<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 entry represents a beta-barrel domain found in the protease inhibitors staphostatin [<cite idref="PUB00029436"/>] and metalloprotease inhibitor [<cite idref="PUB00006323"/>]. </p><p>Staphostatin A (SAV1910, SA1726) and B (SspC) are cysteine protease inhibitors belonging to MEROPS inhibitor families I57 and I58 (clan IK). They have a beta-barrel topology that act as specific inhibitors of staphopains, the major secreted cysteine proteases of <taxon tax_id="1280">Staphylococcus aureus</taxon> [<cite idref="PUB00032579"/>]. These inhibitors compete with substrate for binding at the active site of the staphopain, acting to protect intracellular proteins against proteolytic damage by prematurely folded and activated staphopains [<cite idref="PUB00034485"/>]. They are also involved in growth capacity, viability and bacterial morphology.</p><p>Monomeric serralysin inhibitors are metalloprotease inhibitors that interact with specific metalloproteases synthesised by serralysin secretors and characterised by being plant, insect and animal pathogens. It is probable that the serralysin inhibitors protect the host from proteolysis during export of the protease. The members of this family belong to MEROPS proteinase inhibitor family I38, clan IK.</p> Protease inhibitor, beta-barrel domain