Can someone explain it very clearly how the hydrolysis of ATP into ADP + Pi provides energy for reactions to take place? I'm having difficulty visualising it in terms of Gibbs free energy. I can maybe understand how the free phosphate group might bind to an enzyme and phosphorylate it, invoking some sort of conformational change which would make it more favourable for a reaction to take place, but that doesn't seem to be how people generally explain it.
ATP hydrolysis provides energy that can drive otherwise unfavorable reactions. Although ATP is often described as having a “high-energy phosphate bond,” the energy does not come simply from breaking the bond. Instead, ATP hydrolysis to ADP + Pi is thermodynamically favorable because the products have lower free energy than ATP.The hydrolysis reaction has a negative ΔG, meaning it releases free energy. An unfavorable reaction, on the other hand, has a positive ΔG and requires an input of free energy. Cells can couple these two reactions so that the negative ΔG from ATP hydrolysis offsets the positive ΔG of the unfavorable reaction. If the combined ΔG is negative, the overall coupled reaction can proceed spontaneously.In this way, ATP hydrolysis acts as an energy source that allows many otherwise unfavorable cellular reactions to take place.
>>17040909Thank you.I guess my problem lies in conceptualising what a ΔG looks like, regardless of positive or negative.
>>17040914ΔG = ΔH - TΔSWhere:G = free energyH = enthalpy T = tempS = entropy The repulsion of like charges in the tri phosphate is a higher energy state than the di phosphate and inorganic phosphate (ATP has a higher H then ADP)Their are more ways to arange a two molecule system is than a one molecule system (ADP & Pi have greater S than ATP)For ATP -> ADP & PiΔH decreasesT is constant ΔS increases Therefore by ΔG = ΔH - TΔSΔG decreases (i.e. is negative)So the reaction is spontaneous
>>17040951Also this might help to visualize:ΔH - TΔS = ΔG = - RT ln(K)Where:R = the gas constantT = tempK = the ratio of starting materials vs products at equilibrium Some rxn (say A & B -> C & D) may favor an equilibrium with more starting material vs products. However, when coupled to the ATP <-> ADP & Pi rxn via an enzyme the strong favoring of ADP & Pi (vs ATP) pulls the ratios to favor C & D vs A & B at equilibrium.
>>17040957One last point, thanks for asking a science question. It's so nice to actually discuss science on /sci for a change.
>>17040958Thank you sir. I appreciate it.
I think the actual interesting question isn't how it's theoretically possible for this to occur. ATP has extra energy that you can get away from it in a relatively simple process. It's like saying gasoline burns and that's why cars go wroom. The interesting bit is how on earth does the cell actually redirect that energy into useful purposes.
