How does ATP provide the energy for muscle contraction?
The immediate source of energy for skeletal muscle during work is found in preformed organic compounds containing high-energy phosphates, such as ATP and creatine phosphate (CrP). CK or creatine phosphokinase helps maintain intracellular ATP concentrations by catalyzing the reversible transphosphorylation of creatine and adenine nucleotides and by modulating changes in cytosolic ATP concentrations.
At rest, when there is excess ATP, the terminal phosphate of ATP is transferred to creatine, forming CrP and adenosine diphosphate (ADP) in a reaction catalyzed by CK. CrP serves as a reservoir of high-energy phosphate. With muscle activity and ATP utilization, CK catalyzes the transfer of those phosphates from CrP to rapidly restore ATP levels to normal. CrP stores are sufficient to allow the rephosphorylation of ADP to ATP for only a few minutes of exercise.
Thus, CK along with its products, creatine and CrP, serve as a shuttle mechanism for energy transport between mitochondria, where ATP is generated by oxidative metabolism (Krebs cycle and respiratory/cytochrome chain), and the myofibrils, where ATP is consumed during muscle contraction and relaxation