The Citric Acid Cycle Flashcards
What is the overall purpose of the TCA cycle
convert the acetate moieties derived from the catabolism of fats and carbs and turn them into carbon dioxide
The energy released in the TCA is conserved in the form of ____________ _______________
reduced cofactors (NADH and FADH2)
What makes the TCA cycle oxygen dependent
NADH and FADH2 need to be oxidized back to NAD+ and FAD in the electron transport chain
What did Hans Krebs discover about the TCA cycle that was so important
it is cyclic
What is step 1 of the Krebs cycle
condensation of acetyl CoA with oxaloacetate to form citrate
*only TCA cycle that forms a C-C bond
- acetyl CoA is bound to oxaloacetate with the addition of water
*results in citrate, and the release of CoASH
What is the final product of step 1 of the TCA cycle, and what enzyme and cofactors were required to obtain it
citrate
via citrate synthase
cofactor is H2O - release of CoASH
What is step 2 of the Krebs cycle
isomerization of citrate by aconitase
- citrate is converted into isocitrate (isomers of one another) via the removal of water to form cis-aconitate and then addition of water to form isocitrate
results in isocitrate
What enzyme is required in step 2 of TCA cycle
aconitase
Citrate is a tertiary alcohol, therefore it is a ________ substrate for oxidation
poor substrate
- why it is converted into isocitrate
Isocitrate is a secondary alcohol, therefore it is a ____________ substrate for oxidation
good
What is step 3 of the Krebs cycle
oxidative decarboxylation of isocitrate (isocitrate dehydrogenase)
- oxidation of alcohol to ketone through the transfer of a hydride
results in a-ketogluterate
What is the enzyme used in step 3 of the TCA cycle and what cofactor is present
isocitrate dehydrogenase
NAD+ cofactor - release of NADH
What is step 4 of the Krebs cycle
oxidative decarboxylation of a-ketogluterate
- identical to mechanism of pyruvate dehydrogenase complex
- converts a-ketogluterate to high energy thioester succinyl CoA
results in succinyl CoA
What is the enzyme used in step 4 of the TCA cycle an what cofactors are used
a-ketogluterate dehydrogenase
NAD+ and CoASH cofactors
What deficiency would lower the efficiency and occurrence of the oxidative decarboxylation of a-ketoglutarate (step 4 of Krebs)
thymine
What is step 5 of the Krebs cycle
substrate level phosphorylation (succinyl CoA synthetase)
- high energy thioester bond of succinyl CoA is hydrolyzed
*involves formation of phosphoenzyme intermediate
results in succinate
What enzyme is used in step 5 of TCA cycle and what cofactor is present
succinyl CoA synthetase
GDP + Pi synthesized to GTP as cofactor
(CoASH released)
What is step 6 of the Krebs cycle
oxidation of succinate to fumarate (succinate dehydrogenase)
- oxidizes succinate to the alkene fumarate
*analogous to the acyl-CoA dehydrogenase reaction of beta oxidation
results in fumarate
What enzyme is used in step 6 of the TCA cycle and what cofactor is present
succinate dehydrogenase
FAD cofactor
What is step 7 of the Krebs cycle
hydration of fumarate to malate
- the alkene fumarate is hydrated to the alcohol malate
*analogous to the enoyl-CoA hyratase reaction of beta oxidation
results in malate
What enzyme is used in step 7 of the TCA cycle and what cofactor is present
fumarase
H2O is added (no real cofactor)
What is step 8 of the Krebs cycle
oxidation of malate to oxaloacetate
- malate is oxidized to oxaloacetate
*analogous to the hydroxyacyl-CoA dehydrogenase reaction of beta oxidation
results in oxaloacetate
What enzyme is used in step 8 of the TCA cycle and what cofactor is present
malate dehydrogenase
NAD+ cofactor
How many CO2 formed in one round of the TCA cycle (and where do they come from)
2 CO2 - come from oxidized acetyl groups of acetyl-CoA
How many NADH formed in one round of the TCA cycle
3
How many FADH2 formed in one round of the TCA cycle
1
How many GTP formed in one round of the TCA cycle
1
