Trypsin (Mr 25.000) is produced by the mammalian pancreas. Trypsin is involved in breaking specific peptide bonds, those formed using the carbonyl groups from Lys or Arg. Trypsin has an aspartate residue (189) at the bottom of the pocket (instead of Ser-189 as in chymotrypsin), and this Asp forms a salt bridge with the positively charged group at the end of the substrate lysine and arginine side chains.