8

step 3: 5C to 4C

Answer 8

2-oxoglutarate converted to succinyl-CoA
catalysed by 2-oxoglutarate dehydrogenase complex
decarboxylation from acetyl

Question 9

how is 4C, succinyl CoA rearranged back to its original form so that it can combine with another acetyl CoA to continue the cycle?
2 steps

Answer 9

succinyl CoA converted to succinate/succinic acid
catalysed by succinate thiokinase
hydrolysis of CoA releases energy (ADP+Pi–>ATP)
succinate converted to fumarate by FAD–>FADH2 catalysed by succinate dehydrogenase

Question 10

succinate dehydrogenase

Answer 10

integral membrane protein in mitochondria
has FAD bound

Question 11

last 2 steps of rearrangement of succinyl CoA

Answer 11

fumerase catalyses hydration of fumarate to malate
H2O attacks the C=C
malate dehydrogenase converts malate to oxaloacetate
NAD+–>NADH+H+