Question

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

Answer 1

Answer:

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.