<P> Trypsin (EC 3.4. 21.4) is a serine protease from the PA clan superfamily, found in the digestive system of many vertebrates, where it hydrolyzes proteins . Trypsin is formed in the small intestine when its proenzyme form, the trypsinogen produced by the pancreas, is activated . Trypsin cleaves peptide chains mainly at the carboxyl side of the amino acids lysine or arginine, except when either is followed by proline . It is used for numerous biotechnological processes . The process is commonly referred to as trypsin proteolysis or trypsinisation, and proteins that have been digested / treated with trypsin are said to have been trypsinized . Trypsin was discovered in 1876 by Wilhelm Kühne . </P> <P> In the duodenum, trypsin catalyzes the hydrolysis of peptide bonds, breaking down proteins into smaller peptides . The peptide products are then further hydrolyzed into amino acids via other proteases, rendering them available for absorption into the blood stream . Tryptic digestion is a necessary step in protein absorption, as proteins are generally too large to be absorbed through the lining of the small intestine . </P> <P> Trypsin is produced as the inactive zymogen trypsinogen in the pancreas . When the pancreas is stimulated by cholecystokinin, it is then secreted into the first part of the small intestine (the duodenum) via the pancreatic duct . Once in the small intestine, the enzyme enteropeptidase activates trypsinogen into trypsin by proteolytic cleavage . Autocatalysis does not happen with trypsin, as trypsinogen is a poor substrate, therefore enzymatic damage to the pancreas is avoided . </P> <P> The enzymatic mechanism is similar to that of other serine proteases . These enzymes contain a catalytic triad consisting of histidine - 57, aspartate - 102, and serine - 195 . This catalytic triad was formerly called a charge relay system, implying the abstraction of protons from serine to histidine and from histidine to aspartate, but owing to evidence provided by NMR that the resultant alkoxide form of serine would have a much stronger pull on the proton than does the imidazole ring of histidine, current thinking holds instead that serine and histidine each have effectively equal share of the proton, forming short low - barrier hydrogen bonds therewith . By these means, the nucleophilicity of the active site serine is increased, facilitating its attack on the amide carbon during proteolysis . The enzymatic reaction that trypsin catalyzes is thermodynamically favorable, but requires significant activation energy (it is "kinetically unfavorable"). In addition, trypsin contains an "oxyanion hole" formed by the backbone amide hydrogen atoms of Gly - 193 and Ser - 195, which through hydrogen bonding stabilize the negative charge which accumulates on the amide oxygen after nucleophilic attack on the planar amide carbon by the serine oxygen causes that carbon to assume a tetrahedral geometry . Such stabilisation of this tetrahedral intermediate helps to reduce the energy barrier of its formation and is concomitant with a lowering of the free energy of the transition state . Preferential binding of the transition state is a key feature of enzyme chemistry . </P>

Where is trypsin found in the digestive system