1. The Citric Acid Cycle/The Krebs Cycle


  • after glycolysis, most tof the energy in glucose remains stockpiled in the two molecules of pyruvate
  • in eukaryotic cells, if oxygen is present, the pyruvate enters the mitochondria where the enzymes of the citric acid cycle complete the oxidation of glucose
  • after entering the mitochondria through active transport the pyruvate is converted to a compound called acetyl coenzymme A, or acetyl CoA
    • this step is the junction between glycolysis and the citric acid cycle
    • this is accomplished by a multi-enzyme complex that catalyzes three reactions:
      1. pyruvate's carboxyl group which is already fully oxidized and thus has little chemical energy is removed and given off as a molecule of CO2 (this is the first step in cellular respiration in which CO2 is released)
      2. the remaining two-carbon fragment is oxidized, forming a compound named acetate (the ionized form of acetic acid). An enzyme transfers the extracted electrons to NAD+ , storing energy in the form of NADH
      3. coenzyme A (CoA) a sulfur containing compound derived from a B vitamin is attatched to the acetate by an unstable bond that makes the acetyl group very reactive. because of the nature of the CoA group,, acetyl CoA has a high potential energy. the molecule is now ready to feed its acetyl group inot the citric acid cycle for further oxidation
  • the citric acid cycle is also called the tricarboxylic acid cycle or the Krebs cycle
    • Krebs cycle is in honor of Hans Krebs the German-British scientist who was largely responsible for working out the pathway in the 1930s
  • pyruvate is broken down into three CO2 molecules including the molecule of CO2 released during the conversion of pyruvate to acetyl CoA
  • the cycle generates 1 ATP per turn by substrate-level phosphorylation
  • most of the energy is transferred to NAD+ and a related electron carrier called coenzyme FAD
  • the reduced coenzymes NADH and FADH2 shuttle their high energy electrons to the electron transport chain
  • the citric acid cycle has 8 steps - each is catalyzed by a specific enzyme
  • for each turn of the citric acid cycle, 2 carbons enter in the reduced form of an acetyl group and 2 DIFFERENT carbons leave in the completely oxidized form of CO2 molecules
  • the acetyl group of acetyl CoA joins the cycle by combining with the compound oxaloacetate - this forms citrate
  • the next seven steps decompose the citrate back to oxaloacetate - this regeneration of oxaloacetate is what makes the process a CYCLE
  • energy rich molecules produced by the citric acid cycle for each acetyl group entering the cycle:
    • 3 NAD+ are reduced to NADH
    • FAD accepts 2 electrons and 2 protons to become FADH2
    • in animal tissue cells a guanosine triphosphate molecule is produced by substrate-level phosphorylation in step 5
    • the output of step 5 represents the only ATP directly generated by the citric acid cycle

AN OVERVIEW OF THE CITRIC ACID CYCLE

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CITRIC ACID CYCLE: a close up

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