Why Cellular Di/Triphosphates Preferably Bind Mg2+ and Not Ca2+

Di/triphosphates perform a multitude of essential tasks, being important components of many vital organic cofactors such as adenosine/guanosine di/triphosphate (ADP/GDP, ATP/GTP), flavin adenine dinucleotide, and nicotinamide adenine dinucleotide and its phosphate derivative. They are generally bound to cations inside cells, in particular Mg2+ in the case of ATP/GTP. Yet how their metal-binding modes depend on the number, charge, and solvent exposure of the polyphosphate group and how Mg(2+)and Ca2+ dications that coexist in cellular fluids compete for di/triphosphates in biological systems remain elusive. Using density functional theory calculations combined with a polarizable continuum model, we have determined the relative free energies and stabilities of the different binding modes of di- and triphosphate groups to Mg2+ and Ca2+. We show that the thermodynamic outcome of the competition between Mg2+ and Ca2+ for cellular di/triphosphates depends mainly on the oligomericity/charge and metal-binding mode of the phosphate ligand as well as the solvent exposure of the binding site. Increasing the charge and thus denticity of the phosphate ligand from bi- to tridentate in a buried binding pocket enhances the affinity of the host system for the stronger charge acceptor, Mg2+. The cellular di/triphosphatess intrinsic properties and the protein matrix allowing them to bind a dication bi/tridentately, along with the higher cytosolic concentration of Mg2+ compared to Ca2+, enables Mg2+ to outcompete Ca2+ in binding to these highly charged anions. This suggests an explanation for why nature has chosen Mg2+ but not Ca2+ to perform most of the essential tasks associated with biological triphosphates.