Creatine

Creatine is an organic compound that, in vertebrates, facilitates recycling of adenosine triphosphate (ATP), primarily in muscle and brain tissue. Its phosphorylated form, phosphocreatine, donates phosphate groups to adenosine diphosphate (ADP), turning it back into ATP. Creatine also acts as a buffer. It has the nominal formula (H2N)(HN)CN(CH3)CH2CO2H and in solutions, exists in various tautomers, including a neutral form and zwitterionic forms.