Reversibly catalyzes the transfer of phosphate between ATP and various phosphogens (e.g. creatine phosphate) (PubMed:8186255). 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). Acts as a key regulator of adaptive thermogenesis as part of the futile creatine cycle: localizes to the mitochondria of thermogenic fat cells and acts by mediating phosphorylation of creatine to initiate a futile cycle of creatine phosphorylation and dephosphorylation (By similarity). During the futile creatine cycle, creatine and N-phosphocreatine are in a futile cycle, which dissipates the high energy charge of N-phosphocreatine as heat without performing any mechanical or chemical work (By similarity). 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. Localizes to the mitochondria of thermogenic fat cells via the internal MTS-like signal (iMTS-L) region. The internal MTS-like signal (iMTS-L) mediates targeting to mitochondria thermogenic fat cells. Belongs to the ATP:guanido phosphotransferase family. Creatine kinase entry