But how do we know that the active side contains this yearine molecule that plays the central role in catalysis? Well, basically the way that science has discovered this is they probed the active side with a specific type of Irreversible inhibitor. So they used the specific group inhibitor known as diisopropyl phosphorfluoridate or DIPF. And this is the same Irreversible inhibitor that we spoke about in our lecture on Irreversible inhibitors. So if we take DIPF and we mix it with an enzyme and inside the active site, we have a Serene molecule. And if the reactivity of that serine molecule is quite high, then that DIPF will basically form a covalent bond with that oxygen of the Serene displacing, the H, and that will kick off this fluoride atom and we form a covalent bond between the oxygen and the phosphorus atom.