Question

3. Two amino acids can join together via a peptide bond to make a dipeptide. Below is an example of such a reaction.

glutamate + cysteine  γ-glutamylcysteine

ΔG°’ at 25°C for this reaction is 10 KJ/mol; assume 25°C in answering the following.

a. Does the ΔG°’ indicate that formation of γ-glutamylcysteine is favored? Explain your answer; no calculations required.

b. What is Keq’ for this reaction?

c. In certain conditions, the formation of γ-glutamylcysteine can be coupled to ATP hydrolysis.

ATP  ADP + Pi (ΔG°’=-30.5 kJ/mol).

What is the overall ΔG°’ for the coupled reaction? Is the formation of γ-glutamylcysteine favored?

d. What is Keq’ for the coupled reaction?

e. If glutamate, ATP, ADP, Pi are present at 20 mM, 1 mM, 0.2 mM and 3 mM, what is the equilibrium ratio of γ-glutamylcysteine to cysteine?

f. If γ-glutamylcysteine is removed such that the ratio decreases 10-fold and then 100-fold, what will be the resulting change in ΔG° (Δ ΔG°)?

Answer 1

3.

a. dGo indicated reaction is not favored. +ve sign is non-spontaneous reaction.

b. dGo = -RTlnKeq

10000 = -8.314 x 298 lnKeq

Keq = 0.018

c. Overall dGo for reaction = -30.5 + 10 = -20.5 kJ/mol

Reaction is favored.

d. -20500 = -8.314 x 298 lnKeq

Keq = 3921.50

e. Keq = [y-glutamylcysteine][ADP[Pi]/[glutamate][cysteine][ATP]

3921.50 = [y-glutamycysteine](0.2 x 3)/[cysteine](20 x 1)

[y-glutamycysteine]/[cysteine] = 1.31 x 10^5

f. If glutamylcysteine is removed, the resulting, dGo would increase accordingly 10-fold to 100-fold