And that's why this bond is only a temporary covalent bond. Now, we said earlier that Chimetrypin is an example of a Serene protease. And what that means is inside the active side of that Chimetrypsin, it's the serine molecule, the serine amino acid, that plays the role of the powerful nuclear fide that catalyzes the cleavage of that peptide bond. 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.