* Creatine kinase (CK): This enzyme catalyzes the reversible reaction between creatine phosphate (CP) and ADP to form ATP and creatine. This reaction is the primary source of ATP in the phosphagen system, providing energy for short bursts of intense activity.

* Adenylate kinase (AK): This enzyme catalyzes the transfer of a phosphate group from one ADP molecule to another, producing ATP and AMP. This reaction is less efficient than the CK reaction but can contribute to ATP production during intense exercise.

Therefore, creatine kinase is the primary enzyme responsible for ATP production in the phosphagen system, while adenylate kinase plays a supporting role.