[orgo814]

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 C-14 at its carbonyl carbon, how many rounds of the cycle are required before CO2 with the radiolabeled C-14 is released?

The book says that the C-14O2 comes out during the second round. However, when I go through the process, I find that the labeled carbon ends up on C5 of citrate and ends up being lost as CO2 in the alpha-ketoglutarate dehydrogenase reaction. Can someone please specify what I am missing here? Thanks.

[Babcock_Hall]

Why do you think that it becomes C-5 and not C-1?

[orgo814]

Well either way it's definitely not C4 like my book says.... I think?

[orgo814]

Oh, I see what you mean. So the one originally on the oxaloacetate is the one lost that round

[Babcock_Hall]

I am in a rush, and so I am going to have to go on memory.  I seem to recall that carboxylate carbon atoms at the two ends of the citrate molecule are enantiotopic.  An enzyme can distinguish enantiotopic atoms because the enzyme itself is chiral.