What gives Chimotrypsin the ability to only Cleave on the carboxyl end of specific amino acids, those amino acids that contain bulky hydrophobic side chain groups? Now, to answer this question, we actually have to study the shape of the active side, the structure of that enzyme. If we examine the active side of that enzyme, we're going to find something called the S one pocket. And the S One pocket in China trypsin is basically that region to which that side chain group will actually move into. And if we examine the shape and structure of the S One pocket, we're going to find that it's relatively long, so relatively deep and mostly hydrophobic, so non polar. And because of that structure of the S one pocket, only those amino acids that contain side chain groups that are long, nonpolar, do not have any charges, will actually be able to fit into that pocket, into the active side, without creating too much electric repulsion.