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When ATP is used as the energy currency to make, say, reaction X + Y → Z happen, is what happens on a physical level down at the molecular scale that during the reaction

ATP + H2O → ADP + Pi       ΔG˚ = −30.5 kJ/mol (−7.3 kcal/mol)

that 30.5 kJ/ mol is conferred by ATP molecules physically bumping around the reactants X and Y, the kinetic energy of the above reaction being what does it?

I mean, is the energy coin of ATP conferred to reactions by molecular collisions, or is it an electric field effect in the spatial geometry the way the ATP molecule tends to break apart?

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up vote 18 down vote accepted

Usually in biology (and being ATP, it most probably is biology), it's one of two things.

The gamma-phosphate (the third one, the one farthest from the adenosine) is very unstable, meaning the phosphoanhydride bond is easy to break. The cell "allows" it to break, but only at the cost of moving the phosphate to some other molecule, such as a serine or glycerol or fructose or whatever. This phosphorylation creates a bond with lower energy than the phosphoanhydride, and so is overall favored. Imagine the personification: the gamma phosphate hates being attached to anything, but hates being attached to an ADP the most.

Alternatively, if ATP hydrolysis is coupled via an enzyme, it is usually done through transient storage of the energy is protein conformation. An enzyme binds ATP, which makes the protein structure "bend" or conform around the ATP. This puts loads of strain (energy = A) on the protein which is offset by the stabilization of binding the ATP (energy = B). This strain can make an enzymatic surface open up on the protein which itself takes a lot of energy to make (energy = C). The surface can catalyze some reaction (X+Y->Z in your example) that costs some energy (energy = D). The completion of that reaction alters the enzyme's catalytic site to something new and higher energy (energy = E), which can be alleviated by cleavage of the ATP (-7.3 kcal/mol). Alas, ADP and P do not fit well into that site of the enzyme, so they float out, restoring that original ATP-binding surface to it's original state. Provided A>B>C>D>E>-7.3, the cycle will continue until the ATP is exhausted or you have no more Z to make.

Typing "enzyme catalysis cycle ATP" gives a few examples. Here's a few:
DNA gyrase
Actin-myosin cycle

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Another interesting scheme of the myosin functioning – nico Jan 11 '12 at 7:12
@KAM You're arguing from the basis of a thermodynamic ensemble, which really makes your argument more "on a biochemical level" than "on a physical level". I want to know, electron by electron, particle by particle, exactly mechanistically how the potential energy inherent in ATP flows from point A to point B. Where do I go for that answer? – tel Apr 23 '12 at 18:54
ADP also contains a 'high energy' bond where ΔG˚ for hydrolysis is about −30 kJ/mol. I presume you are not implying otherwise. That is, the 'beta-phosphate' may also be thought of as being 'very unstable'. As I understand things, the (free) energy 'currency' of ATP arises from the fact that the ATP = ADP +Pi reaction is maintained far from equilibrium in the cell. – TomD Nov 1 '13 at 19:17

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