citrate cycle: names in the steps

Question 1

Acetyl-CoA into?

Enzyme?

Intermediates?

Answer 1

Acetyl-CoA + Oxaloacetate -> Citrate

Enzyme: Citrate Synthase

first a reversible conversion to S-Citryl-CoA Intermediate, and then spontaneous irreversible conversion to Citrate

Question 2

Citrate -> into?

Enzyme?

Intermediates?

Answer 2

Citrate -> Isocitrate

Enzyme = Aconitase

First a dehydration into Cis-aconitate Intermediate, then a rehydration into isocitrate

Question 3

Isocitrate -> into?

Enzyme?

Intermediates?

Answer 3

Isocitrate -> Alpha-Ketoglutarate

Enzyme = Isocitrate Dehydrogenase

First an irreversible oxidation to Oxalosuccinate Intermediate (with NAD+ -> NADH & H+), then an irreversible decarboxylation (remove CO2) into alpha-ketoglutarate

Question 4

Alpha-ketoglutarate -> into?

Enzyme?

Intermediates?

Answer 4

Alpha-ketoglutarate + HS-CoA + NAD+ -> Succinyl-CoA (+ NADH + CO2)

Enzyme: Alpha-ketoglutarate dehydrogenase complex

No intermediate!

NAD+ reduces rxn -> NADH & H+, also decarboxylation occurs (CO2 removed)

• now structure has returned to 4 carbons, the rest of cycle just returns to oxaloacetate -

Question 5

Succinyl-CoA -> into?

Enzyme?

Intermediates?

Answer 5

Succiny-CoA + Pi + GDP -> Succinate + GTP

Enzyme: Succinyl-CoA Synthetase

No intermediate!

(example of coupled reaction: hydrolysis of one high E bond to form another)

(GTP can pass Pi to ADP -> ATP via NDP Kinase)

Question 6

Succinate -> into?

Enzyme?

Intermediates?

Answer 6

Succinate + FAD -> Fumarate + FADH2

Enzyme: Succinate Dehydrogenase

No intermediate!

Succinate dehydrogenase is also part of Electron Transport Chain! aka Complex II (only enzyme that particpates in both)

Question 7

Fumarate -> into?

Enzyme?

Intermediates?

Answer 7

Fumarate + H2O -> Malate

Enzyme: Fumarase

No intermediates!

Always L-Malate configuration is formed

Question 8

Malate -> into?

Enzyme?

Intermediates?

Answer 8

Malate + NAD+ -> Oxaloacetate + NADH & H+

Enzyme: Malate Dehydrogenase

No intermediates!

• Not spontaneous! Pushed forward by high [malate] : [oxaloacete] ratio
• Final step, back to oxaloacetate!

Question 9

What are the steps that are irreversible?

(large neg delta G)

Answer 9

Step 1: Citrate synthase

Step 3: Isocitrate Dehydrogenase

Step 4: alpha-Ketoglutarate Dehydrogenase Complex