Creatine (Cr) is made in the liver and kidneys and then
transported to the muscles, where it serves as a high energy
storage compound via phosphorylation by creatine kinase. Two forms
of this enzyme exist, mitochondrial and cytoplasmic. In
mitochondria there is net synthesis of creatine phosphate (CrP). In
the cytosol, the phosphate is transferred from CrP to ADP to supply
ATP to power the muscles. Table 3.2 of your G&G text lists ΔG˚́
for hydrolysis of creatine phosphate: -43.3 kJ/mol at 25°C.
creatine phosphate + H2O —-> creatine + Pi
a. Calculate the Keq́ for this hydrolysis.
c. In mouse muscle cells at 25°C, [Cr] is 10 mM, and [CrP] is 20
mM. What concentration of Pi would be needed to maintain this level
of phosphorylation by reversal of the hydrolysis reaction? Is this
concentration of Pi a reasonable expectation? Why?
d. Coupling ATP hydrolysis (ΔG˚́-35.7 kJ/mol at 25°C) to an
unfavorable reaction is often used to drive biochemical reactions.
Write the chemical equations for the CrP reverse hydrolysis and ATP
hydrolysis and add them together to give the coupled reaction. Then
calculate ΔG˚́for the coupled reaction. Is this reaction favorable?
Justify your answer.
e. Considering ΔǴin the muscle cells rather than under 1M
standard state conditions, what ratio of ATP/ADP would be needed
inside mitochondria to make the coupled reaction for CrP synthesis
produce the concentrations of Cr and CrP observed in mouse muscle?
Is this ratio reasonable? Why?