Reversibly catalyzes the transfer of phosphate between ATP and various phosphogens (e.g. creatine phosphate) (PubMed:5499971). 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 (Probable). ATP + creatine = ADP + H(+) + N-phosphocreatine Dimer of identical or non-identical chains, which can be either B (brain type) or M (muscle type). With MM being the major form in skeletal muscle and myocardium, MB existing in myocardium, and BB existing in many tissues, especially brain. Belongs to the ATP:guanido phosphotransferase family.