Which of the following statements concerning creatine kinase is false?

• A. Rises within 4-6 hours after acute myocardial infarction
• B. Catalyzes the phosphorylation of creatine by ATP
• C. Requires Ca2+ for activity
• D. Found mainly in skeletal and cardiac muscles and in brain tissue

Answer: (C)

Creatine kinase catalyzes the transfer of a phosphate from ATP to creatine, forming phosphocreatine and ADP, and it requires Mg2+ as a cofactor. Calcium ions are not required for this catalytic activity, so CK is not Ca2+-dependent. Clinically, CK rises after myocardial infarction because damaged heart cells release the enzyme into the bloodstream, with the CK-MB fraction increasing early, typically within 4–6 hours. CK is found predominantly in skeletal muscle, cardiac muscle, and brain tissue, corresponding to its isoforms CK-MM, CK-MB, and CK-BB. Therefore, the statement that CK requires Ca2+ for activity is the false one.