So if we count all the amino acids, beginning with the first one along the entire polypeptide chain, this Serene will be labeled as 195. So among the 28 serum residues found on China Trypsin, only Serene 195 actually reacted with this irreversible inhibitor di isopropyl phosphoridate. And so this implied that serine 195 must be the one that plays a major catalytic role in the active side of that enzyme. And so that's how we know that Serene is the one that is involved in actually catalyzing the cleavage of the peptide bonds at these specific bulky hydrophobic amino acids. Now, the next question is what is the general description of the reaction mechanism that takes place inside the active sites? Well, basically, it's a two step catalytic mechanism.