**1. Introduction**

### **1.1. Snake venom serine proteases**

Serine proteases have been isolated from the venoms of viperidae snakes [1, 2] and affect several physiological processes such as the coagulation cascade. These enzymes are called snake venom serine proteases (SVSPs), they are multi-functional proteins with a catalytic triad formed by HDS amino acids [3].

The SVSPs resembles at least in part thrombin, a multifunctional protease that plays a key role in coagulation. Therefore these enzymes are denominated snake venom thrombin-like en‐ zymes (SVTLEs), and are widely distributed in the venoms of several genera [4,5]. While thrombin is able to cleave both fibrinopeptide A (FPA) and fibrinopeptide B (FPB) from fibrino‐ gen leading the formation of fibrin and activating factor XIII, some actions of SVTLEs usually cleave FPA alone and only a few cleave FPB. Thus, without cleavage of both FPA and FPB they are unable to activate factor XIII producing fibrin monomers that are not cross-linked, leading to clots markedly susceptible to digestion by plasmin and are rapidly removed from circulation by either reticuloendothelial phagocitosis and/or normal fibrinolysis. This process causes a breakdown in the fibrinolytic system and effective removal of fibrinogen from the plasma [6].
