text: The chemistry of the Mg ion, as applied to enzymes, uses the full range of this ions unusual reaction chemistry to fulfill a range of functions. Mg interacts with substrates, enzymes, and occasionally both (Mg may form part of the active site). In general, Mg interacts with substrates through inner sphere coordination, stabilising anions or reactive intermediates, also including binding to ATP and activating the molecule to nucleophilic attack. When interacting with enzymes and other proteins, Mg may bind using inner or outer sphere coordination, to either alter the conformation of the enzyme or take part in the chemistry of the catalytic reaction. In either case, because Mg is only rarely fully dehydrated during ligand binding, it may be a water molecule associated with the Mg that is important rather than the ion itself. The Lewis acidity of Mg (pK' 11.4) is used to allow both hydrolysis and condensation reactions (most common ones being phosphate ester hydrolysis and phosphoryl transfer) that would otherwise require pH values greatly removed from physiological values.