Important Differences Between G' and G⁰'

If ATP is the free energy "currency" of a cell, how does a cell synthesize a compound (such as creatine phosphate) with a much higher phosphate transfer potential than that of ATP?

First, values for ATP hydrolysis do not represent actual G' values under likely biological conditions. ATP has a considerably higher G' value at intracellular concentrations of ATP, ADP, and AMP than it has under standard conditions.

For example, consider the following reaction, catalyzed by creatine kinase:

Creatine + ATP <=> Creatine Phosphate + ADP.

This reaction is endergonic ( = +12 kcal/mol) under standard conditions but, because ATP levels are very high within mitochondria, and creatine phosphate levels are relatively low, the reaction is exergonic as written and proceeds to the right. Thus, the cellular concentration of metabolites has a drastic effect on the free energy for a reaction, enabling ATP to create phosphorylated compounds with higher phosphoryl transfer potential than itself under standard conditions.

ATP as Free Energy Currency, Free Energy and Concentration (from Chapter 3), Important Points about G (from chapter 3)