SERPINS: ANTITHROMBIN AND OTHER INHIBITORS OF COAGULATION AND FIBRINOLYSIS. EVIDENCE FROM AMINO ACID SEQUENCES
FACTORS XII AND XI
A number of the key inhibitors of coagulation and fibrinolysis have recently been shown to be members of the same superfamily of serine protease inhibitors, the serpins. The archetypes of the group are alpha-l-antitrypsin and antithrombin and it includes antiplasmin, C1-inhibitor, heparin cofactor II and the newly recognised inhibitors of plasminogen activators and activated Protein C. Alignment of their structures shows that they have the same skeletal three-dimensional conformation and, by
... erence, the same general function mechanisms.The serpins have a reactive centre, primarily dependent on a single amino acid, exteriorly placed on a stressed peptide loop. This functions by offering the cognate protease a high-affinity substrate that resists complete cleavage to form a tight 1:1 complex of inhibitor and protease that is subsequently removed from the circulation. The loop is vulnerable to cleavage with resulting loss of inhibitory activity. This irreversible switch is utilised: pathologically by venom and invasive bacterial proteases; and physiologically by the neutrophil leucocyte to modify local inflammatory responses. These mechanisms contribute to the changes seen in DIC and the shock syndromes.Modelling of antithrombin indicates the likely topological features involved in the binding of heparin, namely a sphere of positive charge centred on the A and D helices and involving Arg 47, Lys 125, Arg 129 and probably Arg 132 and Lys 133.Because the serpins are largely dependent for their specificityon a single amino acid it is now possible to precisely tailor inhibitory activity by site specific mutation. This has been used to produce recombinant antitrypsins that function as an improved inhibitor of neutrophil proteases (valine or leucine reactive centre), or as an analogue of antithrombin (arginine reactive centre). An elegant application of this approach is the engineered mutants of antiplasmin recently described by Holmes, Collen and colleagues (Leuven).