Optimization of triprotonated input structures, 12 and 13, leads to the optimized structures, 12-O and 13-O, which show that the axial hydrogen (H6) transfers from the axial oxygen, O4, to a water molecule (O15) "above" the vanadate as shown in Figure In all of the optimized quadprotonated structures (15-O to 23-O), one finds that a proton has transferred to His404 (i.e., to N26). One can argue that these structures may not enter the catalytic cycle since, as discussed earlier, diprotonation of His404 results in inhibition of the enzyme by substrate. In the other cases (15, 16, 18, 21, and 22), an axial proton (H6 or H7) transfers from axial oxygen, O4, to N26 (e.g., 16-O, Figure ). Whenever the equatorial oxygen, O3, is protonated (with H10) as in the cases of 17, 19, 20, and 23, the optimized structures show that H10 has transferred to N26 (eg., 17-O, Figure ). Even in the case of triprotonated 14, one finds that axial hydrogen, H6, has transferred from O4 to O15 of the water molecule, and then H37 has transferred from O15 to N26. Optimization of 15 results in an unusual transfer of both protons (H6 and H7) on the axial oxygen (O4): H6 transfers from O4 to N26; H7 transfers from O4 to O15, and H36 transfers from O15 to the adjacent water (O16), and then H38 transfers from O16 to O13 of Asp292 (Figure ). As a result of these proton transfers, the vanadate is in diprotonated state, and the energy of 15-O cannot be compared with other structures.