A student is trying to determine the mechanism for a reaction that uses ATP to activate a carboxylate ion, which then reacts with a thiol. If the carboxyl-ate ion attacks the g-phosphorus of ATP, the reaction products are the thioester, ADP, and phosphate. However, whether it attacks the a-phosphorus or the b-phosphorus of ATP cannot be determined from the reaction products because the thioester, AMP, and pyrophosphate would be the products in both reactions. The mechanisms can be distinguished by a labeling experiment in which the enzyme, the carboxylate ion, ATP, and radioactively labeled pyro-phosphate are incubated, and then the ATP is isolated. If the isolated ATP is radioactive, attack occurred on the a-phosphorus. If it is not radioactive, then attack occurred on the b-phosphorus. Explain these conclusions.
A student is trying to determine the mechanism for a reaction that uses ATP to activate a carboxylate ion, which then reacts with a thiol. If the carboxyl-
ate ion attacks the g-phosphorus of ATP, the reaction products are the thioester, ADP, and phosphate. However, whether it attacks the a-phosphorus or
the b-phosphorus of ATP cannot be determined from the reaction products because the thioester, AMP, and pyrophosphate would be the products in both
reactions. The mechanisms can be distinguished by a labeling experiment in which the enzyme, the carboxylate ion, ATP, and radioactively labeled pyro-
phosphate are incubated, and then the ATP is isolated. If the isolated ATP is radioactive, attack occurred on the a-phosphorus. If it is not radioactive, then
attack occurred on the b-phosphorus. Explain these conclusions.
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