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Citric Acid/Kreb's Cycle
Steps and components of the Citric Acid Cycle
8
Biochemistry
Undergraduate 3
03/20/2017

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Term
Step 1 (TCA)
Definition
  • combine acetyl-CoA and oxaloacetate to form citrate
  • enzyme: citrate synthase
  • highly exergonic
  • stimulated by ADP, inhibited by NADH, ATP, and succinyl CoA
  • [image]
Term
Step 2 (TCA)
Definition
  • formation from citrate to isocitrate with cis-aconitate intermediate that is hydrated
  • enzyme: aconitase
  • endergonic
  • [image]
Term
Step 3 (TCA)
Definition
  • oxidation of isocitrate to alpha-ketoglutarate and CO2
  • enzyme: isocitrate dehydrogenase
  • formation of first NADH and CO2 of cycle
  • irreversible due to creation of CO2
  • inhibit by ATP, activate by Ca and ADP
  • [image]
Term
Step 4 (TCA)
Definition
  • oxidation of alpha-ketoglutarate to succinyl CoA and CO2
  • enzyme: alpha-ketoglutarate dehydrogenase complex
  • mechanism identical to PDH
  • 2nd NADH and CO2 of cycle created (irreversible)
  • inhibit by succinyl CoA, stim by Ca
  • [image]
Term
Step 5 (TCA)
Definition
  • Substrate level phosphorylation, producing GTP and forming succinate
  • enzyme: succinyl-CoA synthetase
  • thioester removal provides energy to complete formation of NTP
  • synthetases require nucleoside triphosphates in reactions
  • [image]
Term
Step 6 (TCA)
Definition
  • Succinate dehydrogenation to create fumarate
  • enzyme: succinate dehydrogenase
  • creates only FADH2 of the cycle
  • fumarate is the trans product (cis product maleate is not created)
  • malonate is a competitive inhibitor
  • [image]
Term
Step 7 (TCA)
Definition
  • Hydration of fumarate to form malate
  • enzyme: fumarase
  • carbanion intermediate
  • only forms L-malate, not D-malate
  • exergonic
  • [image]
Term
Step 8 (TCA)
Definition
  • formation of oxaloacetate from L-malate
  • enzyme: malate dehydrogenase
  • 3rd NADH formed
  • highly endergonic, pulled forward by step 1
  • [image]
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