Creatine is a precursor of creatine phosphate, the compound that shuttles high-energy phosphate from mitochondria to sites of muscle contraction.

Creatine is formed by transmethylation of guanidinoacetate (derived from arginine) (reaction 1 below) and has three resonance forms, as follows:

Creatine participates in the reactions that follow:

1. Guanidinoacetate + S-Adenosylmethionine <=> Creatine + S-Adenosylhomocysteine,

2. Creatine + ATP <=> Creatine Phosphate + ADP (catalyzed by Creatine Kinase)

The latter 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 (see here), where it provides the energy for muscle contraction.

High levels of ADP formed in the myofibrils 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.