An enzyme complex that can reversibly convert ATP and creatine to phosphocreatine and ADP. Cytosolic creatine kinases are comprised of homodimers or heterodimers of creatine kinase B-type protein and creatine kinase M-type protein. Mitochondrial creatine kinases are octomers comprised of either four homodimers of creatine kinase U-type, mitochondrial protein or four creatine kinase S-type, mitochondrial protein homodimers. [ ]