Question

The glycolytic reaction glyceraldehyde 3-phosphate + Pi → 1,3-bisposphoglycerate has a
ΔG°′ of +6.3 kJ/mol. In cells however, it is coupled to the subsequent reaction: 1,3- bisposphoglycerate + ADP → ATP + 3-phosphoglycerate which for which G°’= –18.5 kJ/mol Calculate the free energy change for these two reactions if they are coupled together.

This was also given if helpful:

Abbreviations for cofactors such as NADH, ATP etc. are sufficient unless the specific structure for that molecule is requested. Otherwise use complete names. Some potentially useful information:

E' = E°' – (RT/nF) ln Q F=96,484 J/V∙mol R=8.314 J/mol∙K

Answer 1

Delta G for the coupled reaction is described as the sum of delta g of constituent reactions. Here one reaction with G°’= –18.5 kJ/mol is spontaneous and it has been coupled with another reaction which is non-spontaneous with G°’= +6.3 KJ/mol. Hence the free energy change for both reactions coupled together would be

G= -18.5 KJ/mol + 6.3 KJ/mol = -12.2 KJ/mol

hence this reaction after coupling is spontaneous also where one reaction was non-spontaneous earlier.