If acetyl-CoA is labeled with 14C at its methyl group, how many rounds of the cycle are required before 14CO2 is released?

14 CO₂ will be released in the second turn of the cycle

Explanation:

Complete question goes like this, "The CO2 produced in one round of the citric acid cycle does not originate in the acetyl carbons that entered that round. If acetyl-CoA is labeled with 14C at the carbonyl carbon, how many rounds of the cycle are required before 14CO2 is released?"

The answer to this is;

The labeled Acetyl of Acetyl-CoA becomes the terminal carbon (C4) of succinyl-CoA (which becomes succinate that is a symmetrical four carbon diprotic dicarboxylic acid from alpha-ketoglutarate). Succinate converts into fumarate. Fumarate converts into malate, and malate converts into oxaloacetate. Because succinate is symmetrical, the oxaloacetate can have the label at C1 or C4. When these condense with acetyl-CoA to begin the second round of the cycle, both of these carbons are discharged as CO2 during the isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase reactions (formation of alpha-ketoglutarate and succinyl-CoA respectively).

Hence, 14 CO₂ will be released in the second turn of the cycle.