A substrate-like thrombin inhibitor from the saliva of the flea Xenopsylla cheopis is resistant to cleavage by the enzyme

XC-43 is a small peptide identified in the sialome of the flea Xenopsylla cheopis and act as a fast, tight-binding inhibitor of thrombin with a dissociation constant of less than 10 pM. The crystal structure of XC-43 in complex with thrombin shows that despite its substrate-like binding mode, XC-43 is not detectably cleaved by thrombin and that it interacts with the thrombin surface from the enzyme catalytic site through the fibrinogen-binding exosite I. The low rate of hydrolysis is verified in solution experiments with XC-43 which show the substrate to be largely intact after two hours of incubation with thrombin at 37°C. The potential of XC-43 as an anticoagulant is suggested by increased arterial occlusion time, tail bleeding time, and blood coagulation parameters in rat models of thrombosis.