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. So the histidine molecule has a PH that is relatively close to the normal physiological PH. And many enzymes inside our body, as we'll see in the next several lectures, utilize histidine to actually transfer H ion. So active sites may contain residues such as histidine that can participate in transferring hydrogen ions.