Phosphagen (guanidino) kinases such as creatine kinase and similar enzymes.
Eukaryotic creatine kinase-like phosphagen (guanidino) kinases are enzymes that transphosphorylate a high energy phosphoguanidino compound, like phosphocreatine (PCr) in the case of creatine kinase (CK), 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, CKs are found as tissue-specific (muscle, brain), as well as compartment-specific (mitochondrial, cytosolic, and flagellar) isoforms. Mitochondrial and cytoplasmic CKs are dimeric or octameric, while the flagellar isoforms are trimers with three CD domains fused as a single protein chain. CKs are either coupled to glycolysis (cytosolic form) or oxidative phosphorylation (mitochondrial form). Besides CK, one of the most studied members of this family, this model also represents other phosphagen kinases with different substrate specificities, like glycocyamine kinase (GK), lombricine kinase (LK), taurocyamine kinase (TK), and echinoderm arginine kinase (AK).