<p>Peptide proteinase inhibitors can be found as single domain proteins or as single or multiple domains within proteins; these are referred to as either simple or compound inhibitors, respectively. In many cases they are synthesised as part of a larger precursor protein, either as a prepropeptide or as an N-terminal domain associated with an inactive peptidase or zymogen. This domain prevents access of the substrate to the active site. Removal of the N-terminal inhibitor domain either by interaction with a second peptidase or by autocatalytic cleavage activates the zymogen. Other inhibitors interact direct with proteinases using a simple noncovalent lock and key mechanism; while yet others use a conformational change-based trapping mechanism that depends on their structural and thermodynamic properties. </p><p>This group of serine protease inhibitors belong to MEROPS inhibitor family I15, clan IO. They inhibit serine peptidases of the S1 family (<db_xref db="INTERPRO" dbkey="IPR001254"/>) [<cite idref="PUB00014133"/>] and are characterised by a well conserved pattern of cysteine residues. Many of the proteins that belong to this family are anti-coagulants.</p>