Calcium enhances factor Xa activity independent of gamma-carboxyglutamic acid residues

Mechanism of the Ca2+-induced enhancement of the intrinsic factor VIIa activity

The intrinsic activity of coagulation factor VIIa (FVIIa) is dependent on Ca(2+) binding to a loop (residues 210-220) in the protease domain. Structural analysis revealed that Ca(2+) may enhance the activity by attenuating electrostatic repulsion of Glu(296) and/or by facilitating interactions between the loop and Lys(161) in the N-terminal tail. In support of the first mechanism, the mutations E296V and D212N resulted in similar, about 2-fold, enhancements of the amidolytic activity. Moreover, mutation of the Lys(161)-interactive residue Asp(217) or Asp(219) to Ala reduced the amidolytic activity by 40-50%, whereas the K161A mutation resulted in 80% reduction. Hence one of these Asp residues in the Ca(2+)-binding loop appears to suffice for some residual interaction with Lys(161), whereas the more severe effect upon replacement of Lys(161) is due to abrogation of the interaction with the N-terminal tail. However, Ca(2+) attenuation of the repulsion between Asp(212) and Glu(296) keeps the activity above that of apoFVIIa. Altogether, our data suggest that repulsion involving Asp(212) in the Ca(2+)-binding loop suppresses FVIIa activity and that optimal activity requires a favorable interaction between the Ca(2+)-binding loop and the N-terminal tail. Crystal structures of tissue factor-bound FVIIa(D212N) and FVIIa(V158D/E296V/M298Q) revealed altered hydrogen bond networks, resembling those in factor Xa and thrombin, after introduction of the D212N and E296V mutations plausibly responsible for tethering the N-terminal tail to the activation domain. The charge repulsion between the Ca(2+)-binding loop and the activation domain appeared to be either relieved by charge removal and new hydrogen bonds (D212N) or abolished (E296V). We propose that Ca(2+) stimulates the intrinsic FVIIa activity by a combination of charge neutralization and loop stabilization.

Structure and dynamics of zymogen human blood coagulation factor X

The solution structure and dynamics of the human coagulation factor X (FX) have been investigated to understand the key structural elements in the zymogenic form that participates in the activation process. The model was constructed based on the 2.3-A-resolution x-ray crystallographic structure of active-site inhibited human FXa (PDB:1XKA). The missing gamma-carboxyglutamic acid (GLA) and part of epidermal growth factor 1 (EGF1) domains of the light chain were modeled based on the template of GLA-EGF1 domains of the tissue factor (TF)-bound FVIIa structure (PDB:1DAN). The activation peptide and other missing segments of FX were introduced using homology modeling. The full calcium-bound model of FX was subjected to 6.2 ns of molecular dynamics simulation in aqueous medium using the AMBER6.0 package. We observed significant reorientation of the serine-protease (SP) domain upon activation leading to a compact multi-domain structure. The solution structure of zymogen appears to be in a well-extended conformation with the distance between the calcium ions in the GLA domain and the catalytic residues estimated to be approximately 95 A in contrast to approximately 83 A in the activated form. The latter is in close agreement with fluorescence studies on FXa. The S1-specificity residues near the catalytic triad show significant differences between the zymogen and activated structures.

Intravenously and topically applied magnesium in the prevention of arterial thrombosis

Magnesium (Mg) has been shown to reduce platelet aggregation both in vitro and ex vivo, and this antiplatelet effect may be advantageous in the prevention of arterial thrombosis. Previous animal studies have shown an antithrombotic effect of Mg also in vivo, but mainly with higher Mg concentrations ( approximately 3.0-4.0 mM). The objectives of the present study were to evaluate the antithrombotic effect of (1) intravenous Mg at a lower and clinically more relevant concentration and (2) topically applied Mg. The study comprised 30 male rats, randomly assigned into 3 groups: (1) placebo group, (2) intravenous Mg group, and (3) topical Mg group. A thrombogenic lesion was established by making a standardised arteriotomy in the right femoral artery. The vessel was transilluminated and thrombus formation was visualised dynamically by in vivo microscopy and recorded on videotapes. Thrombus area was measured after ended experiment by computer-assisted image analysis. Intravenously administered Mg, elevating the S-Mg level to 2.2 mmol/L, significantly reduced the mean thrombus area (p<0.05) compared to the control group. Topically applied Mg significantly decreased the maximum thrombus area, without any increase in S-Mg level (p<0.05). The Mg-treated groups showed no increase in bleeding complications. A transient fall in blood pressure was seen in the systemic Mg group, but blood pressures were not significantly different between any of the groups at the end of the experiment. In conclusion, topically as well as intravenously infused Mg reduce arterial thrombus formation in this in vivo rat model without compromising haemostasis.

Interaction of calcium with native and decarboxylated human factor X. Effect of proteolysis in the autolysis loop on catalytic efficiency and factor Va binding

Human factor X is a two-chain, 58-kDa, vitamin K-dependent blood coagulation zymogen. The light chain of factor X consists of an NH2-terminal gamma-carboxyglutamic acid (Gla) domain, followed by a few helical hydrophobic residues and the two epidermal growth factor-like domains, whereas the heavy chain contains the serine protease domain. In this study, native factor X was found to contain three classes of Ca2+-binding sites: two high affinity (Kd 100 +/- 30 microM), four intermediate affinity (Kd 450 +/- 70 microM), and five to six low affinity (Kd 2 +/- 0.2 mM). Decarboxylated factor X in which the Gla residues were converted to Glu retained the two high affinity sites (Kd 140 +/- 20 microM). In contrast, factor X lacking the Gla domain as well as a part of the helical hydrophobic residues (des-44-X) retained only one high affinity Ca2+-binding site (Kd 130 +/- 20 microM). Moreover, a synthetic peptide composed of residues 238-277 (58-97 in chymotrypsinogen numbering) from the protease domain of factor X bound one Ca2+ with high affinity (Kd 150 +/- 20 microM). From competitive inhibition assays for binding of active site-blocked factor Xa to factor Va in the prothrombinase complex, the Kd for peptide-Va interaction was calculated to be approximately 10 microM as compared with 30 pM for factor Xa and approximately 1.5 microM for decarboxylated factor Xa. A peptide containing residues 238-262(58-82) bound Ca2+ with reduced affinity (Kd approximately 600 microM) and did not inhibit Xa:Va interaction. In contrast, a peptide containing residues 253-277(73-97) inhibited Xa:Va interaction (Kd approximately 10 microM) but did not bind Ca2+. In additional studies, Ca2+ increased the amidolytic activity of native and des-44-Xa toward a tetrapeptide substrate (benzoyl-Ile-Glu-Gly-Arg-p-nitroanilide) by approximately 1.6-fold. The half-maximal increase was observed at approximately 150 microM Ca2+ and the effect was primarily on the kcat. Ca2+ also significantly protected cleavage at Arg-332-Gln-333(150-151) in the protease domain autolysis loop. Des-44-Xa in which the autolysis loop was cleaved possessed </=5% of the amidolytic activity of the noncleaved form; however, the S1 binding site was not affected, as determined by the p-aminobenzamidine binding. Additionally, autolysis loop-cleaved, active site-blocked native factor Xa was calculated to have approximately 10-fold reduced affinity for factor Va as compared with that of the noncleaved form.

Hemostatic changes after early versus late intracoronary magnesium during acute myocardial infarction in swine

There has been some debate regarding the benefit of magnesium (Mg) in the treatment of acute myocardial infarction (AMI) because of conflicting results from recent clinical trials. Several different hypotheses have been advanced to explain the cardioprotective properties of Mg, including the influence of the timing of Mg administration during AMI. This experiment was designed to assess the effect of intracoronary Mg on certain hemostatic parameters that are known to change during an AMI. Yorkshire swine underwent thoracotomy and 50 min left anterior descending artery (LAD) occlusion, followed by 3 h of reperfusion. In the early group, 250 mg of MgSO4 was delivered at the onset of reperfusion (n = 6, Mg-early group). In the second group, MgSO4 was given after 1 h of reperfusion (n = 6, Mg-late group). Six animals received saline instead of Mg and served as controls. The dynamics of plasma antithrombin-III (AT-III), protein C, total protein S, fibronectin, endothelin-1 (ET-1), as well as the stable metabolites of thromboxane (TXB2) and prostacyclin (6-keto-PGFla) were determined at baseline, twice during occlusion, and three times during reperfusion. Mg given at reperfusion onset was associated with a diminished ET-1 (32.9%), decreased fibronectin level (21.7-25.2%), and increased protein C concentrations (31.9-52.3%) when compared with both the control and late Mg group. In summary, intracoronary Mg administered at the onset of reperfusion favorably influenced hemostasis in swine. The beneficial effects of early Mg supplementation in an expanding array of clinical conditions, including AMI, may be directly related to the improved hemostatic profile in such patients.