danny_boy

When you go from:

Glyceraldehyde-3-Phosphate --->1,3 bisphosphoglycerate

There are 2NAD+ and 2Pi also reacting, and 2NADH as a product.

When calculating the free energy:

G=standard energy + RT ln (Keq)

To get Keq, we need concentrations of products and reactants. Now, nowhere, in any calculation have I ever seen a concentration for NAD+ or NADH used in these calculations. Are they so small, that we neglect them, or is an actual number used to represent their concentration ?

Thanx.

Poloboy

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)