How does the mood stabilizer lithium bind ATP, the energy currency of the cell: Insights from solid-state NMR

Background: Lithium, in the form of a salt, is a mood stabilizer and a leading drug for the treatment of bipolar disorder. It has a very narrow therapeutic range and a variety of side effects. Lithium can replace magnesium and other cations in enzymes and small molecules, among them ATP, thereby affecting and inhibiting many biochemical pathways. The form of binding of lithium ions to ATP is not known. Methods: Here we extract the binding environment of lithium in solid ATP using a multi-nuclear multi-dimensional solid-state NMR approach. Results: We determine that the coordination sphere of lithium includes, at a distance of 3.0(±0.4) Å, three phosphates; the two phosphates closest to the ribose ring from one ATP molecule, and the middle phosphate from another ATP molecule. A water molecule most probably completes the fourth coordination. Despite the use of excess lithium in the preparations, sodium ions still remain bound to the sample, at distances of 4.3–5.5 Å from Li, and coordinate the first phosphate and two terminal phosphates. Conclusions: Solid-state NMR enables to unravel the exact coordination of lithium in ATP showing binding to three phosphates from two molecules, none of which are the terminal gamma phosphate. General significance: The methods we use are applicable to study lithium bound to a variety of ATP-bound enzymes, or to other cellular targets of lithium, consequently suggesting a molecular basis for its mode of action.