Why is ATP synthesis reversible on the F1 subunit??? What is the point of this?

dwilmer

Why is ATP synthesis reversible on the F1 subunit??? What is the point of this?

They did some experiment a long time ago where they took F1 subunit and mixed it with ATP and water, with the Oxygen radioisotope marked.

Then they watched the reaction and noticed that after ADP with the Pi was made, all 4 oxygens on the Pi had the marker on them.

So they surmised that reaction must be reversible because how else could all the oxygens be the same?

Ok, i understand that, i think. But heres what confuses me...

The whole point of ATP synthase is to MAKE ATP. So why did they do the experiment starting with with ATP and water, unless they already suspected rxn was reversible and they were just tryng to prove it? I mean, they were studying how ATP is made right? So wouldnt the normal thing to do would be to start with reactants ADP + Pi + F1 subunit and then see what happens? Instead, they started with exact opposite reactants (ATP and water and F1)..

My other question is: What is the advatage of the reaction being reversible, --for the molecule i mean?

thank you for any help..

alxm

Why wouldn't it be reversible? This is what you'd expect from the basic chemical thermodynamics of the thing.

Monique

Do you have a reference to the experiment that was performed?

As for the reversibility of the reaction, that should be obvious. What use would there be in storing energy, if the energy could never be released again? Of course the container in which the energy is stored should be stable until the energy is actually required, this is probably what puzzles you?

With the reaction set up the researchers probably were trying to learn something about the kinetics of the reaction, but I can't say for sure without a reference.

Ygggdrasil

It would be more convenient for the enzyme to work only in the forward dirrection, but unfortunately, enzymes cannot break the laws of nature. Enzymes work by lowering the energy of the transition state which can only increase the rate of the reaction (in both dirrections) but cannot alter the thermodynamics of the reaction (i.e. whether products or reactants are preferred).

Obviously it is more favorable to hydrolyze ATP than to synthesize ATP, else other reactions in the cell could not derive free energy from the hydrolysis of ATP. ATP synthase is able to synthesize ATP because it couples the energy of proton flow across the mitochondrial inner membrane to drive ATP synthesis (similar to how a dam harnesses the energy of water flowing downhill to generate electricity). Since it is technically difficult to setup a similar proton gradient in a lab, most experiments on ATP synthase examine the reverse reaction. In this case, the enzyme will be hydrolyzing ATP to pump protons through the enzyme.

PTM19

Whether a given reaction is reversible or not mostly depends on the energy content of the substrate and product molecules.

In the case where the product has less chemical energy the reaction can proceed spontaneously and extra energy is released as kinetic and vibrational energy which quickly spreads among molecules heating up reaction medium.

In the case where the product has more chemical energy then the substrate the energy has to be provided and reaction can only take place if the difference is smaller then the thermal energy available to the molecules in the medium (kinetic and vibrational energy which depends on the temperature).

This means that all reactions in which difference in energy between the substrate and the product is smaller then available thermal energy will be reversible and able to run both ways when catalyzed by enzymes. The actual direction will depend on substrate and product concentrations and affinity of the enzyme for substrate and product.

When cells need to force reversible reactions is one particular direction or perform reactions which cannot occur spontaneously they couple them to other irreversible reactions like hydrolysis of ATP so that the summary reaction releases more energy then is available in the medium.

To force ATP synthesis cells couple it to another irreversible reaction - flow of protons through the membrane from the side of higher concentration to the side of lower concentration. Proton concentration is kept high on one side of the membrane by respiration. Those protons flow through a channel in the ATP synthase enzyme which is located in the membrane and their flow drives the enzyme to rotate in the direction forcing synthesis of ATP.

However when F1 subunit is decoupled from the proton channel there is nothing to force the reaction in the synthesis direction and ATP is being hydrolyzed.

So the hydrolysis is unavoidable but during normal operation the enzyme is tightly controlled by the cell to make sure it synthesizes much more ATP then it hydrolyzes.