Inactivation of creatine kinase is due to the conformational changes of the active sites during thermal denaturation.

The conformational changes of the active site of creatine kinase (ATP: creatine N-phosphotransferase EC 2.7.3.2.) during thermal denaturation was followed by changes in fluorescence at the active site of the enzyme labeled by o-phthalaldehyde. Conformational changes of the active site occurred at the same time as inactivation of the enzyme. The active site changes occurred before the denaturation of the enzyme molecule as a whole was detected. The above results showed that the thermal inactivation of the creatine kinase was due to the conformational changes of its active sites.