Mg²⁺‐dependent, Li⁺‐sensitive phosphatases are a widely distributed family of enzymes with significant importance throughout the biological kingdom. Inositol monophosphatase (IMPase) is an important target of Li⁺‐based therapeutic agents in manic depressive disorders. However, despite decades of intense research efforts, the precise mechanism of Li⁺‐induced inhibition of IMPase remains obscured. Here we describe a structural investigation of the Li⁺ binding site in staphylococcal IMPase I (SaIMPase I) using X–ray crystallography. The biochemical study indicated common or overlapping binding sites for Mg²⁺ and Li⁺ in the active site of SaIMPase I. The crystal structure of the SaIMPase I ternary product complex shows the presence of a phosphate and three Mg²⁺ ions (namely Mg1, Mg2 and Mg3) in the active site. As Li⁺ is virtually invisible in X–ray crystallography, competitive displacement of Mg²⁺ ions from the SaIMPase I ternary product complex as a function of increasing LiCl concentration was used to identify the Li⁺ binding site. In this approach, the disappearing electron density of Mg²⁺ ions due to Li⁺ ion binding was traced, and the Mg²⁺ ion present at the Mg2 binding site was found to be replaced. Moreover, based on a detailed comparative investigation of the phosphate orientation and coordination states of Mg²⁺ binding sites in enzyme–substrate and enzyme–product complexes, inhibition mechanisms for Li⁺ and Mg²⁺ are proposed.

The atomic coordinates for the SaIMPase I ternary complex, SaIMPase I in 50 mm LiCl, SaIMPase I in 100 mm LiCl and SaIMPase I in 0 mm MgCl2 have been submitted to the Protein Data Bank under accession numbers 4G61, 4I40, 4I3Y and 4PTK, respectively.

• SaIMPase-I and SaIMPase-I bind by x-ray crystallography (View interaction)