Isolation of intact modules from noncatalytic parts of vitamin K-dependent coagulation factors IX and X and protein C

Phosphatidylserine-induced factor Xa dimerization and binding to factor Va are competing processes in solution

A soluble, short chain phosphatidylserine, 1,2-dicaproyl-sn-glycero-3-phospho-l-serine (C6PS), binds to discrete sites on FXa, FVa, and prothrombin to alter their conformations, to promote FXa dimerization (K(d) ~ 14 nM), and to enhance both the catalytic activity of FXa and the cofactor activity of FVa. In the presence of calcium, C6PS binds to two sites on FXa, one in the epidermal growth factor-like (EGF) domain and one in the catalytic domain; the latter interaction is sensitive to Na(+) binding and probably represents a protein recognition site. Here we ask whether dimerization of FXa and its binding to FVa in the presence of C6PS are competitive processes. We monitored FXa activity at 5, 20, and 50 nM FXa while titrating with FVa in the presence of 400 μM C6PS and 3 or 5 mM Ca(2+) to show that the apparent K(d) of FVa-FXa interaction increased with an increase in FXa concentration at 5 mM Ca(2+), but the K(d) was only slightly affected at 3 mM Ca(2+). A mixture of 50 nM FXa and 50 nM FVa in the presence of 400 μM C6PS yielded both Xa homodimers and Xa·Va heterodimers, but no FXa dimers bound to FVa. A mutant FXa (R165A) that has reduced prothrombinase activity showed both weakened dimerization (K(d) ~ 147 nM) and weakened FVa binding (apparent K(d) values of 58, 92, and 128 nM for 5, 20, and 50 nM R165A FXa, respectively). Native gel electrophoresis showed that the GLA-EGF(NC) fragment of FXa (lacking the catalytic domain) neither dimerized nor formed a complex with FVa in the presence of 400 μM C6PS and 5 mM Ca(2+). Our results demonstrate that the dimerization site and FVa-binding site are both located in the catalytic domain of FXa and that these sites are linked thermodynamically.

Localization of phosphatidylserine binding sites to structural domains of factor Xa

Binding of short chain phosphatidylserine (C6PS) enhances the proteolytic activity of factor X(a) by 60-fold (Koppaka, V., Wang, J., Banerjee, M., and Lentz, B. R. (1996) Biochemistry 35, 7482-7491). In the present study, we locate three C6PS binding sites to different domains of factor X(a) using a combination of activity, circular dichroism, fluorescence, and equilibrium dialysis measurements on proteolytic and biosynthetic fragments of factor X(a). Our results demonstrate that the structural responses of human and bovine factor X(a) to C6PS binding are somewhat different. Despite this difference, data obtained with fragments from both human and bovine factor X(a) are consistent with a common hypothesis for the location of C6PS binding sites to different structural domains. First, the gamma-carboxyglutamic acid (Gla) domain binds C6PS only in the absence of Ca(2+) (k(d) approximately 1 mm), although this PS site does not influence the functional response of factor X(a). Second, a Ca(2+)-dependent binding site is in the epidermal growth factor domains (EGF(NC)) that are linked by Ca(2+) and C6PS binding to the Gla domain. This site appears to be the lipid regulatory site of factor X(a). Third, a Ca(2+)-requiring site seems to be in the EGF(C)-catalytic domain. This site appears not to be a lipid regulatory site but rather to share residues with the substrate recognition site. Finally, the full functional response to C6PS requires linkage of the Gla, EGF(NC), and catalytic domains in the presence of Ca(2+), meaning that PS regulation of factor X(a) involves linkage between widely separated parts of the protein.

Calcium-binding EGF-like modules in coagulation proteinases: function of the calcium ion in module interactions

Epidermal growth factor (EGF)-like modules are involved in protein-protein interactions and are found in numerous extracellular proteins and membrane proteins. Among these proteins are enzymes involved in blood coagulation, fibrinolysis and the complement system as well as matrix proteins and cell surface receptors such as the EGF precursor, the low density lipoprotein receptor and the developmentally important receptor, Notch. The coagulation enzymes, factors VII, IX and X and protein C, all have two EGF-like modules, whereas the cofactor of activated protein C, protein S, has four EGF-like modules in tandem. Certain of the cell surface receptors have numerous EGF modules in tandem. A subset of EGF modules bind one Ca(2+). The Ca(2+)-binding sequence motif is coupled to a sequence motif that brings about beta-hydroxylation of a particular Asp/Asn residue. Ca(2+)-binding to an EGF module is important to orient neighboring modules relative to each other in a manner that is required for biological activity. The Ca(2+) affinity of an EGF module is often influenced by its N-terminal neighbor, be it another EGF module or a module of another type. This can result in an increase in Ca(2+) affinity of several orders of magnitude. Point mutations in EGF modules that involve amino acids which are Ca(2+) ligands result in the biosynthesis of biologically inactive proteins. Such mutations have been identified, for instance, in factor IX, causing hemophilia B, in fibrillin, causing Marfan syndrome, and in the low density lipoprotein receptor, causing hypercholesterolemia. In this review the emphasis will be on the coagulation factors.

Localization of the specific binding site for magnesium(II) ions in factor IX

We demonstrated recently that coagulation factor IX has a specific binding site(s) for Mg2+ ions, independent of the (Ca2+)-binding sites, and that binding of Mg2+ ions is very important for expression of the functional conformation of this protein. We report here the localization of this Mg2+-specific binding site. We prepared three Gla-containing fragments of bovine factor IX, namely GlaEGF(NC) (residues 1-144+286-296), GlaEGF(N) (1-83) and the Gla domain peptide (1-46). Fragments GlaEGF(NC) and GlaEGF(N) retained the ability to undergo a conformational change upon binding of Mg2+ ions in the presence of excess Ca2+ ions. This change could be detected by a conformation-specific antibody. Furthermore, the Gla domain peptide was capable of binding Mg2+ ions, as determined by the metal ion-induced quenching of the intrinsic fluorescence. It appears that the (Mg2+)-specific binding site of factor IX is located in the N-terminal Gla domain.

Regulation of the tertiary structure and function of coagulation factor IX by magnesium (II) ions

The indispensable role of Ca2+ ions in the maintenance of the functional tertiary structures of vitamin K-dependent coagulation factors has been definitively established but the participation of Mg2+ ions, another alkaline-earth metal that is present abundantly in blood plasma, in such a process is not yet understood. We show here that the Ca(2+)-stabilized conformation of coagulation factor IX undergoes a further conformational change upon binding of Mg2+ ions using three independent structural probes. The probes we used were (i) IX/X-bp, a snake venom anticoagulant that recognizes the Gla domains in coagulation factors IX and X, (ii) conformation-specific polyclonal antibodies against bovine factor IX, and (iii) monoclonal antibodies against the Gla domain of human factor IX. The binding of all these probes had an absolute requirement for Ca2+ ions, and Mg2+ ions alone were ineffective. However, when added together with Ca2+ ions, Mg2+ ions at physiological concentrations greatly augmented the binding of these probes to factor IX; the required concentration of Ca2+ ions was much reduced, and the affinity of each probe for factor IX was increased even in the presence of an excess of Ca2+ ions. These results suggest the presence of a Mg(2+)-specific binding site that does not interact with Ca2+ ions in factor IX. Furthermore, Mg2+ ions potentiated the susceptibility of factor IX to activation by factor XIa, concomitant with their effect on the conformation. Similarly, the required Ca2+ concentration was reduced by Mg2+ ions, and the rate of conversion to factor IXa was increased by Mg2+ ions in the presence of an excess of Ca2+ ions. At a saturating concentration of Ca2+ ions (5 mM), addition of 1 mM Mg2+ reduced the apparent Km value for factor IX from 0.31 to 0.18 microM, and in the presence of a physiological concentration of Ca2+ ions (1 mM), the reduction in Km by Mg2+ ions was far more striking (from 0.91 to 0.24 microM). The apparent Vmax values were hardly affected by Mg2+ ions. Our present data reveal a hitherto novel physiological role of the Mg2+ ions in plasma. Not only Ca2+ ions but also Mg2+ ions are important regulators of the stabilization of the native conformation of factor IX as well as of its efficient activation.