atp synthase and gibbs free energy

[jessa234]

Hi,

I'm trying to answer a question that i'm stuck on. The question is as follows:

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If the Gibbs free energy change (deltaG) for ATP hydrolysis in a cell is -57 kJ/mol and the free energy change for transporting a proton from the cytoplasm/inter-membrane space into the mitochondrial matrix is -21.5 kJ/mol

a) What is the minimum number of c-subunits that an ATP synthase needs in order to synthesize rather than hydrolyze ATP.
b) If the ATP synthase that you described in part (a) had its OSCP subunit removed, would it still synthesize ATP in the conditions described above? Why or why not?

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For a), I put that the minimum number of c-subunits is 3 because since atp hydrolysis has a deltaG of -57 kJ/mol, then atp synthesis must have deltaG = +57 kJ/mol. We need the total free energy from transporting protons to be greater in magnitude than the deltaG of atp synthesis - eg. the overall process must be negative. so -21.5 * 3 = -64.5 kJ/mol, and the overall free energy change (from both the proton transport and atp synthesis) is -7.5 kj/mol. Does my reasoning make sense?

i'm not sure how to answer part b). As far as I know, the OSCP subunit anchors the F1 so that it does not rotate along with the gamma subunit, but I'm not sure how removing it would affect ATP synthesis. any help is much appreciated!

[feniouk]

Hi jessa 234,

Originally Posted by jessa234

Hi,
If the Gibbs free energy change (deltaG) for ATP hydrolysis in a cell is -57 kJ/mol and the free energy change for transporting a proton from the cytoplasm/inter-membrane space into the mitochondrial matrix is -21.5 kJ/mol
a) What is the minimum number of c-subunits that an ATP synthase needs in order to synthesize rather than hydrolyze ATP.
b) If the ATP synthase that you described in part (a) had its OSCP subunit removed, would it still synthesize ATP in the conditions described above? Why or why not?

The hydrophilic catalytic F1-domain of ATP synthase has 3 catalytic sites. So one 360 degree revolution of the c-ring oligomer with subunits gamma-epsilon-etc. corresponds to 3 ATP (or -57*3= -171 kJ per 3 mol).
If every c-subunit translocates 1 proton, and every mol of protons gives -21.5kJ: -171/-21.5 = 7.9535, so 8 c-subunits are the minimal number per one ATP synthase complex.


Regards,

Boris.

[feniouk]

Hi jessa 234,

Originally Posted by jessa234

Hi,
If the Gibbs free energy change (deltaG) for ATP hydrolysis in a cell is -57 kJ/mol and the free energy change for transporting a proton from the cytoplasm/inter-membrane space into the mitochondrial matrix is -21.5 kJ/mol
a) What is the minimum number of c-subunits that an ATP synthase needs in order to synthesize rather than hydrolyze ATP.
b) If the ATP synthase that you described in part (a) had its OSCP subunit removed, would it still synthesize ATP in the conditions described above? Why or why not?

The hydrophilic catalytic F1-domain of ATP synthase has 3 catalytic sites. So one 360 degree revolution of the c-ring oligomer with subunits gamma-epsilon-etc. corresponds to 3 ATP (or -57*3= -171 kJ per 3 mol).
If every c-subunit translocates 1 proton, and every mol of protons gives -21.5kJ: -171/-21.5 = 7.9535, so 8 c-subunits are the minimal number per one ATP synthase complex.
See http://www.atpsynthase.info for more details.

Regards,

Boris.