Krebs Cycle

• Krebs Cycle is also known as citric acid cycle or tricarboxylic cycle
• citric acid is the first product of the krebs cycle
• Citric acid plays a crucial role in cell production
• Hans Adolf Krebs

  • discovered krebs cycle in the 1930s
• citric acid is composed of three carboxyl groups (COO−)
• In eukaryotic organisms, the Krebs cycle occurs in the mitochondrion.
• In prokaryotic organisms, the Krebs cycle occurs in the cytoplasm.
• After glycolysis, the 2 pyruvate molecules undergo oxidation to form 2 molecules of acetyl CoA
• Transition Reaction Glycolysis to Krebs Cycle

  1. decarboxylation of pyruvate molecules which results in the formation of a two-carbon molecule (CO2)
  2. Two-carbon molecule undergoes oxidation turning to acetate (NADH is produced)
  3. Coenzyme A (sulfur containing enzyme) is added turning acetate to Acetyl-CoA
• The transition reaction from glyco to krebs is catalyzed by pyruvate dehydrogenase
• STEP 1

  Krebs cycle starts when the acetyl-CoA reacts with oxaloacetate (a four-carbon molecule)

  • making citric acid or citrate, a six-carbon molecule
  • catalyzed by citrate synthase
• STEP 2

  Citrate undergoes isomerization and becomes isocitrate via the enzyme aconitase.
• Isomerization involves the rearrangement of the atoms
• STEP 3

  Isocitrate is then oxidized into alpha-ketoglutarate via the enzyme isocitrate dehydrogenase.

  • oxidation of isocitrate causes the release of carbon dioxide and the reduction of NAD+ to NADH.
• STEP 4

  
  

  Alpha-ketoglutarate is oxidized via the enzyme alpha-ketoglutarate dehydrogenase.
• STEP 5

  
  

  Alpha-ketoglutarate loses CO2 and NAD is reduced to NADH making it to four-carbon molecule that reacts with coenzyme A and forms succinyl-CoA.

  • catalyzed by succinyl CoA synthetase
• STEP 6

  • coenzyme A from succinyl-CoA is replaced by a phosphate group
  • phosphate group is immediately transferred to GDP (guanosine diphosphate) to form GTP (guanosine triphosphate) (2 GTP)
  • forms a four-carbon molecule succinate
• GTP (guanosine triphosphate)

  • a molecule similar to ATP in terms of function
• STEP 7

  • succinate oxidizes to form fumarate, releasing two hydrogen molecules
  • these hydrogen molecules are picked up by FAD, reducing it to form FADH2
  • catalyzed by succinic dehydrogenase
• STEP 8

  • fumarate receives a water molecule, which results in the formation of malate
  • catalyzed by fumarase
• STEP 9

  • malate is oxidized and becomes oxaloacetate, reducing NAD+ to NADH
  • catalyzed malate dehydrogenase
• When oxaloacetate reacts with acetyl-CoA, another round of Krebs cycle takes place.
• one Krebs cycle always regenerates oxaloacetate
• In the Krebs cycle, there are a total of six NADH molecules, four carbon dioxide, and two FADH2 molecules produced.
• The NADH and FADH2 molecules will enter the ETC while CO2 is released to the environment through the lungs.
• GTP is equivalent to two ATP molecules
• Transition reaction

  2 pyruvate + 2 coenzyme A + 2 NAD+ → 2 acetyl CoA + 2 CO2 + 2 NADH
• Transition reaction

  2 pyruvate + 2 coenzyme A + 2 NAD+ → 2 acetyl CoA + 2 CO2 + 2 NADH
• Krebs Cycle

  2 acetyl CoA + 6 NAD + + 2 FAD + 2 ADP + 2 Pi → 4 CO2 + 2 CoA + 6 NADH + 2 FADH2 + 2 GTP
• Krebs Cycle

  2 acetyl CoA + 6 NAD + + 2 FAD + 2 ADP + 2 Pi → 4 CO2 + 2 CoA + 6 NADH + 2 FADH2 + 2 GTP