<p>This group of serine proteases belong to the MEROPS peptidase family S1 (chymotrypsin family, clan PA(S))and to peptidase family S6 (Hap serine peptidases). The catalytic activity of the serine proteases from the chymotrypsin family is provided by a charge relay system involving an aspartic acid residue hydrogen-bonded to a histidine, which itself is hydrogen-bonded to a serine. The sequences in the vicinity of the active site serine and histidine residues are well conserved in this family of proteases [<cite idref="PUB00004028"/>].</p><p>The chymotrypsin family is almost totally confined to animals, although trypsin-like enzymes are found in actinomycetes of the genera Streptomyces and Saccharopolyspora, and in the fungus <taxon tax_id="5507">Fusarium oxysporum</taxon> [<cite idref="PUB00003576"/>]. The enzymes are inherently secreted, being synthesised with a signal peptide thattargets them to the secretory pathway. Animal enzymes are either secreteddirectly, packaged into vesicles for regulated secretion, or are retainedin leukocyte granules [<cite idref="PUB00003576"/>].</p><p>The Hap family, 'Haemophilus adhesion and penetration', are proteins that play a role in the interaction with human epithelial cells. The serine protease activity is localized at the N-terminal domain, whereas the binding domain is in the C-terminal region. </p>
Peptidase S1/S6, chymotrypsin/Hap, active site