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The problem states:
The following reaction is catalyzed by the enzyme phosphoglycerate kinase
1,3-bisphosphoglycerate + ADP > 3-phosphoglycerate + ATP

Change in Standard free energy for this reaction is -4.5 kcal/mol. Assuming that the value of [ATP]/[ADP] is 10 and the temperature is 25degreesC, calculate the concentration ratio of 3-phosphoglycerate to 1,3-bisphosphoglycerate at equilibrium.

I tried working out Keq. I can use the standard free energy formula SFE= -RTlnKeq but i'm getting confused on the units. SFEnergy is in kcal/mol. the gas constant R, when in those same units is .0019. Temp is 25degC (or298K). But when plugged in to the SFE formula, it comes out to be 1808. Is that an ok value for Keq?

And then, how do i use that to calculate the concentrations? It has something to do with the atp/adp =10 but i can't figure out how it all fits together. Would i use Keq= AxB/CxD and since B is 1 and D is 10 (the 10 from ATP/ADP)  say 1x/10x=.01x then plug in...
keq=.01x
1808=.01x
x=180800
Then that times 1 gives the concentration of 1,3-bisphosphoglycerate, and that times 10 gives concentration of 3-phosphoglycerate? Surely it can't be that simple...

This is for my Cell Bio class. Chemistry and math aren't really my best subjects = ) Any help would be very much appreciated.

[Dan]

the gas constant R, when in those same units is .0019. Temp is 25degC (or298K).

R = 1.9858775 cal K^-1 mol^-1

This rounds to 0.0020 kcal K^-1 mol^-1 not 0.0019

Notice the units of R are in kcal K^-1 mol^-1, so temperature should be in K, not ^oC

But when plugged in to the SFE formula, it comes out to be 1808. Is that an ok value for Keq?

I get 2.0 x 10³ (using R = 1.99 cal K^-1 mol^-1), this is a plausible magnitude.

Would i use Keq= AxB/CxD and since B is 1 and D is 10 (the 10 from ATP/ADP)  

Yes you're on the right track, but remember for a reaction A+B->C+D:

K=[C][D]/[A][B ]   ie. [Products]/[Reactants]

The question is asking for a the concentration ratio [C]/[A], it is given that [D]/[B ] = 10