Which CK isoenzyme is the mitochondrial form?

• A. CK-MM
• B. CK-BB
• C. CK-MV
• D. Mitochondrial CK

Answer: (D)

Creatine kinase has several forms that match where they work in the cell. The main cytosolic forms are CK-MM in skeletal muscle, CK-MB in heart, and CK-BB in brain. These operate in the cytosol and help shuttle phosphate groups for quick energy use. There is also a distinct mitochondrial form of creatine kinase located in the mitochondria, which plays a key role in the phosphocreatine shuttle that transfers high-energy phosphate between mitochondria and the cytosol to keep ATP supply rapid where it’s needed. Since the question asks for the isoenzyme that is mitochondrial, the mitochondrial CK designation is the correct one. The other forms are tied to specific tissues and are cytosolic, not located in mitochondria.