Biologically relevant phosphoranes: Hypervalent phosphorus as applied to phosphoryl transfer enzymes

The coordination tendencies of phosphorus to form a hexacoordinated state from a pentacoordinated state, which might assist in describing the mechanistic action of phosphoryl transfer enzymes, are delineated. In view of the work reported here and recent work on enzyme promiscuity and moonlighting activities, it is suggested that donor action should play a role in determining active site interactions in phosphoryl transfer enzyme mechanisms. Biochemists studying phosphoryl transfer enzymes outline mechanisms of nucleophilic attack at phosphorus that take place by way of proposed trigonal bipyramidal intermediates or transition states. However; recent work has shown the ready availability of higher coordinate forms of phosphorus, particularly the ease of formation of hexacoordinate phosphorus. Our recent work established the X-ray structure of several biorelevant phosphoranes. Included are the structures of a xylofuranose based phosphorane 1 and a thymidine based phosphorane. Dynamic equilibrium between two isomeric forms exists in solution for 1. In addition, bicyclic phosphorane 3 exists in equilibrium between pentacoordinated and hexacoordinated isomeric forms. The rapid exchange process between these two geometries reorients the nucleotidyl or carbohydrate component of the trigonal bipyramidal phosphorane. At an active site, this type of pseudorotational behavior provides a mechanism that could bring another active site residue into play and account for a means for phosphoryl transfer enzymes to express promiscuous behavior. Pseudorotation, a well-founded process in non-enzymatic phosphorus chemistry may have an application in the future of phosphoryl transfer enzyme chemistry.