... distance of 3.5 A˚, and the shortestdistance between the imidazole ring of His 210 and the atoms of Asp 113 is 3.2 A˚. The substrate binding subsite in API is composed of His 210 -Gly 211 -Gly 212 , ... Ser 214 –Trp 215 –Gly 216 and His 210 –Gly 211 –Gly 212 , respectively. S1pocket is the substrate binding site for the side-chain of Lys (API) or Lys and Arg (trypsin). The aromatic stacking between Trp169 ... W169Y, W169F, and W169H mutants, the side-chain at position 16 9 remainedparallel with the side-chain of His 210 . On the other hand, asmall side-chain at position 16 9, typically W169V and W169A,...