It is generally accepted that enzymes catalyze reactions by lowering the apparent activation energy by transition state stabilization or through destabilization of ground states. A more controversial proposal is that enzymes can also accelerate reactions through barrier compression - an idea that has emerged from studies of H-tunneling reactions in enzyme systems. The effects of barrier compression on classical (over-the-barrier) reactions, and the partitioning between tunneling and classical reaction paths, have largely been ignored. We performed theoretical and computational studies on the effects of barrier compression on the shape of potential energy surfaces/reaction barriers for model (malonaldehyde and methane/methyl radical anion) and enzymatic (aromatic amine dehydrogenase) proton transfer systems. In all cases, we find that barrier compression is associated with an approximately linear decrease in the activation energy. For partially nonadiabatic proton transfers, we show that barrier compression enhances, to similar extents, the rate of classical and proton tunneling reactions. Our analysis suggests that barrier compression - through fast promoting vibrations, or other means - could be a general mechanism for enhancing the rate of not only tunneling, but also classical, proton transfers in enzyme catalysis. © 2010 by the Biophysical Society.