[Imo]

I have to relate the breakdown of ATP in ADP + P to this figure

{MOD Edit -- Remove image link}

How can i do this? Charges differents are in equilibrium?

[ramboacid]

Well, it looks to me that the magnesium cation stabilizes the charges on the phosphate groups. Since ATP releases energy by releasing a phosphate group, if the magnesium makes it less likely to decompose by stabilizing the phosphate groups, then I'd imagine that the presence of magnesium would shift the dissociation equilibrium to the left in the reaction ATP  ADP + Pi.

However, I am not a biochemist, so it would be prudent to get a second opinion on this topic. 

[Babcock_Hall]

My recollection is that Mg(II) binds to ATP more strongly than it binds to ADP; therefore it makes deltaG°'  for hydrolysis slightly less negative than it would be in the absence of Mg(II).  My recollection is that Mg(II) is not strongly catalytic with respect to the rate of nonenzymatic hydrolysis.  It does shift the pKa value of (H)ATP^3-.  However, most enzymes that catalyze phosphoryl group transfer use Mg(II), occasionally along with another metal ion, such as Zn(II) in alkaline phosphatase.