A Chymotrypsin folds bringing together Asp102, His 57, Ser 195 in this sequence in space.
Asp 102, His 57 and Ser 195 lie in this order forming a charge relay; The negatively charged aspartate carboxylate residue pulls the Ser –OH proton through His, leaving it with a negative charge. Ser195 becomes acidic due to the unique constellation of the three amino acid residues because the protein has folded uniquely in space.