Phosphagen

The phosphagens are energy storage compounds, also known as high-energy phosphate compounds, are chiefly found in muscular tissue in animals. They allow a high-energy phosphate pool to be maintained in a concentration range, which, if it all were ATP, would create problems due to the ATP consuming reactions in these tissues. As muscle tissues can have sudden demands for lots of energy; these compounds can maintain a reserve of high-energy phosphates that can be used as needed, to provide the energy that could not be immediately supplied by glycolysis or oxidative phosphorylation.

The actual biomolecule used as a phosphagen is dependent on the organism. The majority of animals use arginine/phosphoarginine as phosphagens; however, the phylum Chordata (i.e., animals with spinal cords) use creatine. Creatine phosphate, or phosphocreatine, is made from ATP by the enzyme creatine kinase in a reversible reaction:

• Creatine + ATP $\leftrightarrow$ creatine phosphate + ADP (this reaction is Mg⁺⁺-dependent)

However, annelids (segmented worms) use a set of unique phosphagens; for example, earthworms use the compound lombricine.