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ATP Hydrolysis (the goal)
The end reaction that drives energy requiring processes such as muscle contraction is the hydrolysis of ATP. ATP and water are reacted to produce adenosine diphospate (ADP), inorganic phosphate (Pi), a proton (hydrogen ion) and the energy that drives the desired process. Note that at the pH level of normal cell conditions the proton is released as shown. This is often omitted in biochemistry texts which means a major cause of acidosis is overlooked. The protons acidify the cellular environment and contribute to fatigue.
ATP hydrolysis
When the cell pH falls below 6.9 the proton can attach to the Pi molecule. However, this happens only in small amounts.
Regeneration of ATP
Creatine Kinase Reaction (CK)
This reaction uses cellular stores of creatine phosphate (CrP) to convert ADP back into ATP. It also consumes a proton and thus provides buffering against acidosis. The CK reaction is the quickest way to generate ATP in the cell but also the most limited as it uses up stores of CrP. It is thought to be useful for brief intense activity or as a buffer for a sudden change to a higher intensity. The CK reaction works both ways depending on the environment within the cell. The reaction reverses to replenish CrP during recovery. The CK reaction is also thought to be useful in phosphate management - "creatine phosphate shuttle", but this is not discussed here.
Creatine Kinase Reaction (CK)
Adenylate Kinase Reaction (AK)
With increasing exercise intensity the AK reaction also increases its activity. This reaction combines 2 ADP molecules to produce an ATP molecule and an adenosine monophosphate (AMP) molecule. The presence of AMP in the cell helps stimulate increased rates of glycolysis (discussed later).
Adenylate Kinase Reaction (AK)
AMP Deaminase Reaction
Some of the AMP produced by the AK reaction is converted into inosine monophosphate (IMP) and ammonia (NH4) with the addition of a proton. Because of the consumption of a proton this reaction also provides some buffering to acidosis.
AMP Deaminase Reaction
The generation of AMP is associated with an intense level of activity and a failure of the mitochondrial respiration (discussed later) to supply enough ATP.
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