Protease inhibitors may be classified either by the type of protease they inhibit, or by their mechanism of action. In 2004 Rawlings and colleagues introduced a classification of protease inhibitors based on similarities detectable at the level of amino acid sequence. This classification initially identified 48 families of inhibitors that could be grouped into 26 related superfamily (or clans) by their structure. According to the MEROPS database there are now 81 families of inhibitors. These families are named with an I followed by a number, for example, I14 contains hirudin-like inhibitors.
Proteinase propeptide inhibitors (sometimes referred to as activation peptides) are responsible for the modulation of folding and activity of the peptidase pro-enzyme or zymogen. The pro-segment docks into the enzyme, shielding the substrate binding site, thereby promoting inhibition of the enzyme. Several such propeptides share a similar topology, despite often low sequence identities. The propeptide region has an open-sandwich antiparallel-alpha/antiparallel-beta fold, with two alpha-helices and four beta-strands with a (beta/alpha/beta)x2 topology.
The peptidase inhibitor I9 family contains the propeptide domain at the N-terminus of peptidases belonging to MEROPS family S8A, subtilisins. The propeptide is removed by proteolytic cleavage; removal activating the enzyme.
The inhibitor I29 domain, which belongs to MEROPS peptidase inhibitor family I29, is found at the N-terminus of a variety of peptidase precursors that belong to MEROPS peptidase subfamily C1A; these include cathepsin L, papain, and procaricain. It forms an alpha-helical domain that runs through the substrate-binding site, preventing access. Removal of this region by proteolytic cleavage results in activation of the enzyme. This domain is also found, in one or more copies, in a variety of cysteine peptidase inhibitors such as salarin.
The structure of SMPI has been determined. It has 102 amino acid residues with two disulphide bridges and specifically inhibits metalloproteinases such as thermolysin, which belongs to MEROPS peptidase family M4. SMPI is composed of two beta-sheets, each consisting of four antiparallel beta-strands. The structure can be considered as two Greek key motifs with 2-fold internal symmetry, a Greek key beta-barrel. One unique structural feature found in SMPI is in its extension between the first and second strands of the second Greek key motif which is known to be involved in the inhibitory activity of SMPI. In the absence of sequence similarity, the SMPI structure shows clear similarity to both domains of the eye lens crystallins, both domains of the calcium sensor protein-S, as well as the single-domain yeast killer toxin. The yeast killer toxin structure was thought to be a precursor of the two-domain beta gamma-crystallin proteins, because of its structural similarity to each domain of the beta gamma-crystallins. SMPI thus provides another example of a single-domain protein structure that corresponds to the ancestral fold from which the two-domain proteins in the beta gamma-crystallin superfamily are believed to have evolved.
Inhibitor family I48 includes clitocypin, which binds and inhibits cysteine proteinases. It has no similarity to any other known cysteine proteinase inhibitors but bears some similarity to a lectin-like family of proteins from mushrooms.
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