Question

Although not part of glycolysis, endogenous stores of phosphocreatine in skeletal muscle are suggested to provide a reserve of ATP through substrate-level phosphorylation, as shown below:

a)       Phosphocreatine + H₂O ? Creatine + P_i + 10.3 kcal/mol

b)      ADP + P_i + 7.3 kcal/mol ? ATP + H₂O

Add these two reactions together (including the energy terms) to show the net, coupled reaction

Answer 1

a) Phosphocreatine + H₂O ? Creatine + Pi + 10.3 kcal/mol

b)      ADP + Pi + 7.3 kcal/mol ? ATP + H2O

1) Adding the two reactions,

Phosphocreatine + H₂0 + ADP + Pi +7.3 K cal/mlàCreatine+ Pi + ATP + H₂0 + 10.3 Kcal/mol

2) Deleting H₂0 from both sides of the reaction as it is common,

Phosphocreatine + ADP + Pi +7.3 K cal/mlàCreatine + Pi + ATP + 10.3 Kcal/mol

3) Inorganic phosphate is common on both sides, hence can be deleted from both sides

Phosphocreatine + ADP + 7.3 K cal/mlàCreatine + ATP + 10.3 Kcal/mol

4) This is an exothermic reaction, as more energy is released than utilized. The amount of excess energy is -10.3+7.3= -3.0 Kcal/mol. This energy will be shown in right side of reaction, as it is released (due to -ve sign). Energy is released for the reaction to occur.

Phosphocreatine + ADP àCreatine ATP + 3.0 Kcal/mol

5) Hence, net reaction of the two reaction shown is,

Phosphocreatine + ADP àCreatine + ATP + 3.0 Kcal/mol

It can also be written as:

Phosphocreatine + ADP àCreatine + ATP        ?Go’ = 3.0 Kcal/mol