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    Carbon Tracking in TCA Cycle

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    C14-Glucose labeled at carbon 5 enters glycolysis, is made into pyruvate, then acetyl CoA, and enters the TCA cycle. How much of the C14 will be released as CO2 during the second turn of the TCA cycle?

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    What happens to the [14C]-Glucose labeled at carbon 5 as it goes through glycolysis?

    The important point is that when the six carbon fructose-1,6-bisphosphate splits into DHAP and G3P, and that when DHAP converts into another G3P, that C5 (and C2 for that matter) both end up being the middle carbon in G3P. You can go through the individual steps on your own to confirm this, but it can be easy to remember:

    C6 of glucose gets phosphorylated, right?
    When it becomes fructose, the same C6 is phosphorylated.
    Then, fructose gets phosphorylated on C1 too. Therefore, both ends are phosphorylated, C1 and C6.
    When you get to the two G3P molecules, just remember that the carbons with the phosphates on them are the same carbons that ...