Krebs Cycle

• The citric acid cycle is also known as the tricarboxylic acid (TCA) cycle or the krebs cycle.
• Who discovered Krebs Cycle?

  Hans Krebs (in the 1930s)
• First product of Krebs Cycle

  citric acid (COO-)
• Transition reaction

  pyruvate is converted to acetyl-CoA
• Pyruvate dehydrogenase


  formation of acetyl-CoA from pyruvate
• Citrate synthase

  acetyl-CoA rects with oxaloacetate, which forms citrate / citric acid (6 carbon molecule)
• Aconitase


  Citrate undergoes isomerization and becomes isocitrate
• Isocitrate dehydrogenase


  isocitrate undergoes oxidation, which results in a-ketoglutarate
• a-ketoglutarate dehydrogenase

  oxidation of a-ketoglutarate. CO2 is released and NAD+ becomes NADH, which leaves a four-carbon molecule behind. succinyl-CoA is formed
• Succinyl CoA synthase

  A four-carbon molecule reacts with coenzyme A, which forms succinate.
• Succinate

  the coenzyme A from succinyl-CoA is replaced by a phosphate group. However, the phosphate group is immediately transferred to GDP to form GTP.
• succinic dehydrogenase

  succinate is oxidized to form fumarate.

  this oxidation process causes 2 hydrogen molecules to be released, and are transferred to FAD to create FADH2
• fumarase

  the fumarate receives a water molecule to form malate
• malate dehydrogenase


  malate is oxidized and becomes oxaloacetate. this process causes the reduction of NAD+ to NADH.
• One Krebs cycle always regenerates

  oxaloacetate
• Krebs Cycle

  2 acetyl CoA + 6 NAD+ + 2 FAD + 2 ADP -> 4 CO2 + 2 CoA + 6 NADH + 2 FADH2 + 2 GTP
• Transition Reaction


  2 pyruvate + 2 coenzyme A + 2 NAD+ -> 2 acetyl CoA + 2 CO2 + 2 NADH