And what active sites of enzymes provide is they provide that small region of space, that microenvironment that brings the substrate close enough for the collisions to actually take place at a high enough frequency. In addition, the active sites may also orient the molecules in the proper orientation for that bond to actually form and for us to form those products. Now, method number three is called acid based catalysis. And in acid based catalysis, we basically have a transfer of an Hion. Now, there are many residues that are involved or there are specific residues found in active sites that might be involved in a transfer of an H ion. And one specific residue is the histidine amino acid.