Question

If a small amount of ATP labeled with radioactive phosphorus in the terminal position, $\left[y-^{32} \mathrm{P}\right] \mathrm{ATP},$ is added to a yeast extract, about half of the $^{32} \mathrm{P}$ activity is found in $\mathrm{P}_{\mathrm{i}}$ within a few minutes, but the concentration of ATP remains unchanged. Explain. If the same experiment is carried out using ATP labcled with $^{32} \mathrm{P}$ in the central position, $\left[\beta-^{32} \mathrm{P}\right] \mathrm{ATP},$ the $^{32} \mathrm{P}$ does not appear in $\mathrm{P}_{\mathrm{i}}$ within such a short time. Why?

Name: Problem 23, Chapter 13: Principles of Biochemistry
Uploaded: 2021-02-16T18:39:48-08:00
Duration: 7 min 2 s
Channel: Niamat Khuda

   If a small amount of ATP labeled with radioactive phosphorus in the terminal position, $\left[y-^{32} \mathrm{P}\right] \mathrm{ATP},$ is added to a yeast extract, about half of the $^{32} \mathrm{P}$ activity is found in $\mathrm{P}_{\mathrm{i}}$ within a few minutes, but the concentration of ATP remains unchanged. Explain. If the same experiment is carried out using ATP labcled with $^{32} \mathrm{P}$ in the central position, $\left[\beta-^{32} \mathrm{P}\right] \mathrm{ATP},$ the $^{32} \mathrm{P}$ does not appear in
$\mathrm{P}_{\mathrm{i}}$ within such a short time. Why?

Principles of Biochemistry

David L. Nelson,… 7th Edition

Chapter 13, Problem 23

Instant Answer

Solved by Expert Niamat Khuda

02/16/2021

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The terminal phosphate is represented as $\gamma$, the middle one as $\beta$, and the initial one as $\alpha$. Show more…

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If a small amount of ATP labeled with radioactive phosphorus in the terminal position, $\left[y-^{32} \mathrm{P}\right] \mathrm{ATP},$ is added to a yeast extract, about half of the $^{32} \mathrm{P}$ activity is found in $\mathrm{P}_{\mathrm{i}}$ within a few minutes, but the concentration of ATP remains unchanged. Explain. If the same experiment is carried out using ATP labcled with $^{32} \mathrm{P}$ in the central position, $\left[\beta-^{32} \mathrm{P}\right] \mathrm{ATP},$ the $^{32} \mathrm{P}$ does not appear in $\mathrm{P}_{\mathrm{i}}$ within such a short time. Why?

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Key Concepts

Isotopic Exchange Reactions in Metabolism

Enzymes in metabolic pathways can catalyze reversible phosphoryl transfer reactions that cause rapid exchange of the gamma phosphate between ATP and Pi without net synthesis or degradation of ATP. This exchange explains why a radioactive label on the gamma phosphate appears quickly in the Pi pool. In contrast, the beta phosphate is not involved in such rapid exchange processes, so a radioactive label on the beta position does not appear in Pi over a short time.

ATP Structure and Phosphoryl Group Lability

ATP consists of three phosphate groups—alpha, beta, and gamma—with the gamma (terminal) phosphate being the most reactive due to its position and bonding characteristics. This inherent lability makes the gamma phosphate more readily transferred during enzymatic reactions, leading to its rapid exchange with inorganic phosphate (Pi) even under conditions where the overall ATP concentration remains steady.

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