Biochemistry question (delta G)

[shineornot]

Hi everyone,

For this question, I know you're supposed to flip the sign for the deltaG of GDP to GTP (so it'll be 3.3), but then do you add that to 33.8 or subtract? I think we're supposed to add:

During the metabolic conversion of succinyl-CoA to succinate in the TCA cycle, GTP is generated from GDP and inorganic phosphate: Succinyl-CoA + GDP + Pi → Succinate + GTP + CoA where ΔG°’ = -3.3 kJ/mol. Calculate the biochemical standard-state free energy change for the breakdown of GTP into GDP and inorganic phosphate, given that the ΔG°’ for Succinate + CoA → Succinyl-CoA is 33.8 kJ/mol.

I am getting 37.1 kJ/mol whereas if you do 3.3-33.8, you'd get -30.5 kJ/mol. I am just confused because if you add, then deltaG is positive making the reaction nonspontaneous and that doesn't sound right since it's being coupled to another reaction.

Which one is the right way to do it?

Thank you!

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[theonlytycrane]

Succinate + GTP + CoA → Succinyl-CoA + GDP + Pi : 3.3 kJ / mol

Succinyl-CoA → Succinate + CoA : -33.8 kJ / mol

Net = -33.8 kJ /mol + 3.3 kJ / mol = -30.5 kJ / mol. The free energy release should be negative, as you mentioned, because the phosphorylation (energy release) pays for another reaction.

 

[shineornot]

Succinate + GTP + CoA → Succinyl-CoA + GDP + Pi : 3.3 kJ / mol

Succinyl-CoA → Succinate + CoA : -33.8 kJ / mol

Net = -33.8 kJ /mol + 3.3 kJ / mol = -30.5 kJ / mol. The free energy release should be negative, as you mentioned, because the phosphorylation (energy release) pays for another reaction.

 

[theonlytycrane]

But I thought we are going from Succinate + CoA to Succinyl-CoA? Why would we have to flip this reaction? Aren't we just going backwards from the first reaction given with all the components?

The question is just asking for deltaG (std) for GTP -> GDP + Pi.

I rearranged the equations so that the sum would give deltaG (std) for GTP hydrolysis.

 

[aldol16]

Set up these sorts of problems using Hess's law. Write it out - there are few "laws" in science because few things obey some rule completely without exception. Hess's law always works.

 

[shineornot]

Set up these sorts of problems using Hess's law. Write it out - there are few "laws" in science because few things obey some rule completely without exception. Hess's law always works.

But there are two unrelated reactions. How would you use Hess's law here?

 

[shineornot]

The question is just asking for deltaG (std) for GTP -> GDP + Pi.

I rearranged the equations so that the sum would give deltaG (std) for GTP hydrolysis.

Okay, so we are just doing Succinyl-CoA + GTP --> Succinate + CoA + GDP + Pi? Because that's when it should be 3.3+ (-33.8)
So, we're doing the forward reaction for Succinyl as given in the first equation, and the backwards for GTP/GDP?

 

[shineornot]

Nevermind I got it now! Thank you all for all your help! I'm sorry it took me so long to figure it out!

Thanks again!!