The phosphagens are energy storage compounds, high energy phosphate compounds used especially by muscular tissue in animals to maintain a high energy phosphate pool in a concentration range, where, if it all were ATP, would create problems because of all the ATP consuming reactions in these tissues. Muscle tissues will have sudden demands for lots of energy. These compounds can maintain a reserve of high energy phosphates that can kick in as needed, to provide that rapid energy that could not be supplied by immediately by glycolysis or oxidative phosphorylation.
The compound used as a phosphagen is dependent on the organism. In mammals, the compound used is creatine. Creatine phosphate, or phosphocreatine, is made from ATP by the enzyme creatine kinase in a reversible reaction:
- Creatine + ATP <--> creatine phosphate + ADP (this reaction is Mg++ dependent)
