Krebs Cycle

• Krebs cycle


  Other main name for the citric acid cycle, along with tricarboxylic acid cycle
• Krebs cycle is a closed loop; it reforms the exact same molecule you had in the beginning
• Krebs cycle step 1


  Acetyl CoA joins with 4-carbon molecule (oxaloacetate) releasing the CoA group and creating a 6-carbon molecule (citrate)
• Krebs cycle step 2


  Citrate is converted into its isomer, isocitrate, by removing a water molecule then adding it somewhere else
• Krebs cycle step 3


  Isocitrate dehydrogenase catalyzes the oxidization of isocitrate, releases CO2, leaving 5-carbon molecule a-ketoglutarate. NAD+ is reduced to forn NADH because of the oxidization
• Krebs cycle step 4


  A-ketoglutarate dehygrogenase catalyzes a-ketoglutarate getting oxidized, reducing NAD+ to NADH and releasing CO2. Remaining 4-carbon molecule picks up a coenzyme a and becomes succinyl CoA
• Krebs cycle step 5


  CoA in succinyl CoA is replaced with a phosphate group, which is then transferred to ADP or GDP to make ATP or GTP. Resulting 4-carbon molecule is succinate
• Krebs cycle step 6


  Succinate is oxidized to form 4Cm fumarate and gives 2 protons to FAD, making FADH2. Enzyme that does this is in the matrix membrane so e- goes directly onto the ETC
• Krebs cycle step 7


  Water is added to 4Cm fumarate, converting it into 4Cm malate
• Krebs cycle step 8


  Malate is oxidized to form oxaloacetate, the starting molecule that binds to acetyl CoA in step 1. NAD+ is reduced to NADH in the process
• Krebs cycle reactants: Acetyl CoA, 2H2O, 3NAD+, FAD, GDP, Pi

  Krebs cycle products: 2CO2, 3NADH, 3H, FADH2, CoASH, GTP
• Every glucose molecule that is processed yields 2 acetyl CoA, so the Krebs cycle happens twice
• Energy output for one glucose molecule in the citric acid cycle is 1 ATP, 6 NADH, and 2 FADH2
• 3 ATP molecules are synthesized per 1 NADH molecule that enters the ETC
• 2 ATP molecules are synthesized per 1 FADH2 molecule that enters the ETC
• FAD is more electronegative than NAD+, so it's more useful when accepting electrons from more electronegative molecules, while NAD+ is better at giving them to the ETC when it's time