Discussion
Intracellular concentrations of ATP, ADP, and inorganic phosphate in four cell types are given below. Which one of these cell types has the most negative ΔG for ATP hydrolysis?
| Cell type | ATP (mM) | ADP (mM) | Inorganic phosphate (mM) |
|---|---|---|---|
| L | 3.0 | 1.8 | 5.0 |
| K | 3.9 | 1.3 | 3.0 |
| B | 2.7 | 0.7 | 2.7 |
| M | 7.2 | 0.9 | 8.0 |
Answer: C
The cell type is B.
\[ \begin{aligned} \Delta G &= \Delta G^{\circ\prime} \\ &+ RT \ln\frac{[ADP][P_{i}]}{[ATP]} \end{aligned} \]
Since \(\Delta G^{\circ\prime}\) is the same for all four, \(\Delta G\) is most negative where the mass-action ratio is smallest. In mM:
- L: \(1.8 \times 5.0 / 3.0 = 3.0\)
- K: \(1.3 \times 3.0 / 3.9 = 1.0\)
- B: \(0.7 \times 2.7 / 2.7 = 0.7\)
- M: \(0.9 \times 8.0 / 7.2 = 1.0\)
B has the smallest ratio, so the logarithm term is most negative there. M has the most ATP, but its high phosphate level brings its ratio up to that of K; L has the largest ratio and the least negative \(\Delta G\).
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