Pseudocerastes persicus trypsin inhibitor (PPTI), Kunitz-type serine protease inhibitor bitisilin, and similar proteins
This group contains Pseudocerastes persicus trypsin inhibitor (PPTI), Bitis gabonica Kunitz-type serine protease inhibitor bitisilin-1 (BG-11), -2 (BG-15) and -3 (two-Kunitz protease inhibitor), Oxyuranus scutellatus scutellatus taicatoxin, and serine protease inhibitor component (TSPI, also called venom protease inhibitor 1 or venom protease inhibitor 2), among others. PPTI from P. persicus venom shows inhibitory effect against trypsin proteolytic activity and has similarities to dendrotoxins (DTXs), with corresponding functionally important residues. Studies have shown the ability of PPTI to inhibit voltage-gated potassium channels, and consequently have dual functionality. Bitilisins 1, 2, and 3 are serine protease inhibitors expressed in snake venom glands; bitsilin-3 consists of two Kunitz protease inhibitor domains. Taicatoxin inhibits trypsin, tissue kallikrein, elastase, plasmin and factor Xa, and is also known to block the voltage-dependent L-type calcium channels from the heart, and the small conductance calcium-activated potassium channels (KCa) in chromaffin cells and in the brain. The structures of these Kunitz-type proteins are similar to other Kunitz-type proteinase inhibitors such as BPTI (bovine pancreatic trypsin inhibitor), showing an alpha/beta fold with irregular secondary structure stabilized by three disulfide bonds.
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