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 Exists as an octamer composed of four MTCK homodimers. Highly expressed in brain and gut, weakly in testis. Also found in cell bodies of neural cells of the gray matter. Mitochondrial creatine kinase binds cardiolipin. Belongs to the ATP:guanido phosphotransferase family.