Reversibly catalyzes the transfer of phosphate between ATP and various phosphogens (e.g. creatine phosphate) (PubMed:8525081, PubMed:20026305). 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. Expressed in almost all tissues and found enriched in various region of the brain, retina, heart, gizzard, gut and sperm. Ba-CK and Bb-CK are phosphorylated. The N-terminus of BA-CK is blocked. Belongs to the ATP:guanido phosphotransferase family.