Proteolytic enzymes that exploit serine in their catalytic activity are ubiquitous, being found in viruses, bacteria and eukaryotes []. They include a wide range of peptidase activity, including exopeptidase, endopeptidase, oligopeptidase and omega-peptidase activity. Many families of serine protease have been identified, these being grouped into clans on the basis of structural similarity and other functional evidence []. Structures are known for members of the clans and the structures indicate that some appear to be totally unrelated, suggesting different evolutionary origins for the serine peptidases [].Not withstanding their different evolutionary origins, there are similarities in the reaction mechanisms of several peptidases. Chymotrypsin, subtilisin and carboxypeptidase C have a catalytic triad of serine, aspartate and histidine in common: serine acts as a nucleophile, aspartate as an electrophile, and histidine as a base []. The geometric orientations of the catalytic residues are similar between families, despite different protein folds []. The linear arrangements of the catalytic residues commonly reflect clan relationships. For example the catalytic triad in the chymotrypsin clan (PA) is ordered HDS, but is ordered DHS in the subtilisin clan (SB) and SDH in the carboxypeptidase clan (SC) [, ].This entry contains serine peptidases belonging to MEROPS peptidasesubfamily S1A (chymotrypsin A, clan PA(s)). The type example ischymotrypsin A from Bos taurus (Bovine).Tysnd1 acts in the peroxisomes, where beta-oxidation of fatty acids takes place. Tysnd1 is responsible both for the removal of the leader peptide from PTS2 proteins and for the specific processing of PTS1 proteins []. PTS1 and PTS2 are peroxisomal signal targeting sequences. The N-terminal fragment of Tysnd1 is removed, suggesting that this fragment has an inhibitory function. The expression of Tysnd1 is induced by the proliferator-activated receptor alpha agonist bezafibrate, along with an increase in its substrates [].
