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. And so what happens is a nearby histidine in the active side participates in actually taking away that H atom. And so we see that the H atom is transferred onto this nitrogen and the positive charge is now essentially delocalized among these different atoms in the histidine side chain. But this one now contains a full negative charge.