So by transferring hydrogen ion the active site may activate a nucleophile that is required in that catalysis process. Now, by transferring an H ion we can also actually stabilize different types of groups that might be found inside the active site that contain charges. And the transfer of H ions can also be used to increase the electrostatic interactions that take place within that active site and that can interstabilize things like the transition state inside that chemical reaction. Now, one particular example of an enzyme that uses acid based catalysis is Chimetrypsin. Inside Chimotrypsin, inside the active side, we have a Serene residue that acts as a nuclear foile. But to create a strong nuclear file what must happen is the H atom the H ion from the oxygen of Serene must be taken away.