Question

An experiment is performed using glucose labeled with ¹⁴C at C-4. If the reaction is monitored in real time, meaning that substrates, products, and enzyme-bound intermediates could be detected. After the transition reaction, which of the following molecules will contain ¹⁴C?

A. acetyl CoA
b. CO₂
c. PEP
d. 2PG

Answer 1

The ¹⁴C label at the C-4 position of glucose will be found in CO₂ following the transition reaction, as carbons are released as CO₂ at this reaction stage. Option number a is correct.

Step-by-step explanation:

Following the transition reaction, where glucose labeled with ¹⁴C at C-4 is used, the radioisotope will be present in CO₂. This is because in the transition reaction, a multi-enzyme complex converts pyruvate into one acetyl group and one molecule of CO₂. Since the labeled carbon was at the C-4 position in glucose, and during glycolysis and the transition reaction carbons are released as CO₂, the ¹⁴C from the original glucose would have been released as CO₂.

Neither acetyl CoA, phosphoenolpyruvate (PEP), nor 2-phosphoglycerate (2PG) will contain the ¹⁴C after the transition step of cellular respiration.