Snake venom metalloproteases, in addition to their contribution to the digestion of the prey, affect various physiological functions by cleaving specific proteins. They exhibit their activities through activation of zymogens of coagulation factors, and precursors of integrins or receptors. Based on their structure–function relationships and mechanism of action, we have defined classification and nomenclature of functional sites of proteases. These metalloproteases are useful as research tools and in diagnosis and treatment of various thrombotic and hemostatic conditions. They also contribute to our understanding of molecular details in the activation of specific factors involved in coagulation, platelet aggregation and matrix biology. This review provides a ready reference for metalloproteases that interfere in blood coagulation, fibrinolysis and platelet aggregation.

Tick saliva contains potent antihemostatic molecules that help ticks obtain their enormous blood meal during prolonged feeding. We isolated thrombin inhibitors present in the salivary gland extract from partially fed female Amblyomma variegatum, the tropical bont tick, and characterized the most potent, variegin, one of the smallest (32 residues) thrombin inhibitors found in nature. Full-length variegin and two truncated variants were chemically synthesized. Despite its small size and flexible structure, variegin binds thrombin with strong affinity (K_i ∼10.4 pm) and high specificity. Results using the truncated variants indicated that the seven residues at the N terminus affected the binding kinetics; when removed, the binding characteristics changed from fast to slow. Further, the thrombin active site binding moiety of variegin is in the region of residues 8–14, and the exosite-I binding moiety is within residues 15–32. Our results show that variegin is structurally and functionally similar to the rationally designed thrombin inhibitor, hirulog. However, compared with hirulog, variegin is a more potent inhibitor, and its inhibitory activity is largely retained after cleavage by thrombin.

Summary: To obtain blood meals, haematophagous animals are armed with potent pharmacological molecules to overcome several of their hosts’ response systems. Among them, a large number of exogenous anticoagulants which target the haemostatic system have been identified and characterised. Studies on these anticoagulants have expanded our knowledge on the blood coagulation system, and provided a valuable source of antithrombotic therapeutic agents. Advances in genomic, transcriptomic, structural and proteomic tools greatly accelerated the discovery and analysis of the exogenous anticoagulants in recent years. The molecular diversity observed in these molecules is huge and is constantly expanding. In this review, we will provide an overview on the structure, function and mechanism of the exogenous anticoagulants from haematophagous animals and rationalise their molecular diversity.