TCA cycle

Question 1

what is the TCA cycle?

Answer 1

major energy generating pathway
oxidation of acetyl CoA produces ATP
occurs aerobically in mitochondria

acetyl CoA from:
glucose in glycolysis
fatty acids and lipid metabolism
protein/amino acid metabolism

ATP from:
substrate level phosphate
oxidative phosphorylation

Question 2

step 1

Answer 2

condensation

catalysed by citrate synthase

acetyl CoA joins with oxaloacetate to form citrate

Question 3

step 2

Answer 3

rearrangement

catalysed by aconitase

successive dehydration and hydration reactions

cofactor = iron

citrate rearranged into isocitrate

Question 4

step 3

Answer 4

oxidative decarboxylation

catalysed by isocitrate dehydrogenase

cofactor = Mn2+

produces NADH
CO2 released

isocitrate decarboxylated and oxidised into a-ketoglutarate

Question 5

step 4

Answer 5

oxidative decarboxylation

catalysed by a-ketoglutarate dehydrogenase

coenzyme = CoA-SH
cofactor = magnesium

produces NADH
CO2 released

carbon removed replaced with coenzyme CoA-SH

a-ketoglutarate decarboxylated and replaced with CoA-SH and oxidised into succinyl-CoA

Question 6

step 5

Answer 6

hydrolysis reaction linked to substrate level phosphorylation

catalysed by succinylcholine CoA synthase

cofactor = magnesium

exchange of phosphoric group between GTP and ADP

succinyl-CoA hydrolysed (bond between CoA broken) to create succinate
CoA released

energy released used to phosphorylate GDP into GTP
GTP used to used phosphorylate ADP into ATP

Question 7

step 6

Answer 7

oxidation

catalysed succinate dehydrogenase

takes place at mitochondrial inner membrane
FAD reduced to FADH
- produces 2 ATP per FADH molecule in oxidative phosphorylation

succinate oxidised into fumarate

Question 8

step 7

Answer 8

hydration

catalysed by fumarase

water added

fumarate hydrated into malate

Question 9

step 8

Answer 9

oxidation

catalysed by malate dehydrogenase
NAD reduced

NADH releases 3 ATP per molecule in oxidative phosphorylation in electron transport chain

malate oxidised into oxaloacetate

Question 10

why does the TCA cycle create so much energy?

Answer 10

produced through substrate level phosphorylation

lots of energy produced as processes combined
- TCA cycle in matrix and electron transport chain in inner membrane

TCA cycle produces high energy electrons - reduced NAD and FAD
used in electron transport chain
creates proton gradient which drives chemiosmotic synthesis of ATP

TCA cycle allows energy from many different molecules
- acetyl CoA can come from glucose, lipids/fatty acids