The work reported by Kwong et al and Bos et al provide important mechanistic data that assist in explaining why such proteins with similar catalytic activity have evolved to perform quite different physiological functions. The focus here is the prothrombinase, the FXa-FVa complex, which functions in the blood to respond to injury and minimise bleeding. This functional role is highly conserved in a variety of species including snakes. In snakes a closely related prothrombinase complex has been recruited into the venom gland as a toxin to assist in immobilising prey by inducing a massive thrombotic event such as a disseminated intravascular coagulation. This evolution has been achieved by distinct processes for FXa- and FVa-like proteins.