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Well, RTln(Keq) = standard free energy, making DeltaG zero, as it always is at equilibrium (by definition).
I think you mean DeltaG = standard energy + RTln(Q) where Q is the concentration of prod/react. under the given conditions. In this case, to calculate the actual free energy change (DeltaG) for the G3P dehydrogenase reaction, I think you certainly do need to include the concentrations of NAD/NADH and Pi. I know that the NAD/NADH ratio in particular has a significant impact on the flow of this reaction step under cytosolic conditions. Where did you find sample calculations not including these?
As I understand it, the following should be the means of calculating the actual free energy change for an oxidation-reduction reaction like this:
1. Find the standard reduction potentials (E-prime-naught, I don't know how to do superscript on this forum) for the reduction of 1,3-BPG to G3P and the reduction of NAD to NADH
2. Subtract one reaction from the other to get the DeltaE`0 (that's E-prime-naught, the overall standard reduction potential of the reaction).
3. Use this to calculate the standard free energy change of the reaction, according to this equation:
DeltaG`0 = -n*F*DeltaE`0
where n is # of electrons (2 in this reaction) and F is the Faraday: 96.5 kJ/(V mol)
4. Now that you have the standard free energy change (DeltaG`0) you can find the actual free energy change by using the correct form of the reaction you referenced:
DeltaG = DeltaG`0 +RTln(Q)
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