Krebs cycle

Question 1

After Glycolysis (Oxidation of pyruvate to Acetyl CoA)

Answer 1

Pyruvate: generate further energy from glucose

Pyruvate transported to mitochondrion

Pyruvate oxidized (pyruvate dehydrogenase enzyme complex):

1) Decarboxylation: carboxyl group removed —> CO2 released
2) Reduction: NAD+ reduced —> NADH (high E electron carrier)
3) Energy from 2): Acetyl group + coenzyme A (acetyl thioester bond)= Acetyl CoA (starting molecule of Krebs)

Coenzyme A: triglyceride (fat) synthesis, transport acetyl groups

Recap: Oxidation of Pyruvate
2 CO2
2 NADH
2 Acetyl CoA (from 2 pyruvate = 1 glucose)

Question 2

Krebs cycle (overview)

Answer 2

Mitochondria matrix (NOT cristae) ´
Glucose completely oxidized (max E in glucose —-> high E electron carriers-NADH & FADH2)

Question 3

Krebs cycle (9 steps)

Answer 3

1. Condensation
   Acetyl CoA —> Acetyl group (leaves CoA)
   —> Acetyl group + oxaloacetate = citrate

2-3. Isomerization
Citrate —> Isocitrate (isomer in 2 steps)

4. First Oxidation
   1) Isocitrate oxidized —> NADH + E (from NAD+)
   2) Isocitrate decarboxylated (from E of
   Redox) —>CO2
   Leads to alpha-ketogluterate

2 NADH, 2 CO2 (2 pyruvate=1 glucose)

5. Second Oxidation
   1) alpha-ketogluterate oxidized—> NADH + E
   2) decarboxylation (E from redox)—> CO2
   3) leads to Succinate + E from redox—> succinate bind to CoA

2 NADH, 2 CO2 (2 pyruvate = 1 glucose)

6. Substrate level phosphorylation
   1) bond b/w succinate & CoA broken
   —> high E + succinate
   2) E used to form GTP (guanosine triphosphate)
   3) GTP transfer phosphate to ADP
   —> ATP (substrate level phosphorylation)

2 ATP (2 pyruvate = 1 glucose)

7. Third Oxydation
   Succinate oxidized—> fumerate + FADH2 (from FAD)
   [high E electron carrier (lower E than NADH), NOT FREE to move in cell- stay in mitochondrion]

2 FADH2 (2 pyruvate = 1 glucose)

8. Regeneration of oxaloacetate
   Fumerate + 1 water = malate
9. Regen. of oxaloacetate
   Malate oxidized —-> oxaloacetate + NADH

2 NADH (2 pyruvate = 1 glucose)

Restart cycle: oxaloacetate + Acetyl group (pyruvate) —> citrate

Question 4

Krebs cycle (recap)

Answer 4

Recap: Krebs cycle (1 glucose, 2 pyruvate)
6 NADH
4 CO2
2 ATP
2 FADH2

All 6 C glucose transformed to 6 CO2 
2 CO2 (oxidation of pyruvate) + 4 CO2 (Krebs cycle) = 6 CO2 

Energy from bonds of glucose:
2 FADH2 (Krebs cycle)
10 NADH (2-glycolysis, 2-oxidation of pyruvate, 6-Krebs cycle)
4 ATP (2-glycolysis, 2-Krebs cycle)