Functions of individual gamma-carboxyglutamic acid (Gla) residues of human protein c. Determination of functionally nonessential Gla residues and correlations with their mode of binding to calcium

Activated protein C action in inflammation

Activated protein C (APC) is a natural anticoagulant that plays an important role in coagulation homeostasis by inactivating the procoagulation factor Va and VIIIa. In addition to its anticoagulation functions, APC also has cytoprotective effects such as anti-inflammatory, anti-apoptotic, and endothelial barrier protection. Recently, a recombinant form of human APC (rhAPC or drotrecogin alfa activated; known commercially as 'Xigris') was approved by the US Federal Drug Administration for treatment of severe sepsis associated with a high risk of mortality. Sepsis, also known as systemic inflammatory response syndrome (SIRS) resulting from infection, is a serious medical condition in critical care patients. In sepsis, hyperactive and dysregulated inflammatory responses lead to secretion of pro- and anti-inflammatory cytokines, activation and migration of leucocytes, activation of coagulation, inhibition of fibrinolysis, and increased apoptosis. Although initial hypotheses focused on antithrombotic and profibrinolytic functions of APC in sepsis, other agents with more potent anticoagulation functions were not effective in treating severe sepsis. Furthermore, APC therapy is also associated with the risk of severe bleeding in treated patients. Therefore, the cytoprotective effects, rather than the anticoagulant effect of APC are postulated to be responsible for the therapeutic benefit of APC in the treatment of severe sepsis.

Mouse recombinant protein C variants with enhanced membrane affinity and hyper-anticoagulant activity in mouse plasma

Mouse anticoagulant protein C (461 residues) shares 69% sequence identity with its human ortholog. Interspecies experiments suggest that there is an incompatibility between mouse and human protein C, such that human protein C does not function efficiently in mouse plasma, nor does mouse protein C function efficiently in human plasma. Previously, we described a series of human activated protein C (APC) Gla domain mutants (e.g. QGNSEDY-APC), with enhanced membrane affinity that also served as superior anticoagulants. To characterize these Gla mutants further in mouse models of diseases, the analogous mutations were now made in mouse protein C. In total, seven mutants (mutated at one or more of positions P(10)S(12)D(23)Q(32)N(33)) and wild-type protein C were expressed and purified to homogeneity. In a surface plasmon resonance-based membrane-binding assay, several high affinity protein C mutants were identified. In Ca(2+) titration experiments, the high affinity variants had a significantly reduced (four-fold) Ca(2+) requirement for half-maximum binding. In a tissue factor-initiated thrombin generation assay using mouse plasma, all mouse APC variants, including wild-type, could completely inhibit thrombin generation; however, one of the variants denoted mutant III (P10Q/S12N/D23S/Q32E/N33D) was found to be a 30- to 50-fold better anticoagulant compared to the wild-type protein. This mouse APC variant will be attractive to use in mouse models aiming to elucidate the in vivo effects of APC variants with enhanced anticoagulant activity.

Regulation of protein C inhibitor (PCI) activity by specific oxidized and negatively charged phospholipids

Protein C inhibitor (PCI) is a serpin with affinity for heparin and phosphatidylethanolamine (PE). We analyzed the interaction of PCI with different phospholipids and their oxidized forms. PCI bound to oxidized PE (OxPE), and oxidized and unoxidized phosphatidylserine (PS) immobilized on microtiter plates and in aqueous suspension. Binding to OxPE and PS was competed by heparin, but not by the aminophospholipid-binding protein annexin V or the PCI-binding lipid retinoic acid. PS and OxPE stimulated the inhibition of activated protein C (aPC) by PCI in a Ca++-dependent manner, indicating that binding of both, aPC (Ca++ dependent) and PCI (Ca++ independent), to phospholipids is necessary. A peptide corresponding to the heparin-binding site of PCI abolished the stimulatory effect of PS on aPC inhibition. No stimulatory effect of phospholipids on aPC inhibition was seen with a PCI mutant lacking the heparin-binding site. A heparin-like effect of phospholipids (OxPE) was not seen with antithrombin III, another heparin-binding serpin, suggesting that it is specific for PCI. PCI and annexin V were found to be endogenously colocalized in atherosclerotic plaques, supporting the hypothesis that exposure of oxidized PE and/or PS may be important for the local regulation of PCI activity in vivo.

Multifunctional specificity of the protein C/activated protein C Gla domain

Activated protein C (APC) has potent anticoagulant and anti-inflammatory properties that are mediated in part by its interactions with its cofactor protein S and the endothelial cell protein C receptor (EPCR). The protein C/APC Gla domain is implicated in both interactions. We sought to identify how the protein C Gla domain enables specific protein-protein interactions in addition to its conserved role in phospholipid binding. The human prothrombin Gla domain, which cannot bind EPCR or support protein S cofactor activity, has 22/45 residues that are not shared with the human protein C Gla domain. We hypothesized that the unique protein C/APC Gla domain residues were responsible for mediating the specific interactions. To assess this, we generated 13 recombinant protein C/APC variants incorporating the prothrombin residue substitutions. Despite anticoagulant activity similar to wild-type APC in the absence of protein S, APC variants APC(PT33-39) (N33S/V34S/D35T/D36A/L38D/A39V) and APC(PT36/38/39) (D36A/L38D/A39V) were not stimulated by protein S, whereas APC(PT35/36) (D35T/D36A) exhibited reduced protein S sensitivity. Moreover, PC(PT8/10) (L8V/H10K) displayed negligible EPCR affinity, despite normal binding to anionic phospholipid vesicles and factor Va proteolysis in the presence and absence of protein S. A single residue variant, PC(PT8), also failed to bind EPCR. Factor VIIa, which also possesses Leu-8, bound soluble EPCR with similar affinity to wild-type protein C, collectively confirming Leu-8 as the critical residue for EPCR recognition. These results reveal the specific Gla domain residues responsible for mediating protein C/APC molecular recognition with both its cofactor and receptor and further illustrate the multifunctional potential of Gla domains.

Selective modulation of protein C affinity for EPCR and phospholipids by Gla domain mutation

Uniquely amongst vitamin K-dependent coagulation proteins, protein C interacts via its Gla domain both with a receptor, the endothelial cell protein C receptor (EPCR), and with phospholipids. We have studied naturally occurring and recombinant protein C Gla domain variants for soluble (s)EPCR binding, cell surface activation to activated protein C (APC) by the thrombin-thrombomodulin complex, and phospholipid dependent factor Va (FVa) inactivation by APC, to establish if these functions are concordant. Wild-type protein C binding to sEPCR was characterized with surface plasmon resonance to have an association rate constant of 5.23 x 10(5) m(-1).s(-1), a dissociation rate constant of 7.61 x 10(-2) s(-1) and equilibrium binding constant (K(D)) of 147 nm. It was activated by thrombin over endothelial cells with a K(m) of 213 nm and once activated to APC, rapidly inactivated FVa. Each of these interactions was dramatically reduced for variants causing gross Gla domain misfolding (R-1L, R-1C, E16D and E26K). Recombinant variants Q32A, V34A and D35A had essentially normal functions. However, R9H and H10Q/S11G/S12N/D23S/Q32E/N33D/H44Y (QGNSEDY) variants had slightly reduced (< twofold) binding to sEPCR, arising from an increased rate of dissociation, and increased K(m) (358 nm for QGNSEDY) for endothelial cell surface activation by thrombin. Interestingly, these variants had greatly reduced (R9H) or greatly enhanced (QGNSEDY) ability to inactivate FVa. Therefore, protein C binding to sEPCR and phospholipids is broadly dependent on correct Gla domain folding, but can be selectively influenced by judicious mutation.

Role of the gamma-carboxyglutamic acid domain of activated factor X in the presence of calcium during inhibition by antithrombin-heparin

BACKGROUND

Factor (F)Xa has 11 gamma-carboxylated glutamic acid (Gla) residues that are involved in calcium-dependent membrane binding. The serpin antithrombin (AT) is an important physiological regulator of FXa activity in an inhibition reaction that is enhanced by heparin. Recently, Rezaie showed that calcium further enhanced the heparin-catalyzed AT inhibition of FXa by promoting 'ternary complex' formation, and these results showed a role for the gamma-carboxyl-glutamate (Gla)-domain of FXa.

OBJECTIVES

In this study, we used recombinant FXa mutants to assess the role of individual Gla residues in augmenting or antagonizing the AT-heparin inhibition reaction in the presence of calcium.

RESULTS AND CONCLUSIONS

In the absence of heparin, AT inhibition of plasma and the recombinant FXas were essentially equivalent. Similar to plasma-derived FXa, calcium increased about 3-fold the inhibition rate of wild-type recombinant FXa by AT-heparin over that in the presence of EDTA. Interestingly, three different effects were found with the recombinant FXa Gla-mutants for AT-heparin inhibition: (i) Gla-->Asp 14 and 29 were enhanced without calcium; (ii) Gla-->Asp 16 and 26 were not enhanced by calcium; and (iii) Gla-->Asp 19 was essentially the same as wild-type recombinant FXa. These results support a theory that mutating individual Gla residues in FXa alters the calcium-induced conformational changes in the Gla region and affects the antithrombin-heparin inhibition reaction.

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.

Vitamin K-dependent proteins

Vitamin K is required for the synthesis of gamma-carboxyglutamate (Gla) during postribosomal protein modification. Substrates include blood clotting proteins, bone proteins, cell signaling, and receptor proteins. In addition, Gla is a component of short toxin peptides from the marine snail Conus. Studies of structure-function relationships are the most advanced for the blood coagulation proteins. Reviews of vitamin K action and blood coagulation are presented. Special focus is on the structure-function role of Gla in blood coagulation and the impact of this amino acid on enzyme reaction kinetics. This amino acid forms calcium and membrane binding sites for these proteins. Two proposed mechanisms of protein-membrane attachment are reviewed. One involves membrane attachment by protein insertion into the hydrocarbon region of the membrane, while another considers attachment by specific interactions with phospholipid head groups. Membrane attachment generates the potential for several forms of nonclassical enzyme kinetic behaviors, all of which have been observed in vitro. For example, the reaction may be limited by properties of the enzyme active site, a condition that allows use of classic steady-state enzyme kinetic parameters. However, the reaction may be limited by substrate binding to the membrane, by substrate flux through solution, and/or by solvent flow rates across the membrane surface. These states provide special mechanisms that are not anticipated by classical steady-state kinetic derivations. They may be used to regulate coagulation in vivo. Overall, vitamin K research spans the spectrum of biological research and experience. Exciting new ideas and findings continue to emanate from vitamin K-related research.

Undercarboxylation of recombinant prothrombin revealed by analysis of gamma-carboxyglutamic acid using capillary electrophoresis and laser-induced fluorescence

The gamma-carboxyglutamic acid (Gla) content of several variants of human prothrombin has been measured by using capillary electrophoresis and laser-induced fluorescence (CE-LIF). Both plasma-derived prothrombin and recombinant prothrombin contain ten residues of Gla per molecule of protein. In contrast, a variant of human prothrombin (containing the second kringle domain of bovine prothrombin) was separated into two populations that differed in their Gla content. Direct measurement of the Gla content showed an association with the presence or absence of the calcium-dependent conformational change that is required for prothombinase function. Thus, the CE-LIF assay is useful in determining the carboxylation status of recombinant proteins.

Enhancement of human protein C function by site-directed mutagenesis of the gamma-carboxyglutamic acid domain

This study reports properties of site-directed mutants of human protein C that display enhanced calcium and/or membrane binding properties. Mutants containing the S11G modification all showed increased affinity for membranes at saturating calcium concentration. Ser-11 is unique to human protein C, whereas all other vitamin K-dependent proteins contain glycine. This site is located in a compact region of the protein, close to a suggested membrane contact site. Additional changes of H10Q or S12N resulted in proteins with lower calcium requirement for membrane contact but without further increase in membrane affinity at saturating calcium. Mutations Q32E and N33D did not, by themselves, alter membrane affinity to a significant degree. These mutations were included in other mutant proteins and may contribute somewhat to higher function in these mutants. This family of mutants helped discriminate events that are necessary for protein-membrane binding. These include calcium binding to the free protein and subsequent protein-membrane contact. Depending on conditions of the assay used, the mutants displayed increased activity of the corresponding activated protein C (APC) derivatives. The degree of enhanced activity (up to 10-fold) was dependent on the concentration of phospholipid and quality of phospholipid (+/- phosphatidylethanolamine) used in the assay. This was expected, because APC is active in its membrane-associated form, which can be regulated by changes in either the protein or phospholipid. As expected, the largest impact of the mutants occurred at low phospholipid concentration and in the absence of phosphatidylethanolamine. The anticoagulant activity of all proteins was stimulated by protein S, with the greatest impact on the enhanced mutants. Whereas plasma containing Factor V:R506Q was partially resistant to all forms of APC, the enhanced variants were more active than normal APC. Protein C variants with enhanced function present new reagents for study of coagulation and may offer improved materials for biomedical applications.

The functions of the first epidermal growth factor homology region of human protein C as revealed by a charge-to-alanine scanning mutagenesis investigation

Variant proteins containing charge-to-alanine mutations of single amino acid residues and clusters of such groups contained in the epidermal growth factor 1 (EGF1) homology unit of human protein C (PC) have been accomplished, resulting in the following recombinant (r) mutant proteins: r-[E56A/H57A]PC; r-[H66A]PC; r-[D71A]PC; r-[D79A/R81A]PC; r-[E85A/R87A]PC; and r-[R91A/E92A]PC. Studies of the mutant proteins with a variety of Ca2+-dependent and Ca2+-independent monoclonal antibodies not only led to identification of the epitopes of these antibodies, but also confirmed the importance of D/beta-hydroxyaspartic acid (Hya)71 as one probable coordination site for Ca2+. Employing these antibodies, it was also revealed that Ca2+ binding to its site in the EGF1 region of PC did not influence Ca2+ binding or adoption of the Ca2+-dependent conformation of the gamma-carboxyglutamic acid domain of this same protein. In addition, the Ca2+-induced inhibition of PC activation by thrombin, and the kinetic constants for activation of PC by the thrombin/thrombomodulin complex, were only modestly affected by any of the mutations. The mutants r-[E56A/H57A]APC and r-[H66A]APC displayed at least 70% of wild type r-APC activity in a fVIII inactivation assay, while r-[D79A/R81A]APC, r-[E85A/R87A]APC and r-[R91A/E92A]APC possessed only approximately 40% activity in that same assay. The special role of D/Hya71 in this process was confirmed by showing that r-[D71A]APC was inactive in the fVIII-inactivation assay. These findings demonstrate that some of the charged residues of EGF1, most notably those in the carboxy-terminal region of this domain, participate as partial determinants of the anticoagulant activity of APC. Overall, with the exceptions noted, the data generally suggest that the charged residues of the EGF1 domain of PC, and the Ca2+ binding site contained within this module, are likely more involved with maintenance of the overall structural integrity of this module rather than with its direct functional interactions with effectors, activators, or substrates of PC and APC. Lastly, functional Ca2+ binding to the Gla domain of PC is not significantly influenced by the binding of Ca2+ to the EGF1 module.

Prediction of solution structures of the Ca2+-bound gamma-carboxyglutamic acid domains of protein S and homolog growth arrest specific protein 6: use of the particle mesh Ewald method

The solution structures of the N-terminal domains of protein S, a plasma vitamin K-dependent glycoprotein, and its homolog growth arrest specific protein 6 (Gas6) were predicted by molecular dynamics computer simulations. The initial structures were based on the x-ray crystallographic structure of the corresponding region of bovine prothrombin fragment 1. The subsequent molecular dynamics trajectories were calculated using the second-generation AMBER force field. The long-range electrostatic forces were evaluated by the particle mesh Ewald method. The structures that stabilized over a 400-ps time interval were compared with the corresponding region of the simulated solution structure of bovine prothrombin fragment 1. Structural properties of the gamma-carboxyglutamic acid (Gla) domains obtained from simulations and calcium binding were found to be conserved for all three proteins. Analysis of the predicted solution structure of the Gla domain of Gas6 suggests that this domain should bind with negatively charged phospholipid surfaces analogous to bovine prothrombin fragment 1 and protein S.

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.

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.

Dynamic features of prothrombin interaction with phospholipid vesicles of different size and composition: implications for protein--membrane contact

The dynamics of prothrombin interaction with membrane vesicles of different size and composition was investigated to ascertain the impact of membrane surface characteristics and particle size on this interaction. Dissociation rates were highly sensitive to membrane composition and varied from about 20/s for membranes of 10% PS to 0.1/s for membranes of 50% PS. Overall affinity also varied by more than two orders of magnitude. Very small differences between prothrombin binding to SUV versus LUV were found. Association with large unilamellar vesicles (LUV of 115 nm diameter) was about 4-fold slower, when expressed on the basis of binding sites, than association with small unilamellar vesicles (SUV, 30 nm diameter) of the same composition. Both reactions proceeded at less than 25% of the collisional limit so that the differences were largely due to intrinsic binding properties. Vesicles of 325 nm diameter showed even slower association velocities. Dissociation rates from LUV were about 2-fold slower than from SUV. Again, these differences arose primarily from intrinsic binding properties. Dissociation conformed to a single first order reaction over a wide range of protein occupancy on the membrane. At very high packing density, the dissociation rate increased by about 2-fold. At equilibrium, prothrombin preferred binding to SUV over LUV by about 2-fold. This very small difference, despite substantial differences in phospholipid headgroup packing and hydrocarbon exposure, appeared inconsistent with an important role for protein insertion into the hydrocarbon region of the membrane. However, prothrombin-membrane interaction may arise from a series of interaction forces that have compensating features at equilibrium. The small differences in prothrombin binding to SUV versus LUV, together with differences in the number of protein binding sites per vesicle, were important to identify mechanisms of substrate delivery to the active site of the prothrombinase enzyme [Lu, Y., & Nelsestuen, G. L. (1996) Biochemistry 35, 8201-8209].

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.

Structurally and functionally distinct Ca2+ binding sites in the gamma-carboxyglutamic acid-containing domain of factor VIIa

The structural and functional effects of Ca2+ binding to vitamin-K-dependent coagulation factor VIIa were investigated. Conformational changes with a midpoint around 0.7 mM Ca2+ quenched the intrinsic protein fluorescence of a fragment of factor VIIa comprising only the light chain and this coincided with an increase in factor VIIa amidolytic activity in the absence of tissue factor. Ca2+ binding to sites in factor VIIa and in the fragment with an apparent dissociation constant of 1.3-1.4 mM induced binding to phospholipids. A similar Ca2+ dependency was not observed with factor VIIa lacking the N-terminal 38 or 44 residues of the light chain and the observed effects could thus be attributed to gamma-carboxyglutamic-acid-dependent Ca2+ binding. Mg2+ appeared to bind to the site(s) of relatively higher affinity since, although it was less efficient than Ca2+, it stimulated the amidolytic activity and induced quenching of the intrinsic fluorescence. In contrast, Mg2+ did not induce expression of the phospholipid-interactive structure. The binding properties of two monoclonal antibodies that recognized epitopes in the gamma-carboxy-glutamic-acid-rich domain of factor VIIa corroborated the occurrence of two Ca(2+)-induced, sequential structural changes and only one of the antibodies recognized the Mg(2+)-induced structure. Thus Ca2+ binding to the gamma-carboxyglutamic-acid-containing domain appeared to result in at least two distinct structural transitions with different functional consequences. The two (sets of) sites responsible for the observed effects could be distinguished based upon differences in Ca2+ affinity and metal ion selectivity. The interaction between factor VIIa and tissue factor was monitored by means of a direct binding assay and an amidolytic assay. In both systems, half-maximal Ca2+ enhancement was observed at 0.25 mM. This coincided with a Ca(2+)-induced conformational change in factor VIIa associated with fluorescence quenching. The same effect on amidolytic activity was observed with the two N-terminally truncated forms of factor VIIa and it is presumably mediated by Ca2+ binding to a site located in the serine protease part.

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.

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)