In the cases where the rear equatorial O3 is protonated with H10 (i.e., 5, 7, 11, 12, 13, 17, 19, 20, 22, 23), one finds either that there is a strong hydrogen bond between H10 and N26 of His404 or that H10 has transferred to N26 (as in the quadprotonated states). The average value of ρ for this hydrogen bond (H10-N26) is 0.044. As mentioned above, diprotonation of His404 leads to the enzyme being inhibited by substrate. One finds in the literature the statement that His496 is covalently bonded to vanadium; however, we find that the electron density, ρ, at the bond critical point between V and N5 is actually much less than what one would expect for a covalent bond; for a covalent bond, one would expect ρ to exceed 0.2 electrons/bohr 3 . The ρ value is actually in the range of 0.0221-0.0801 electrons/bohr 3 , and this range is more characteristic of hydrogen bonding. The weakness of this bond agrees with the result that the vanadate cofactor can be removed by dialysis against phosphate and then reincorporated. Some other structurally significant hydrogen bonds become evident from the AIM study.