Tick anticoagulant peptide (TAP) is a competitive inhibitor of factor Xa, a serine protease that converts prothrombin into thrombin, which acts to convert fibrinogen into fibrin, the insoluble matrix of blood clots. TAP is one of the antihemostatic components evolved in the blood-feeding tick. This subfamily also includes the N-terminal Kunitz inhibitor domain of ornithodorin which binds to the active site of thrombin, while the C-terminal domain binds at the fibrinogen recognition exosite. The TAP structure is similar to that of 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.
Jiyao W et al.(2023). "The conserved domain database in 2023.",