A model of the transition state in the alkaline phosphatase reaction.

A high resolution crystal structure of Escherichia coli alkaline phosphatase in the presence of vanadate has been refined to 1.9 A resolution. The vanadate ion takes on a trigonal bipyramidal geometry and is covalently bound by the active site serine nucleophile. A coordinated water molecule occupies the axial position opposite the serine nucleophile, whereas the equatorial oxygen atoms of the vanadate ion are stabilized by interactions with both Arg-166 and the zinc metal ions of the active site. This structural complex supports the in-line displacement mechanism of phosphomonoester hydrolysis by alkaline phosphatase and provides a model for the proposed transition state in the enzyme-catalyzed reaction.