[galpinj]

Quick question about ATP formation from ADP

There are a lot of diagrams on the formation of ATP from ADP through substrate level phosphorylation, but I haven't found anything to explain the chemistry behind the change. From what I understand, there is a high-energy intermediate with a phosphate that transfers its phosphate to ADP (with the help of an enzyme) to form ATP.

My question is, even if the high-energy intermediate is unstable, it will take energy to break the bond (as it takes energy to break any bond) between the phosphate and this intermediate. I'm also aware that it's very unfavorable to attach the phosphate to ADP (negative charges repulse the two, loss of resonance, etc), so where exactly is the energy coming from to make ATP? If the "high energy intermediate" needs energy to break the bond, how and from where is it supplying energy to form ATP?

[subro]

I think the energy comes from the other reaction. For example:



In this case, the exergonic keto-enol tautomerization of piruvate can drive the reaction to the right and form ATP.

[galpinj]

Hey Subro,

I think I'm a little bit out of my comfort zone here; this is the first time I've ever heard of tautomerization! So, basically, you believe the energy is provided from the conversion of the enol (O^-) to the ketone (C=O)? Why is this conversion more stable or energetically favorable?

Thanks for the help

[subro]

Substrate level phosphorylation happens during an enzymatic reaction that involves a substrate (succynil-CoA, PEP, and so on). When the substrate turns into a product, if the reaction is exergonic (spontaneus), it "releases" some energy that can be used to turn ADP into ATP. Actually, there is no energy released, but a complex mechanism that involves some intermediates that turns the ADP phosphorylation exergonic.

Keto form are usually more stable that enol form, so this fact makes the PEP reaction exergonic.

[Babcock_Hall]

It seems to me that we are mixing together two very different things.  Chemical reactions have an activation energy, and this activation energy is lower for catalyzed reactions than uncatalyzed ones, but the activation energy is not zero.  Pyruvate kinase is the catalyst for the conversion of PEP and ADP to pyruvate and ATP. 

The concept of substrate-level phosphorylation is concerned with the equilibrium constant for the reaction (which is intimately related to ΔG°'), not with the kinetics of the reaction.  For your specific example, we can try to answer the question of why the equilibrium constant for reaction catalyzed by pyruvate kinase is much larger than one.  The textbook explanation is that the tautomerization of the enol form to the ketone form of pyruvate is responsible for much of the favorable change in standard free energy.  If you search through posts here of about 1-3 years old, you will find a reference to an article that supports the standard explanation.
EDT
There are other substrate-level phosphorylations, such as in the reaction catalyzed by phosphoglycerate kinase.  Tautomerization does not play a role in that reaction, however.