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. Now, why would we want to transfer an H ion? Well, in some cases, if we transfer an H ion from one molecule to another molecule we basically create a strong nucleophile. And that strong nucleophile might be needed in that particular biological reaction.