In the case of VCPO, the results from X-ray diffraction studies give only partial information about the resting state. Nevertheless, it is clear that the active site of the enzyme consists of a vanadate in a trigonal bipyramidal configuration with His496 covalently bonded to one of the axial positions of the vanadate, and the vanadate is hydrogen bonded to a sphere of amino acid residues surrounding it, namely, Lys353, Ser402, Gly403, His404, Arg490, and Arg360 (Figure ). Furthermore, Asp 292 forms a salt bridge to Arg490. The resolution of the X-ray crystal structure ( 0.24 Å) is insufficient to reveal the protonation state of the vanadate, and computational studies have been carried out to determine the resting state of VCPO. .