Reversibly catalyzes the transfer of phosphate between ATP and various phosphogens (e.g. creatine phosphate). Creatine kinase isoenzymes play a central role in energy transduction in tissues with large, fluctuating energy demands, such as skeletal muscle, heart, brain and spermatozoa. ATP + creatine = ADP + H(+) + N-phosphocreatine Dimer of identical or non-identical chains. With MM being the major form in skeletal muscle and myocardium, MB existing in myocardium, and BB existing in many tissues, especially brain. This electric ray muscle-specific creatine kinase (MM isozyme) is isolated from the electric organ, which derives embryologically from skeletal muscle. It may be involved in the electrical discharge process. Belongs to the ATP:guanido phosphotransferase family.