ABSTRACT
Crystallographic studies have revealed in many cases that Mg2+ is bound to enzymes or substrates in part through water-mediated interactions. It is necessary to ask what the structural and functional implications are for water as a Mg2+ ligand in Mg2+- dependent enzymes. The first-order rate of exchange of Mg2-1-with water is of the order of 105s_1, which compares with 108 for Ca2+ and 3 x 107 for Mn2+ [2]. The slower exchange rate of Mg2+ with water compared with either Ca2+ or Mn2+ has direct implications for its role in enzymatic catalysis. The extensive study of metalligand interactions in relevant small molecules has shown Mg2+ to always have six coordinating oxygen ligands with a mean Mg2+-to-0 distance of 2.07 A [3].
