TCA Cycle

Question 1

Conversion of Pyruvate to Acetyl-CoA

Answer 1

reaction: pyruvate => acetyl-CoA + CO2
enzyme: pyruvate dehydrogenase complex
ΔG’°= -33.4 kJ/mol
first carbons to be fully oxidized

Question 2

Pyruvate Dehydrogenase Complex (E1)

Answer 2

step 1: decarboxylate pyruvate to aldehyde

step 2: oxidation of aldehyde to carboxylic acid

Question 3

Pyruvate Dehydrogenase Complex (E2)

Answer 3

step 3: formation of acetyl-CoA

Question 4

Pyruvate Dehydrogenase Complex (E3)

Answer 4

step 4: deoxidation of lipamide cofactor

step 5: regeneration of oxidized FAD cofactor to form NADH

Question 5

TPP

Answer 5

• coenzyme
• cleaves bonds next to carbonyl groups
• thiazolium ring can deprotonate to form a carbanion and
  acts as an electron sink

Question 6

Lipolysine

Answer 6

• prosthetic group in pyruvate dehydrogenase

- electron carrier and an acyl carrier

Question 7

Coenzyme A

Answer 7

• coenzyme
• accepts and carries acetyl group
• thiol group reacts with acyl group attached to lipotic acid
  and activates it for transacetylation, condensation or redox

Question 8

Step 1

Answer 8

Condensation
reaction: acetyl-CoA + OAA =(H2O=>CoA-SH)=> citrate
enzyme: citrate synthase
- binding of OAA induces conformational change creating binding site
of acetyl-CoA
ΔG’°= -32.2 kJ/mol
- large negative ΔG’° drives TCA cycle, irreversible
- rate limiting step

Question 9

Step 2

Answer 9

Isomerization
reaction: citrate => isocitrate
enzyme: aconitase
ΔG’°= -13.3 kJ/mol

Question 10

Step 3

Answer 10

Oxidative Decarboxylation
reaction: isocitrate => α-ketoglutarate
enzyme: isocitrate dehydrogenase
- step 1: oxidizes isocitrate, NAD(P)+ => NAD(P)H + H+
- step 2: decarboxylation, remove CO2
- step 3: rearrangement of enol intermediate
ΔG’°=

Question 11

Step 4

Answer 11

Oxidative Decarboxylation
reaction: α-ketoglutarate =(CoA-SH + NAD+ => NADH)=> succinyl-CoA
enzyme: α-ketoglutarate dehydrogenase complex
ΔG’°= -33.5 kJ/mol

Question 12

Step 5

Answer 12

Substrate Level Phosphorylation
reaction: succinyl-CoA =(GTP + Pi => GTP + CoA-SH)=> succinate
- [succinate] kept low to drive reaction forward
enzyme: succinyl-CoA synthetase
ΔG’°= -2.9 kJ/mol

Question 13

Step 6

Answer 13

Dehydrogenation
reaction: succinate =(FAD => FADH2)=> fumarate
- [fumarate] kept low to drive reaction forward
enzyme: succinate dehydrogenase
ΔG’°= 0 kJ/mol

Question 14

Step 7

Answer 14

Hydration
reaction: fumarate =(put in OH- and H+)=> L-malate
- addition of H2O across a double bond
- [L-malate] kept low to drive reaction forward
enzyme: fumarase
ΔG’°= -3.8 kJ/mol

Question 15

Step 8

Answer 15

Dehydrogenation
reaction: L-malate =(NAD+ => NADH + H+)=> oxaloacetate
- [OAA] kept VERY low to drive reaction forward
enzyme: malate dehydrogenase
ΔG’°= 29.7 kJ/mol

Question 16

One Turn of TCA Cycle

Answer 16

acetyl-CoA + 3 NAD+ + FAD + GDP + Pi + 2 H20 => 2 CO2 + 3 NADH + 2 FADH2 + GTP + CoA + 3 H+