KNR 280 Study Guide - Glycogen, Cytosol, Citric Acid Cycle

a) Label the following reactions as spontaneous or nonspontaneous:

creatine phosphate + H₂O ⇋ creatine + P_i

ADP + P_i ⇋ ADP + H₂O

b) Calculate the ΔG° and K_eq for the following reactions (i and ii). Are they spontaneous or nonspontaneous?

i) creatine phosphate + ADP ⇋ creatine + ATP

ΔG°:

K_{eq :}

Spontaneous: Yes/No

ii) creatine + ATP ⇋ creatine phosphate + ADP

ΔG°:

K_{eq :}

Spontaneous: Yes/No

The reaction phosphocreatine + ADP creatine + ATP has ΔG (standard conditions) of 10.3 kilocals/mol.Calculate ΔG at 25 degree C, pH 7.0 if the concentration of thereactants and products are: phosphocreatine 4.7mM; creatine 1.0mM;ADP 0.20mM; ATP 2.6mM.

Which value ΔG°' or ΔG is of greater physiologicalsignificance? Why?

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?