In the above discussion, it has been argued that the quadprotonated forms of VCPO are unlikely to enter the catalytic cycle since geometry optimizations of quadprotonated 15-23 show a proton transferring in each case to His404, thus forming diprotonated His404. Since there is evidence that diprotonation of His404 leads to the enzyme becoming inhibited by substrate, we rule out the quadprotonated states as the resting state. With the larger QM Region II, the relative energy of Across the various series, we find that the state in which the rear O3 is protonated is often the lowest energy structure because of strong hydrogen bonding to His404 and because of minimal steric interactions which are present in forms where O1 or O2 is protonated. When O1 or O2 is protonated, steric interactions with Lys353 and Arg360/490 destabilize these protonation states based on an examination of space filling models.