Post‐translational modifications in proteins involved in blood coagulation

Blood coagulation and its anticoagulant counterpart, the protein C (PC) system, proceed via the formation of cellassociatedmacromolecular enzyme complexes that interact in a precisely regulated manner. In the final step of coagulation thrombin is generated, which cleaves soluble fibrinogen to form the insoluble fibrin monomers that constitute the structural foundation of a blood clot [1,2]. Post-translational protein modifications play pivotal roles in both blood coagulation and the PC anticoagulant system. The relatively high concentration of most of the blood coagulation proteins enabled many of them to be purified to homogeneity some 30–40 or more years ago. Advanced chemical characterization and protein sequencing became possible, which in time resulted in the identification of several post-translational amino acid modifications. These developments, and the interest in blood coagulation and coagulation disorders, led to the identification of three types of modified amino acids in coagulation proteins before they were found in proteins belonging to other systems. The first was c-carboxyglutamic acid (Gla) in 1974, which is formed by vitamin K-dependent c-carboxylation of glutamic acid residues [3–5]. Then followed erythro-b-hydroxyaspartic acid (Hya) and erythro-b-hydroxyasparagine (Hyn), which are formed by hydroxylation of aspartic acid and asparagine residues, respectively [6,7]. At that time, blood coagulation was described by the so-called cascade scheme [2,8,9]. It called attention to the signal amplification that is required to obtain the amounts of thrombin necessary to convert soluble fibrinogen to insoluble fibrin and it illustrated the role of enzymatically active macromolecular complexes. Moreover, the importance of binding to cell surfaces, i.e. that the coagulation process to a large extent is an example of chemistry in two (rather than three) dimensions was only beginning to emerge. The identification of Gla, stimulated by a desire to learn about the mode of action of warfarin [10], indirectly led to the purification of PC, and a few years later protein S and thrombomodulin [11–15]. With these proteins identified, and with the novel co-factor role of factor V established as a result of research on resistance to activated protein C (APC), most of the key components of the PC anticoagulant system had been identified [16]. Lately, the coagulation system has been refined because of new insights into the roles of the various cell types involved [17]. Hence, the initiation phase of coagulation is now known to commence on tissue factor (TF)-bearing cells such as macrophages, followed by an amplification phase involving platelet activation and activation of FV and FVIII. In the third phase, the propagation phase, bulk amounts of thrombin are generated on the surface of newly activated platelets [17]. The activity of the serine proteases generated during this sequence of reactions is under rigorous control, mediated by the TF pathway inhibitor and antithrombin; the latter a so-called serpin [18]. Likewise, APC and its co-factors, protein S and FV, regulate the activity of the activated forms of two homologous co-factors, FVIIIa and FVa [19]. In this review, we describe the post-translational modifications that occur in blood coagulation proteins and, where known, their functional implications. In the integrated defense system there are no borders between blood coagulation, the complement system, and the immune system. We have, however, taken a conservative approach and will deal only with those modifications that are found in the traditional coagulation factors, including the PC anticoagulant system and antithrombin. Seven years ago an excellent review of the same field was published [20].