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I am confused as to what the role of PCr in muslce contrraction actually is. I understand that it used to to convert ADP to ATP, however I do not understand where the PCr comes from. Does the PCr not come from respiration itself in a reverse of the reaction described above? Consequently, what is the point of using ATP to produce PCr and then using Pcr to produce ATP? It doesnt make sense to me; please can someone explain?
Creatine-phosphocreatine interconversions are most important in cells that go back and forth between high and low levels of metabolic activity, such as in resting vs active muscle cells. Creatine kinase operates at near equilibrium making the net reaction very sensitive to metabolic flux. In resting muscle, when ATP levels are high, the reaction is driven toward phospocreatine, consuming ATP in the process. Conversely, when the muscle is active ATP levels fall. Now phosphocreatine acts as a high-energy phosphate sink as creatine kinase runs in the other direction, generating needed ATP from phosphocreatine and excess ADP. It isn’t a waste of ATP to consume it in the synthesis of phosphocreatine if you can get it back (for the most part) whenever you need it.
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