The trigonal-bipyramidal NO4 ligand set in biologically relevant vanadium compounds and their inorganic models.

In the present focused review, vanadate-dependent haloperoxidases and vanadate-inhibited enzymes which catalyze the hydrolysis of phosphoester bonds are addressed. In these systems, vanadate [HxVO4](3-x)(-) is covalently coordinated to the imidazolyl moiety of an active site histidine, with a geometrical arrangement close to a trigonal bipyramid. The resulting ligand set, NO4, and ligand arrangement provide peroxidase activity to the haloperoxidases and, to a certain extent, also to vanadate-inhibited phosphatases. The haloperoxidases are responsible for the oxidative halogenation of a variety of organic substrates. They are also active in other oxidation reactions relying on peroxide as the oxidant, such as the oxidative cyclizations of terpenes and, specifically, the oxygenation of (prochiral) sulfides to (chiral) sulfoxides. These functions can be modeled by vanadium complexes. Attracted interest is paid to {V(NO4)} complexes that are functional and structural models of the peroxidases. In the vanadate-inhibited phosphatases - structural analogs of the transition state in phosphoester hydrolysis by the native enzymes - the position of the axial histidine can also be taken by cysteinate or serinate, a fact which has implications for the insulin-mimetic potential of vanadate.