Question

After adding a small amount of ATP labeled with radioactive phosphorus in the terminal position, [y-PJATP, to a yeast extract, a researcher finds about half of the ³²P activity in P; within a few minutes, but the concentration of ATP remains unchanged. She then carries out the same experiment using ATP labeled with ³²P in the central position, (β³²PJATP, but the ³²P does not appear in P, within such a short time.

Which statements explain these results?
a. The terminal (γ) phosphoryl group undergoes a more rapid turnover than the central (β) phosphate group,
b. Yeast cells reincorporate P, released from β³² PJATP into ATP more quickly than P, released from [y³²PJATP.
c. Only the terminal (γ) phosphorous atom acts as an electrophilic target for nucleophilic attack.
d. Yeast cells maintain ATP levels by regulating the synthesis and breakdown of ATP.

Answer 1

The rapid appearance of radioactive Pi when γ-P32ATP is added to yeast extract is explained by the terminal (γ) phosphate group's rapid turnover in biochemical reactions, supporting option a as the correct explanation. Option a. The terminal (γ) phosphoryl group undergoes a more rapid turnover than the central (β) phosphate group, is the correct answer.

Step-by-step explanation:

After a researcher adds ATP labeled with radioactive phosphorus to a yeast extract, the appearance of radioactivity in inorganic phosphate (Pi) but not in ATP suggests specific biochemical activities within the cells. The fact that γ-P32ATP results in radioactive Pi quickly, while β-P32ATP does not, indicates that the terminal (γ) phosphate group of ATP undergoes a more rapid turnover than the central (β) phosphate group. The correct statement that explains this result is option a, 'The terminal (γ) phosphoryl group undergoes a more rapid turnover than the central (β) phosphate group.' This is consistent with the process of phosphorylation, where ATP transfers its γ phosphate to a substrate molecule during a phosphorylation reaction, a key step in various metabolic pathways like glycolysis.

Additionally, yeast cells do actively maintain ATP levels by regulating the synthesis and breakdown of ATP, as mentioned in statement d. However, this regulation does not directly explain the rapid turnover of the γ phosphate group or the immediate appearance of radioactive Pi in the medium. It is important to note that ATP synthesis involves the addition of a phosphate group to ADP in processes such as oxidative phosphorylation or substrate-level phosphorylation, as seen in the glycolytic pathway. Moreover, the electrophilicity of the phosphorus atom in ATP, useful for detecting through radioactive labeling, is reinforced through chelation with Mg2+ ions.

Therefore, the researcher's observations are best explained by the inherent properties of ATP's γ phosphate in cellular reactions and the enzymes involved, which are geared towards rapid transfer of this group during metabolic processes. Consequently, the mention of the correct option in the final answer is option a.