Protease inhibitor I57/I58, staphostatin A/B <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>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"/>, <cite idref="PUB00029436"/>]. 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>