Editing Proteolysis (section)

====Digestion====
In human [[digestion]], proteins in food are broken down into smaller peptide chains by [[digestive enzymes]] such as [[pepsin]], [[trypsin]], [[chymotrypsin]], and [[elastase]], and into amino acids by various enzymes such as [[carboxypeptidase]], [[aminopeptidase]], and [[dipeptidase]]. It is necessary to break down proteins into small peptides (tripeptides and dipeptides) and amino acids so they can be absorbed by the intestines, and the absorbed tripeptides and dipeptides are also further broken into amino acids intracellularly before they enter the bloodstream.<ref>{{cite journal |author=Silk DB |title=Progress report. Peptide absorption in man|journal=Gut |volume=15|issue=6|pages=494–501|year=1974|pmid= 4604970 |pmc=1413009 |doi=10.1136/gut.15.6.494 }}</ref> Different enzymes have different specificity for their substrate; trypsin, for example, cleaves the peptide bond after a positively charged residue ([[arginine]] and [[lysine]]); chymotrypsin cleaves the bond after an aromatic residue ([[phenylalanine]], [[tyrosine]], and [[tryptophan]]); elastase cleaves the bond after a small non-polar residue such as alanine or glycine.

In order to prevent inappropriate or premature activation of the digestive enzymes (they may, for example, trigger pancreatic self-digestion causing [[pancreatitis]]), these enzymes are secreted as inactive zymogen. The precursor of [[pepsin]], [[pepsinogen]], is secreted by the stomach, and is activated only in the acidic environment found in stomach. The [[pancreas]] secretes the precursors of a number of proteases such as [[trypsin]] and [[chymotrypsin]]. The zymogen of trypsin is [[trypsinogen]], which is activated by a very specific protease, [[enterokinase]], secreted by the [[mucosa]] of the [[duodenum]]. The trypsin, once activated, can also cleave other trypsinogens as well as the precursors of other proteases such as chymotrypsin and carboxypeptidase to activate them.

In bacteria, a similar strategy of employing an inactive zymogen or prezymogen is used. [[Subtilisin]], which is produced by ''[[Bacillus subtilis]]'', is produced as preprosubtilisin, and is released only if the signal peptide is cleaved and autocatalytic proteolytic activation has occurred.