The Nonexistence of Specially Stabilized Hydrogen Bonds in Enzymes

Ab initio calculations are used to test a recent suggestion that enzymic catalysis can be aided by strengthening of a hydrogen bond in a key intermediate, occurring when this bond is shortened and the pKa’s of the two groups are equalized. The requisite amount of energy is not available in electrically neutral H-bonds; no additional strengthening can be accomplished by shortening such a bond. Interaction energies where one subunit is charged, on the other hand, can be very high. These bonds are intrinsically very short, and the proton transfer profile contains a very low energy barrier. There is no special stabilization associated with the disappearance of the transfer barrier or equalization of the pKa’s.