Most serpins are protease inhibitors, targeting extracellular, chymotrypsin-like serine proteases. These subunits belong to atypical serine proteases. Bacterial exotoxic proteases destroy extracellular structures. The activity of proteases is inhibited by protease inhibitors. The coagulation factors are generally serine proteases (enzymes), which act by cleaving downstream proteins. PMSF does not inactivate all serine proteases. Alteration in pH causes inhibition of lysosomal acidic proteases causing a diminished proteolysis effect. Caspases are proteins that are highly conserved, cysteine-dependent aspartate-specific proteases. The enzymatic mechanism is similar to that of other serine proteases. Specifically, mannose binding triggers recruitment of MBL-associated serine proteases (MASPs). Host organisms must ensure that the activity of serine proteases is adequately regulated. Plasmin, like trypsin, belongs to the family of serine proteases. Within each superfamily, families are designated by their catalytic nucleophile, (S = serine proteases). The main player in the catalytic mechanism in the serine proteases is the catalytic triad. Enzymes that help such reactions are called proteases.