Phosphagen (guanidino) kinases are enzymes that transphosphorylate a high energy phosphoguanidino compound, like phosphocreatine (PCr) in the case of creatine kinase (CK) or phosphoarginine in the case of arginine kinase, which is used as an energy-storage and -transport metabolite, to ADP, thereby creating ATP. The substrate binding site is located in the cleft between the N and C-terminal domains, but most of the catalytic residues are found in the larger C-terminal domain. In higher eukaryotes, CK exists in tissue-specific (muscle, brain), as well as compartment-specific (mitochondrial and cytosolic) isoforms. They are either coupled to glycolysis (cytosolic form) or oxidative phosphorylation (mitochondrial form). Besides CK and AK, the most studied members of this family are also other phosphagen kinases with different substrate specificities, like glycocyamine kinase (GK), lombricine kinase (LK), taurocyamine kinase (TK) and hypotaurocyamine kinase (HTK). The majority of bacterial phosphagen kinases appear to lack the N-terminal domain and have not been functionally characterized.