The importance of specific gamma-carboxyglutamic acid residues in prothrombin. Evaluation by site-specific mutagenesis

A binding site expressed on the surface of activated human platelets is shared by factor X and prothrombin

We have demonstrated the presence of a saturable, reversible, and Ca(2+)-dependent binding site for 125I-labeled factor X ([125I]factor X) on human platelets (16000 +/- 2000 sites per platelet, Kd = 320 +/- 40 nM, n = 12) activated with either thrombin or the thrombin receptor agonist peptide, SFLLRN-amide, but not with ADP. Bound [125I]factor X could be completely removed by the addition of a Ca2+ chelator or an excess of unlabeled factor X. Antibodies that inhibit binding of factor X to the MAC-1 integrin receptor of monocytes and those directed against human factor V, failed to disrupt [125I]factor X binding to platelets. Prothrombin, but neither factor VII, factor IX, protein C, nor protein S, was an effective competitor of [125I]factor X binding with a K1 approximately Kd. [125I]Prothrombin also binds to activated (but not unactivated) platelets in a saturable, reversible, and Ca(2+)-dependent manner (20500 +/- 1500 sites, Kd = 470 +/- 110 nM, n = 3). Annexin V potently inhibited the binding of both [125I]factor X and [125I]prothrombin (IC50 approximately 3 nM). Factor X, prothrombin, and prothrombin fragment 1 (residues 1-155) were equipotent inhibitors of [125I]prothrombin and [125I]factor X binding, whereas Gla-domain-less factor X was unable to compete with [125I]factor X for platelet binding sites. Thus, it is the Gla-domains of factor X and prothrombin that appear to contain the regions necessary for platelet binding. The results of studies utilizing artificial phospholipid surfaces have led to the hypothesis that the substrates (FX and prothrombin) for the intrinsic pathway FXase and prothrombinase complexes are bound to the phospholipid surface. The factor X/prothrombin binding site we have described on the surface of activated platelets permits the utilization of surface-bound substrates by these complexes when they are assembled on a physiologic surface.

Identification of the phospholipid binding site in the vitamin K-dependent blood coagulation protein factor IX

The blood coagulation and regulatory proteins that contain gamma-carboxyglutamic acid are a part of a unique class of membrane binding proteins that require calcium for their interaction with cell membranes. Following protein biosynthesis, glutamic acids on these proteins are converted to gamma-carboxyglutamic acid (Gla) in a reaction that requires vitamin K as a cofactor. The vitamin K-dependent proteins undergo a conformational transition upon metal ion binding, but only calcium ions mediate protein-phospholipid interaction. To identify the site on Factor IX that is required for phospholipid binding, we have determined the three-dimensional structure of the Factor IX Gla domain bound to magnesium ions by NMR spectroscopy. By comparison of this structure to that of the Gla domain bound to calcium ions, we localize the membrane binding site to a highly ordered structure including residues 1-11 of the Gla domain. In the presence of Ca2+, Factor IX Gla domain peptides that contain the photoactivatable amino acid p-benzoyl-L-phenylalanine at positions 6 or 9 cross-link to phospholipid following irradiation, while peptides lacking this amino acid analog or with this analog at position 46 did not cross-link. These results indicate that the NH2 terminus of the Gla domain, specifically including leucine 6 and phenylalanine 9 in the hydrophobic patch, is the contact surface on Factor IX that interacts with the phospholipid bilayer.

Site-directed mutagenesis but not gamma-carboxylation of Glu-35 in factor VIIa affects the association with tissue factor

Factor VIIa is a vitamin K-dependent enzyme whose gamma-carboxyglutamic acid (Gla)-containing domain is important for calcium ion-dependent binding to the cofactor tissue factor and membrane surfaces. This domain contains 10 Gla residues, the individual roles and importance of which are not known. Comparisons with the homologous protein C, factor IX and prothrombin may provide functional information on the first nine Gla residues, whereas no data can be extrapolated to Gla-35 in factor VIIa. Therefore, the effects of posttranslational gamma-carboxylation and site-directed mutagenesis of Glu-35 were investigated. Mutations to Asp, Gln or Val all lead to a lower affinity for tissue factor by decreasing the rate of association, in the case of the Val mutant by a factor of 200, as measured by surface plasmon resonance. In contrast, Glu or Gla side chains at position 35 appear to fulfil the functional roles equally well.

Biospecific interactions of Vitamin K-dependent factors with phospholipid-like polystyrene derivatives. Part I: Factor II

Phosphorylated polystyrene derivatives with different compositions in phosphate groups were shown to be either recognized as phospholipidic or as DNA-like surfaces by antibodies from Systemic Lupus Erythematosus patients. In order to check whether these polymers were able to interact with Vitamin K-dependent coagulation factors, phosphorylated resins of various compositions in phosphate groups were assessed with regard to their interactions with Factor II, one of the Vitamin K-dependent factors. These studies were performed either in the presence or the absence of calcium ions, and with or without albumin precoating of the polymers. The results show that the affinity of the protein for the polymer is increased in the presence of calcium ions and depends on the composition of the polymer. The protein-polymer interactions involve the formation of binary or ternary complexes and the domains of predominance of these complexes were determined as a function of the calcium ion concentration in the assay. This allowed us to propose optimal conditions for Factor II purification by highly specific liquid chromatography using phosphorylated polystyrene resins of given compositions as stationary phases.

Homology modeling and molecular dynamics simulation of human prothrombin fragment 1

The crystallographic structure of bovine prothrombin fragment 1 bound with calcium ions was used to construct the corresponding human prothrombin structure (hf1/Ca). The model structure was refined by molecular dynamics to estimate the average solution structure. Accommodation of long-range ionic forces was essential to reach a stable solution structure. The gamma-carboxyglutamic acid (Gla) domain and the kringle domain of hf1/Ca independently equilibrated. Likewise, the hydrogen bond network and the calcium ion coordinations were well preserved. A discussion of the phospholipid binding of the vitamin K-dependent coagulation proteins in the context of the structure and mutational data of the Gla domain is presented.

Structure of the calcium ion-bound gamma-carboxyglutamic acid-rich domain of factor IX

We have determined the Ca(II)-bound structure of factor IX, residues 1-47, by nuclear magnetic resonance (NMR) spectroscopy. The amino-terminal 47 residues include the gamma-carboxyglutamic acid-rich and aromatic amino acid stack domains, and this region is responsible for Ca(II)-dependent phospholipid binding in factor IX. Protons in the 1-47 amino acid sequence were assigned using standard two-dimensional homonuclear NMR experiments. A total of 851 distance restraints and 57 torsion angle restraints were used to generate 17 final structures by distance geometry and simulated annealing methods. The backbone RMSD to the geometric average is 0.6 +/- 0.1 A. The Ca(II)-bound structure is substantially more ordered with increased helical content compared to the apo-factor IX (1-47) structure. The global fold is similar to the crystal structure of the Ca(II)-bound Gla domain of prothrombin fragment I from residues 12 to 47 (RMSD approximately 1.3 A), but the backbone conformation differs in the first 11 residues, particularly between residues 3 and 6. The amino-terminal nine Gla residues are oriented to the interior of the protein and suggest an internal Ca(II) binding pocket. The carboxyl-terminal three Gla residues are exposed to solvent. The majority of hydrophobic residues are required to stabilize a globular core in the carboxyl-terminal three-quarters of the molecule. However, a hydrophobic surface patch in the amino-terminal region may represent a phospholipid binding site in factor IX.

Structure of the Ca(2+)-free Gla domain sheds light on membrane binding of blood coagulation proteins

Reversible membrane binding of gamma-carboxyglutamic acid (Gla)-containing coagulation factors requires Ca(2+)-binding to 10-12 Gla residues. Here we describe the solution structure of the Ca(2+)-free Gla-EGF domain pair of factor x which reveals a striking difference between the Ca(2+)-free and Ca(2+)-loaded forms. In the Ca(2+)-free form Gla residues are exposed to solvent and Phe 4, Leu 5 and Val 8 form a hydrophobic cluster in the interior of the domain. In the Ca(2+)-loaded form Gla residues ligate Ca2+ in the core of the domain pushing the side-chains of the three hydrophobic residues into the solvent. We propose that the Ca(2+)-induced exposure of hydrophobic side chains is crucial for membrane binding of Gla-containing coagulation proteins.

Structure of the metal-free gamma-carboxyglutamic acid-rich membrane binding region of factor IX by two-dimensional NMR spectroscopy

The gamma-carboxyglutamic acid-rich domain of blood coagulation Factor IX is required for the binding of the protein to phospholipid membranes. To investigate the three-dimensional structure of this domain, a synthetic peptide corresponding to residues 1-47 of Factor IX was studied by 1H NMR spectroscopy. In the absence of metal ions, the proton chemical shift dispersion in the one-dimensional NMR spectrum indicated that the peptide contains regular structural elements. Upon the addition of Ca(II) or Mg(II), large chemical shift changes were observed in the amide proton and methyl proton regions of the spectrum, consistent with the conformational transitions that metal ions are known to induce in native Factor IX. The apopeptide was studied by two-dimensional NMR spectroscopy at 500 MHz to determine its solution structure. Protons were assigned using total correlation spectroscopy, nuclear Overhauser effect spectroscopy, and double quantum-filtered correlation spectroscopy experiments. Intensities of cross-peaks in the nuclear Overhauser effect spectrum were used to generate a set of interproton distance restraints. The structure of the apopeptide was then calculated using distance geometry methods. There are three structural elements in the apopeptide that are linked by a flexible polypeptide backbone. These elements include a short amino-terminal tetrapeptide loop (amino acids 6-9), the disulfide-containing hexapeptide loop (amino acids 18-23), and a carboxyl-terminal alpha helix (amino acids 37-46). Amide hydrogen exchange kinetics indicate that the majority of the peptide is solvent accessible, except in the carboxyl-terminal element. The structured regions in the apopeptide are insufficient to support phospholipid binding, indicating the importance of additional structural features in the Ca(II)-stabilized conformer.

Properties of recombinant chimeric human protein C and activated protein C containing the gamma-carboxyglutamic acid and trailing helical stack domains of protein C replaced by those of human coagulation factor IX

The properties of a recombinant (r) chimeric human protein C (PC) containing replacement of its gamma-carboxyglutamic acid (Gla) and helical stack (HS) domains by those of human coagulation factor IX (fIX) have been examined. Titration with Ca2+ of the divalent cation-induced intrinsic fluorescence quenching of this chimera (r-GDIX/PC) allowed determination of the [Ca2+], of 1.8 mM, required to produce this alteration in 50% of the protein molecules. These values were 0.41 and 0.61 mM for wtr-PC and fIX, respectively. The chimera did not react with a Ca(2+)-dependent, Gla domain-directed conformational monoclonal antibody (MAb) to r-PC but did interact with a similar MAb (H5B7) to fIX. The [Ca2+] required to induced H5B7 binding to 50% of the r-GDIX/PC molecules was 6.6 mM, while this same value for fIX was a nearly identical 7.2 mM. The [Ca2+] needed for binding of 50% of r-GDIX/PC to acidic phospholipid (PL) vesicles was 0.58 mM, while that for wtr-PC and fIX were 1.2 and 0.55 mM, respectively. The [protein] required for 50% binding of r-GDIX/PC to PL at 20 mM Ca2+ was 0.29 microM. These same values for r-PC and fIX were 0.38 and 1.8 microM, respectively. The Ca(2+)-mediated inhibition of the thrombin-catalyzed activation of r-GDIX/PC was characterized by a Ki of 118 microM, a value similar to that of 125 microM obtained for this same inhibition of wtr-PC activation. The thrombin-catalyzed activation of both r-GDIX/PC and wtr-PC was stimulated by soluble r-thrombomodulin. Similar to the case of wtr-PC, Ca2+ initially enhanced and, at higher concentrations, inhibited the activation of r-GDIX/PC. The Km and kcat values for this latter activation at optimal [Ca2+] (100 microM) were 4.1 microM and 2.5 s-1, respectively. These same kinetic constants for activation of wtr-PC were 4.3 microM and 2.9 s-1, respectively. These results show that many of the features needed for functional integrity of the Ca2+-bound Gla/HS domains of PC are also present in those same modules of fIX, a finding that points to a generalized functional role for the Ca2+-induced conformation of the structural unit consisting of the Gla and HS domains. The data also suggest that the Ca2+-bound form of the Gla/HS region is an independently folded unit in PC and perhaps in fIX.(ABSTRACT TRUNCATED AT 400 WORDS)