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

Human protein C and activated protein C Components of the human anticoagulation system

Protein C, the zymogen form of the anticoagulant protein, activated protein C, is a member of the vitamin K-dependent class of proteins that function in generation and control of formation of blood clots. This plasma protein consists of a series of domain regions that are qualitatively similar to those present in human coagulation factors VII, IX, and X, and that appear to be related to specific properties of these proteins. With the recent advent of rigorous application of genetic engineering strategies to this system, and the continuing discoveries and characterization of genotypes and phenotypes of protein C in patients, great progress has been made in understanding structure-function relationships of protein C and activated protein C. This review is a summary and synthesis of recent pertinent studies with an emphasis on these topics.

Interaction of bovine coagulation factor X and its glutamic-acid-containing fragments with phospholipid membranes. A surface plasmon resonance study

The interaction of blood coagulation factor X and its Gla-containing fragments with negatively charged phospholipid membranes composed of 25 mol% phosphatidylserine (PtdSer) and 75 mol% phosphatidylcholine (PtdCho) was studied by surface plasmon resonance. The binding to 100 mol% PtdCho membranes was negligible. The calcium dependence in the membrane binding was evaluated for intact bovine factor X (factor X) and the fragment containing the Gla-domain and the N-terminal EGF (epidermal growth factor)-like domain, Gla-EGFN, from factor X. Both proteins show the same calcium dependence in the membrane binding. Calcium binding is cooperative and half-maximum binding was observed at 1.5 mm and 1.4 mm, with the best fit to the experimental data with three cooperatively bound calcium ions for both the intact protein and the fragment. The dissociation constant (Kd) for binding to membranes containing 25 mol% PtdSer decreased from 4.6 microm for the isolated Gla-domain to 1 microm for the fragments Gla-EGFN and Gla-EGFNC (the Gla-domain and both EGF-like domains) fragments and to 40 nm for the entire protein as zymogen, activated enzyme or in the active-site inhibited form. Analysis of the kinetics of adsorption and desorption confirmed the equilibrium binding data.

Modeling zymogen protein C

A solution structure for the complete zymogen form of human coagulation protein C is modeled. The initial core structure is based on the x-ray crystallographic structure of the gamma-carboxyglutamic acid (Gla)-domainless activated form. The Gla domain (residues 1-48) is modeled from the x-ray crystal coordinates of the factor VII(a)/tissue factor complex and oriented with the epidermal growth factor-1 domain to yield an initial orientation consistent with the x-ray crystal structure of porcine factor IX(a). The missing C-terminal residues in the light chain (residues 147-157) and the activation peptide residues 158-169 were introduced using homology modeling so that the activation peptide residues directly interact with the residues in the calcium binding loop. Molecular dynamics simulations (Amber-particle-mesh-Ewald) are used to obtain the complete calcium-complexed solution structure. The individual domain structures of protein C in solution are largely unaffected by solvation, whereas the Gla-epidermal growth factor-1 orientation evolves to a form different from both factors VII(a) and IX(a). The solution structure of the zymogen protein C is compared with the crystal structures of the existing zymogen serine proteases: chymotrypsinogen, proproteinase, and prethrombin-2. Calculated electrostatic potential surfaces support the involvement of the serine protease calcium ion binding loop in providing a suitable electrostatic environment around the scissile bond for II(a)/thrombomodulin interaction.

Secretion, gamma-carboxylation, and endoplasmic reticulum-associated degradation of chimeras with mutually exchanged Gla domain between human protein C and prothrombin

Warfarin, an antagonist of vitamin K, causes diminution of vitamin K-dependent coagulation factors in the circulation. Although all vitamin K-dependent factors have Gla domains, the warfarin-induced decrease in their plasma concentration differs among factors. In warfarin-treated HepG2 cells, we found modest and severe intracellular degradation of prothrombin and protein C, respectively. To investigate the structural features of these proteins that contribute to their warfarin sensitivity, chimeric prothrombin containing the prepropeptide and Gla domain of protein C was expressed in baby hamster kidney (BHK) cells. This chimera showed similar secretion kinetics and warfarin sensitivity to those of wild-type prothrombin, demonstrating that the Gla domain cannot solely explain the warfarin sensitivity of protein C. In contrast, two chimeric protein Cs containing either the Gla domain alone or the prepropeptide and Gla domain of prothrombin showed impaired secretion. Even though gamma-carboxylation proceeded normally, both chimeras were degraded intracellularly by the proteasome. From these results, we conclude that not only the folding of the Gla domain, but the entire structure and conformation of protein C and prothrombin, contribute to their quality control and susceptibility to warfarin-induced ER (endoplasmic reticulum)-associated degradation.

A chimeric protein C containing the prothrombin Gla domain exhibits increased anticoagulant activity and altered phospholipid specificity

To determine the structural basis of phosphatidylethanolamine (PE)-dependent activated protein C (APC) activity, we prepared a chimeric molecule in which the Gla domain and hydrophobic stack of protein C were replaced with the corresponding region of prothrombin. APC inactivation of factor Va was enhanced 10-20-fold by PE. Protein S enhanced inactivation 2-fold and independently of PE. PE and protein S had little effect on the activity of the chimera. Factor Va inactivation by APC was approximately 5-fold less efficient than with the chimera on vesicles lacking PE and slightly more efficient on vesicles containing PE. The cleavage patterns of factor Va by APC and the chimera were similar, and PE enhanced the rate of Arg506 and Arg306 cleavage by APC but not the chimera. APC and the chimera bound to phosphatidylserine:phosphatidylcholine vesicles with similar affinity (Kd approximately 500 nM), and PE increased affinity 2-3-fold. Factor Va and protein S synergistically increased the affinity of APC on vesicles without PE to 140 nM and with PE to 14 nM, but they were less effective in enhancing chimera binding to either vesicle. In a factor Xa one-stage plasma clotting assay, the chimera had approximately 5 times more anticoagulant activity than APC on PE-containing vesicles. Unlike APC, which showed a 10 fold dependence on protein S, the chimera was insensitive to protein S. To map the site of the PE and protein S dependence further, we prepared a chimera in which residues 1-22 were derived from prothrombin and the remainder were derived from protein C. This protein exhibited PE and protein S dependence. Thus, these special properties of the protein C Gla domain are resident outside of the region normally hypothesized to be critical for membrane interaction. We conclude that the protein C Gla domain possesses unique properties allowing synergistic interaction with factor Va and protein S on PE-containing membranes.

The interaction between the endothelial cell protein C receptor and protein C is dictated by the gamma-carboxyglutamic acid domain of protein C

The endothelial cell protein C receptor (EPCR) binds to both protein C and activated protein C (APC) with similar affinity. Removal of the Gla domain of protein C results in the loss of most of the binding affinity. This observation is compatible with at least two models: 1) the Gla domain of protein C interacts with phospholipid on cell surfaces to stabilize interaction with EPCR or 2) the Gla domain of protein C makes specific protein-protein interactions with EPCR. The latter model predicts that chimeric proteins containing the protein C Gla domain should interact with EPCR. To test this, we constructed a prothrombin chimera in which the Gla domain and aromatic stack of prothrombin were replaced with the corresponding region of protein C. The 125I-labeled chimera (Kd = 176 nM) and 125I-APC (Kd = 65 nM) both bound specifically to 293 cells stably transfected with EPCR, but both bound poorly to sham-transfected cells. The chimera also blocked APC binding to EPCR-transfected cells in a dose-dependent fashion (Ki approximately 139 nM) similarly to protein C (Ki approximately 75 nM). Chimera binding to EPCR-transfected cells was blocked by soluble EPCR, demonstrating direct protein-protein interaction between the chimera and EPCR. Consistent with this conclusion, the isolated Gla domain of protein C blocked APC binding to EPCR-transfected cells (IC50 = 2 microM). No inhibition was observed with the isolated prothrombin Gla domain. A protein C chimera with the prothrombin Gla domain and aromatic stack failed to bind to EPCR detectably. These data suggest that the Gla domain of protein C is responsible for much of the binding energy and specificity of the protein C-EPCR interaction.

Highly conserved residue arginine-15 is required for the Ca2+-dependent properties of the gamma-carboxyglutamic acid domain of human anticoagulation protein C and activated protein C

The function of the rigidly conserved amino acid residue R15 in the Ca2+/phospholipid-dependent properties of the gamma-carboxyglutamic acid (Gla)-containing domain (GD) of human Protein C (PC) were investigated through site-directed mutagenesis strategies. A series of recombinant (r) mutants, namely r-[R15K]PC, r-[R15H]PC, r-[R15L]PC, and r-[R15W]PC, were constructed, expressed and purified, and their relevant properties investigated. As revealed by intrinsic fluorescence analysis, all of the variant proteins underwent Ca2+-dependent structural transitions. Nonetheless, they displayed altered binding properties to acidic phospholipid vesicles, and also did not interact with a monoclonal antibody specific for the type of Ca2+-dependent conformation of the GD that characterizes the wild-type protein. On conversion into their activated forms, these variant enzymes possessed less than 10% of the ex vivo plasma anticoagulant activity of wild-type r-PC. Similar activities were found when the r-active PC mutants were assayed directly for inactivation of factor Va and factor VIII, in the complete prothrombinase and tenase complexes respectively. We conclude that R15 is a critical residue in allowing the GD of PC, and probably of other proteins of this class, to adopt a Ca2+-dependent conformation that allows functional phospholipid binding, thus explaining the strict conservation of this amino acid residue in GD modules of various proteins. As a result of an analysis of structural models of the Ca2+-GD complex of PC, it is postulated that hydrogen bonds between the side chain of R15 and the functionally important Gla16 residue, as well as between the side chain of R15 and the carbonyl oxygen in the peptide bond of H10, are critical for adoption of a Ca2+-dependent conformation of the GD that allows functional phospholipid binding.

gamma-Carboxyglutamic acids 36 and 40 do not contribute to human factor IX function

The gamma-carboxyglutamic acid (Gla) domains of the vitamin K-dependent blood coagulation proteins contain 10 highly conserved Gla residues within the first 33 residues, but factor IX is unique in possessing 2 additional Gla residues at positions 36 and 40. To determine their importance, factor IX species lacking these Gla residues were isolated from heterologously expressed human factor IX. Using ion-exchange chromatography, peptide mapping, mass spectrometry, and N-terminal sequencing, we have purified and identified two partially carboxylated recombinant factor IX species; factor IX/gamma 40E is uncarboxylated at residue 40 and factor IX/gamma 36,40E is uncarboxylated at both residues 36 and 40. These species were compared with the fully gamma-carboxylated recombinant factor IX, unfractionated recombinant factor IX, and plasma-derived factor IX. As monitored by anti-factor IX:Ca (II)-specific antibodies and by the quenching of intrinsic fluorescence, all these factor IX species underwent the Ca(II)-induced conformational transition required for phospholipid membrane binding and bound equivalently to phospholipid vesicles composed of phosphatidylserine, phosphatidylcholine, and phosphatidylethanolamine. Endothelial cell binding was also similar in all species, with half-maximal inhibition of the binding of 125I-labeled plasma-derived factor IX at concentrations of 2-6 nM. Functionally, factor IX/gamma 36,40E and factor IX/gamma 40E were similar to fully gamma-carboxylated recombinant factor IX and plasma-derived factor IX in their coagulant activity and in their ability to participate in the activation of factor X in the tenase complex both with synthetic phospholipid vesicles and activated platelets. However, Gla 36 and Gla 40 represent part of the epitope targeted by anti-factor IX:Mg(II)-specific antibodies because these antibodies bound factor IX preferentially to factor IX/gamma 36,40E and factor IX/gamma 40E. These results demonstrate that the gamma-carboxylation of glutamic acid residues 36 and 40 in human factor IX is not required for any function of factor IX examined.

Hydrophobic amino acid residues of human anticoagulation protein C that contribute to its functional binding to phospholipid Vesicles

The contributions to functional phospholipid (PL) binding of the cluster of amino acid side chains of human protein C (PC) comprising F4, L5, and L8 have been assessed by construction of mutants of PC and activated protein C (APC) designed wherein a hydrophilic side chain replaced the wild-type hydrophobic groups at these positions. The PL-dependent plasma-based anticoagulant activities of [F4Q]-r-APC and [L8Q]r-APC were severely reduced to 5% and < 2%, respectively, of wild-type r-APC. Activity losses of the mutants toward inactivation of coagulation factor VIII, measured in the complete in vitro tenase system, have also been observed. As evidenced through Ca(2+)-induced intrinsic fluorescence changes, both [F4Q]r-PC and [L8Q]r-PC were able to adopt Ca(2+)-dependent conformations that appeared similar to that of wtr-PC, ruling out shortcomings associated with such Ca(2+)-induced transitions as the basis for their anticoagulant activity losses. However, despite this, [L8Q]r-PC showed greatly defective macroscopic binding properties to PL vesicles, as did to a lesser extent [F4Q]r-PC. These findings were similar to those reported previously for [L5Q]r-PC/APC [Zhang, L., & Castellino, F. J. (1994) J. Biol. Chem. 269, 3590-3595]. We thus propose that the PL-dependent activity losses of these mutants are related to their suboptimal binding to PL or to their misorientation on the PL surface leading to poor alignment of the active sites of the r-APC mutants with the complementary cleavage sites on fVIII/fVIIIa and fV/fVa.(ABSTRACT TRUNCATED AT 250 WORDS)

Transfer of specific endothelial cell-binding properties from the procoagulant protein human factor IX into the anticoagulant protein human protein C

A series of recombinant (r) chimeric mutants of human coagulation protein C (PC) and activated protein C (APC) containing replacements of homologous PC domains by those of human coagulation factor IX (fIX) were generated, with the intention of determining whether the specific bovine aortic endothelial cell (BAEC) receptor-binding characteristics of fIX could be incorporated into the chimeric r-PC while maintaining the essential properties of PC and APC. Using a competitive BAEC displacement assay with [125I]fIX, we found that a chimeric r-PC (r[delta PC1-46/delta fIX1-47]PC), consisting of the entire gamma-carboxyglutamic domain ([GDIX], residues 1-38) and helical stack ([HSIX], residues 38-47) of fIX as replacements for these same domains of PC, provided an IC50 for fIX-related BAEC binding of 13 nM, as compared to 10 nM for that of unlabeled fIX. This showed that all of the BAEC tight binding determinants for fIX existed within the [GDIX/HSIX]. Additionally, this chimera reacted to the same extent as fIX with the Ca(2+)-dependent, [GDIX]-specific monoclonal antibody H5B7 and lost its reactivity to a similar antibody specific for the [GDPC], JTC1. A synthetic peptide containing residues 1-47 of fIX also competed effectively (IC50 = 16 nM) with intact fIX for BAEC binding. Displacement of [125I]fIX from BAEC did not occur with a chimera containing the [HSIX] alone or with another mutant protein possessing a replacement of the two epidermal growth factor (EGF) homology regions of r-PC (residues 47-137) with those same domains of fIX.(ABSTRACT TRUNCATED AT 250 WORDS)