biochemistry - Why is a magnesium ion essential for ATP activity in enzymic reactions?

The Wikipedia entry on Magnesium in Biology includes the following:

ATP (adenosine triphosphate), the main source of energy in cells, must be bound to a magnesium ion in order to be biologically active. What is called ATP is often actually Mg-ATP.

I don't understand why magnesium is essential for breaking the high energy bond in ATP?

Answer

As far as I know there are two main reasons for this:

• promoting a nucleophilic attack

In general, Mg2+ interacts with substrates through inner sphere coordination, stabilising anions or reactive intermediates, also including binding to ATP and activating the molecule to nucleophilic attack (wiki)

- assists in the leaving a pyrophosphate group and may also facilitate the cleavage of the bond between the α- and β-phosphates by its electron-withdrawing effect. (source)

wouldn't the presence of mg hinder release of phosphate?

As with all reactions this is dependent of the concentration of the molecules involved as stated here:

The Gibbs free energy change (ΔG) due to ATP hydrolysis depends upon the concentrations of the various participants in the reaction. When the concentrations are farther from their equilibrium values, the absolute value of ΔG is greater. Under “standard” conditions (i.e. concentrations of 1M for all reactants except water which is taken at its characteristic concentration of 55M) the Gibbs free energy of ATP hydrolysis varies from -28 to -34 kJ/mol (i.e. ≈12 kBT, BNID 101989) depending on the concentration of the cation Mg2+. The dependence on Mg ions occurs because the positively-charged magnesium ions help to stabilize the ATP molecule. However, in the cell the conditions are never even close to the standard state values. (source)