And so when this is protonated, we basically gain a positive charge. And that positive charge can be delocalized among this region here. So if this gains an H atom, then what happens is this is delocalized among these two atoms, and there will be a positive charge that is delocalized. And so this is also resonant stabilized, just like the argument, because this is essentially an aromatic ring. Now, the thing about histidine, because it contains a lower PKA, and because it can be protonated or deprotonated at the physiological PH, this is a very common amino acid that exists in the active sites of enzymes. Remember, an enzyme is a biological catalyst that speeds up different reactions, and the active side is the location on that enzyme where that reaction actually takes place.