Carbon Labeling: Consider a single radioactive acetyl-CoA (both carbons labeled)
ID: 80130 • Letter: C
Question
Carbon Labeling: Consider a single radioactive acetyl-CoA (both carbons labeled) that enters the citric acid cycle. After one turn of the cycle, both CO2 molecules that leave are not radioactive. During the second turn of the cycle, how many radioactive CO2 molecules leave?
a) 0
b) 1
c) 2
d) it could be 0, 1, or 2
Please EXPLAIN how you get this answer (a DRAWING or DIAGRAM would help). Also HOW does carbon labeleing IN GENERAL work in terms of the CITRIC ACID CYCLE? What if you happened to label some other step in the cycle?
Explanation / Answer
Answer: b) 1 (As explained below)
The Acetyl CoA has two carbons, one is the carbonyl carbon and the other is the methyl carbon. The fate of the two carbons differs. If we label (as in radioactive labeling) the carbonyl carbon as C-1, we see that the when acetyl-CoA condenses with oxaloacetate, the carbonyl carbon is retained and not lost as CO2. The oxaloacetate loses two carbons and forms succinyl-CoA. Now when succinyl-CoA is hydrolyzed to form succinate and oxaloacetate is regenerated, the C-1 atom can be seen in either of the carboxyl carbons. These carboxyl carbons are lost in the second turn.
Now if we label the methyl carbon as C-2, we find that in the first turn, C-2 is evenly distributed between the methylene and carbonyl carbons of oxaloacetate. Unlike the C-1 carbon, which can be seen in the intermediates, the C-2 carbon stays with the oxaloacetate and is not lost to any intermediates in the second turn.
Carbon labelling is simple to understand. In the laboratory, organic molecules can be labeled using carbon labelling. In this, the particular carbons or hydrogens of the molecule (like in our case here, the carbons in Acetyl-CoA molecule) are replaced by their radioactive counterparts, or isotopes, like using 14C instead of the usual 12Cor and 3H instead of 1H. These radioactive active counters are then tracked and measured in the purified intermediate compounds.
If you happened to label some other step in the cycle, then you need to track that particular atom in the molecule. It depends on what you want to track at any step.