Two amino acids can join together via a peptide bond to make a dipeptide. Below
ID: 982195 • Letter: T
Question
Two amino acids can join together via a peptide bond to make a dipeptide. Below is an example of such a reaction.
glutamate + cysteine -> -glutamylcysteine
G°’ at 25°C for this reaction is 10 KJ/mol; assume 25°C in answering the following:
a. Does the G°’ indicate that formation of -glutamylcysteine is favored? Explain your answer; no calculations required.
b. What is Keq’ for this reaction?
c. In certain conditions, the formation of -glutamylcysteine can be coupled to ATP hydrolysis.
*ATP -> ADP + Pi (G°’=-30.5 kJ/mol).*
What is the overall G°’ for the coupled reaction? Is the formation of -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 -glutamylcysteine to cysteine?
f. If -glutamylcysteine is removed such that the ratio decreases 10-fold and then 100-fold, what will be the resulting change in G° ( G°)?
Explanation / Answer
a) Does the G°’ indicate that formation of -glutamylcysteine is favored? Explain your answer; no calculations required.
So the delta G indicates the reaction is not favoured as the delta G is a positive value it means that energy has to be supplied to get the reaction to work.
b) delta G = - RT ln Keq
10000 = - 8.314 x 298 ln Keq
Keq = e-4.03
Keq = 0.017
c) In coupling with ATP the delta G = delta G1+ delta G 2
delta G = 10kJ -30.5 kJ
deta G = -20.5 kJ
Since the delta G is now negative it is a favoured reaction