text: Mg is the fourth-most-abundant metal ion in cells (per moles) and the most abundant free divalent cation — as a result, it is deeply and intrinsically woven into cellular metabolism. Indeed, Mg-dependent enzymes appear in virtually every metabolic pathway: Specific binding of Mg to biological membranes is frequently observed, Mg is also used as a signalling molecule, and much of nucleic acid biochemistry requires Mg, including all reactions that require release of energy from ATP. In nucleotides, the triple-phosphate moiety of the compound is invariably stabilized by association with Mg in all enzymatic processes.