ATP and ADP are respectively the tri- and diphosphate esters of the nucleotide a

Question

ATP and ADP are respectively the tri- and diphosphate esters of the nucleotide adenosine, but the problem can be solved without using the structures, just from the values given of G for the ATP hydrolysis and glutamine forming reaction. Note; the problem mentions the “formation of glutamine from glutamate + NH4+, but of course it is not a ‘formation reaction”!

a) can the hydrolysis of ATP drive the glutamine forming reaction? Show your reasoning.

b) How many moles of ATP are hydrolyzed to form 1 mole of glutamine from glutamate?

Explanation / Answer

Ans. #a. Glutamate + NH4+ -----------> Glutamine + H2O        ; dG0’ = +14.0 kJ/mol

Since the reaction has positive dG0’ value, it is non-spontaneous.

# The dG0’ value of ATP hydrolysis is -30.5 kJ/mol. Since dG0’ of ATP hydrolysis is negative and numerically greater than that of dG0’ of glutamine synthesis, ATP hydrolysis can drive the reaction. The two reactions can be coupled as follow-

            Glutamate + NH4+ -----> Glutamine + H2O         ; dG0’ = +14.0 kJ/mol

            ATP + H2O -------------> ADP + Pi                       ; dG0 = -30.5 kJ/ mol

            ----------------------------------------------------------------------------

Glutamate + NH4+ + ATP ----> Glutamine + ADP + H2O          ; dG0’net = -16.5 kJ/mol

# Since the dG0’ value of coupled reaction (dG0’net) is negative, the overall coupled reaction is spontaneous.

#b. As shown above, 1 mol ATP is hydrolysed to form 1 mol glutamine.

Note that, the dG0’ value of ATP hydrolysis is sufficient to theoretically fuel the synthesis of 2 molecules of glutamine. However, each event of glutamine synthesis independently requires 1 ATP. The large negative dG0’net value of the coupled reaction is necessary to shift the equilibrium towards the right (product).

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