Creatine kinase is an enzyme that catalyzes the reaction,
Creatine + ATP <=> Creatine Phosphate + ADP
The reaction is strongly endergonic as written. However, the level of ATP is very high in mitochondria, so the reaction proceeds to the right. Creatine phosphate then diffuses from mitochondria to the myofibrils, where it provides the energy for muscle contraction. High levels of ADP formed in the myofibrils (see here) during contraction favor the reverse reaction namely, resynthesis of ATP - at the expense of creatine phosphate cleavage to creatine. This example shows that one must consider not only the standard free energy change but also the actual concentrations of all reactants and products when predicting the direction of a reaction in vivo.
