Can a hydroxide ligand trigger a change in the coordination number of magnesium ions in biological systems?

Density functional (B3LYP) calculations indicate that a hydroxide ligand is capable of triggering a reduction in the coordination number of Mg(2+) ions from 6 to 5. Since this could be quite relevant in the mode of action of magnesium-containing enzymes (especially hydrolases in which a metal-bound hydroxide species is believed to play a crucial role), we have performed a systematic deprotonation study of biologically relevant magnesium complexes. We explicitly calculated the preferred coordination number of [MgL(1)(x)L(2)(y)L(3)(z)](2)(-)(n) species at the B3LYP/aug-cc-pVTZ level of theory. L(1), L(2), and L(3) represent combinations of water, hydroxide, carboxylate (models Glu and Asp), ammonia ligands (models Lys and His residues), and fluoride ions. As expected, Mg(2+) exclusively prefers an octahedral coordination geometry with H(2)O, HCO(2)(-), or NH(3). Surprisingly, one hydroxide ligand triggers a change to a trigonal bipyramidal geometry. The isoelectronic fluoride ion behaves similarly. When two OH(-) are present, a tetrahedral coordination geometry is preferred. We postulate that a hydroxide (in addition to its role as an active nucleophile) could be employed by magnesium-containing enzymes to trigger a differential coordination behavior.