Chegg Textbook Solutions for Fundamentals of Biochemistry Life at the Molecular Level 4th Edition: Chapter 17

Chapter: Problem:
  • Step 1 of 1

    The CO2 produced in one round of the citric acid cycle does not originate in the acetyl carbons that entered that round. It is produced from the oxaloacetate that condensed with the acetyl group in the first step of the cycle. The oxaloacetate in turn was synthesized from the acetyl groups that entered the previous rounds of the cycle.

    If acetyl-CoA is labeled with 14C at its carbonyl carbon, the following would happen:

    In the first round, the radiolabeled carbon gets incorporated into citrate and moves along in the cycle. It becomes C4 of succinyl-CoA. In succinate, it can either become the carbon of the in the first or the fourth carboxylic group, as succinate is symmetrical. It finally becomes the C4 or C1 carboxylic carbon in oxaloacetate.

    It then enters another round of the pathway. In this round, that is the second round, the radiolabeled carbon will be lost as 14CO2. This happens either during the reaction catalyzed by isocitrate dehydrogenase or by α-ketoglutarate.

    Hence, two rounds of cycle are required before 14CO2 is released.

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