Heparin binding site, conformational change, and activation of antithrombin

Conformational changes of GDNF-derived peptide induced by heparin, heparan sulfate, and sulfated hyaluronic acid - Analysis by circular dichroism spectroscopy and molecular dynamics simulation

Glial-cell-line-derived neurotrophic factor (GDNF) is a protein that has therapeutic potential in the treatment of Parkinson's disease and other neurodegenerative diseases. The activity of GDNF is highly dependent on the interaction with sulfated glycans which bind at the N-terminus consisting of 19 residues. Herein, we studied the influence of different glycosaminoglycan (i.e., glycan; GAG) molecules on the conformation of a GDNF-derived peptide (GAG binding motif, sixteen amino acid residues at the N-terminus) using both experimental and theoretical studies. The GAG molecules employed in this study are heparin, heparan sulfate, hyaluronic acid, and sulfated hyaluronic acid. Circular dichroism spectroscopy was employed to detect conformational changes induced by the GAG molecules; molecular dynamics simulation studies were performed to support the experimental results. Our results revealed that the sulfated GAG molecules bind strongly with GDNF peptide and induce alpha-helical structure in the peptide to some extent.

Vascular Localization of the Heparin-binding Serpins Antithrombin, Heparin Cofactor II, and Protein C Inhibitor

Heparin is one of the most widely used drugs in the world, acting as an anticoagulant by stimulating the reaction between heparin-binding serpins and the serine proteases of the coagulation cascade. To determine whether the heparin-binding serpins antithrombin (AT), heparin cofactor II (HCII), and protein C inhibitor (PCI) were bound to glycosaminoglycans on the endothelial wall, a bolus of heparin (100 U/kg body weight) was in jected into human volunteers, and serpin concentrations and activities were measured in both pre- and postheparin plasma. No increase in circulating concentrations of AT, HCII, or PCI were observed in postheparin plasma. Sim ilarly, AT and HCII activities did not increase in posthe parin plasma. In contrast, the concentration of another heparin-binding protein, lactoferrin (LF), increased six- fold after heparin injection. Immunohistochemistry of hu man artery was performed using polyclonal antisera to AT, HCII, PCI, LF, and tissue factor pathway inhibitor (TFPI), another heparin-binding protein released by hep arin injection. AT, HCII, and PCI were present in the intima, whereas LF, TFPI, and traces of AT were found on the surface of the vessel wall. The distribution of the proteins in the vessel wall supports the results of the hep arin-injection studies and may give valuable clues to the role of each protein in vascular homeostasis.

Insights into the glycosaminoglycan-mediated cytotoxic mechanism of eosinophil cationic protein revealed by NMR

Protein-glycosaminoglycan interactions are essential in many biological processes and human diseases, yet how their recognition occurs is poorly understood. Eosinophil cationic protein (ECP) is a cytotoxic ribonuclease that interacts with glycosaminoglycans at the cell surface; this promotes the destabilization of the cellular membrane and triggers ECP's toxic activity. To understand this membrane destabilization event and the differences in the toxicity of ECP and its homologues, the high resolution solution structure of the complex between full length folded ECP and a heparin-derived trisaccharide (O-iPr-α-D-GlcNS6S-α(1-4)-L-IdoA2S-α(1-4)-D-GlcNS6S) has been solved by NMR methods and molecular dynamics simulations. The bound protein retains the tertiary structure of the free protein. The (2)S(0) conformation of the IdoA ring is preferably recognized by the protein. We have identified the precise location of the heparin binding site, dissected the specific interactions responsible for molecular recognition, and defined the structural requirements for this interaction. The structure reveals the contribution of Arg7, Gln14, and His15 in helix α1, Gln40 in strand β1, His64 in loop 4, and His128 in strand β6 in the recognition event and corroborates the previously reported participation of residues Arg34-Asn39. The participation of the catalytic triad (His15, Lys38, His128) in recognizing the heparin mimetic reveals, at atomic resolution, the mechanism of heparin's inhibition of ECP's ribonucleolytic activity. We have integrated all the available data to propose a molecular model for the membrane interaction process. The solved NMR complex provides the structural model necessary to design inhibitors to block ECP's toxicity implicated in eosinophil pathologies.

Interaction of heparin and heparin-derived oligosaccharides with synthetic peptide analogues of the heparin-binding domain of heparin/heparan sulfate-interacting protein

BACKGROUND

Although protamine is effective as an antidote of heparin, there is a need to replace protamine due to its side effects. HIP peptide has been reported to neutralize the anticoagulant activity of heparin. The interaction of HIP analog peptides with heparin and heparin-derived oligosaccharides is investigated in this paper.

METHODS

Seven analogues of the heparin-binding domain of heparin/heparan sulfate-interacting protein (HIP) were synthesized, and their interaction with heparin was characterized by heparin affinity chromatography, isothermal titration calorimetry, and NMR.

RESULTS

NMR results indicate the imidazolium groups of the His side chains of histidine-containing Hip analog peptide interact site-specifically with heparin at pH 5.5. Heparin has identical affinities for HIP analog peptides of opposite chirality. Analysis by counterion condensation theory indicates the peptide AC-SRPKAKAKAKAKDQTK-NH2 makes on average approximately 3 ionic interactions with heparin that result in displacement of approximately 2 Na+ ions, and ionic interactions account for approximately 46% of the binding free energy at a Na+ concentration of 0.15 M.

CONCLUSIONS

The affinity of heparin for the peptides is strongly dependent on the nature of the cationic side chains and pH. The thermodynamic parameters measured for the interaction of HIP peptide analogs with heparin are strongly dependent on the peptide sequence and pH.

GENERAL SIGNIFICANCE

The information obtained in this research will be of use in the design of new agents for neutralization of the anticoagulant activity of heparin. The site-specific binding of protonated histidine side chains to heparin provides a molecular-level explanation for the pH-dependent binding of beta-amyloid peptides by heparin and heparan sulfate proteoglycan and may have implications for amyloid formation.

Comparative effects of atrial polypeptide and neurohormone C on the interaction of factor Xa with antithrombin III

The effects of atrial polypeptide and neurohormone C upon the interaction of human factor Xa (FXa) and human antithrombin III (ATIII) were followed by sodium dodecyl sulfate polyacrylamide gel electrophoresis. A pattern of bands consisting of a 1 degree duplex complex (FXaalpha-ATIII band of 109 kDa, FXabeta-ATIII band of 104 kDa), a 2 degree duplex complex (alpha band of 99 kDa, beta band of 95 kDa), a 3 degree duplex complex (alpha band of 66 kDa, beta band of 62 kDa), modified ATIII (ATIIIM, 58 kDa), native ATIII (55 kDa), FXaalpha (52 kDa), FXabeta (47 kDa), and a FXa degradation product (FXagamma, 35 kDa) was detected and quantitated. Preincubation of FXa, ATIII, or mixtures thereof with atrial polypeptide produced a shift from FXaalpha-ATIII to FXabeta-ATIII complexes and increases in both ATIIIM and FXagamma, reflecting degradation of the 1 degree and 2 degree complex to form the 3 degree complex. Atrial polypeptide appeared to promote FXa-ATIII complex formation when preincubated with ATIII, or when added within 1 min to FXa/ATIII mixtures. However, when atrial polypeptide was preincubated with FXa, inhibition of the 1 degree complex formation was suggested. Upon incubation of FXa, ATIII, or mixtures thereof with neurohormone C, there was an increase in total complex formation, a decrease in ATIIIM, a decrease in FXagamma, and little change in the ratio of free FXaalpha to FXabeta, or the ratio of FXaalpha-ATIII to FXabeta-ATIII complexes. Therefore, neurohormone C may act to suppress hydrolysis or proteolytic actions of excess FXa on FXa-ATIII complexes, or autolytic activity of FXa, to the level of FXagamma via an, as yet, unknown mechanism. Additionally, neurohormone C retards the hydrolysis of the FXa-ATIII complexes which form free FXa and ATIIIM. Hence, the role of atrial polypeptide in mixtures with ATIII and in mixtures with FXa is quite contrasting, and may reflect mechanistic effects of the atrial polypeptide molecule, as well as tissue-specific reactions.

Heparin-binding domains in vascular biology

Heparin is a major anticoagulant with activity mediated primarily through its interaction with antithrombin (AT). Heparan sulfate (HS), structurally related to heparin, binds a wide range of proteins of different functionality, taking part in various physiological and pathological processes. The heparin-AT complex, the most well understood facet of anticoagulation, serves as a prototypical example of the important role of heparin/HS in vascular biology. Extensive studies have identified common structural features in heparin/HS-binding sites of proteins. These include the elucidation of consensus sequences in proteins, patterns of clusters of basic and nonbasic residues, and common spatial arrangements of basic amino acids in the heparin-binding sites. Although these studies have provided valuable information, heparin/HS-binding proteins differ widely in structure. The prediction of heparin/HS-binding proteins from sequence information is not currently possible, and elucidation of protein-binding sites requires the individual study of each glycosaminoglycan-protein complex. Thus, x-ray crystallography and site-directed mutagenesis experiments are among the most powerful tools, providing accurate structural information, facilitating the characterization of heparin-protein complexes. Heparin and structurally related heparan sulfate bind a large number of proteins, taking part in a wide range of biological processes, particularly ones involved in vascular biology. Heparin-binding domains share certain common structural features, but there is no absolute dependency on specific sequences or protein folds.

Separation of latent, prelatent, and native forms of human antithrombin by heparin affinity high-performance liquid chromatography

Latent antithrombin (LAT) is a partially denatured form of human antithrombin (AT). LAT does not inhibit clotting of the blood, but has previously been shown to inhibit angiogenesis and carcinogenesis. Another probably partially denatured form is the so-called prelatent AT (P-LAT), described by Larsson et al. [J. Biol. Chem. 276 (2001) 11996]. In the present work, an analytical heparin affinity chromatography method is described that separates an AT form, which is formed during the pasteurization process and which we believe to be identical to the previously described P-LAT, from native AT and LAT. Non-pasteurized AT was shown to contain no P-LAT, while four, heat-treated commercial AT products all contained P-LAT (1-6%, mean=4%). P-LAT has a slightly lower affinity to heparin than does native AT, but exhibits a much stronger heparin affinity when compared to LAT. P-LAT and native AT were shown to have very similar thrombin inhibiting activity, while LAT lacks such activity.

Glycosaminoglycans affect the interaction of human plasma kallikrein with plasminogen, factor XII and inhibitors

Human plasma kallikrein, a serine proteinase, plays a key role in intrinsic blood clotting, in the kallikrein-kinin system, and in fibrinolysis. The proteolytic enzymes involved in these processes are usually controlled by specific inhibitors and may be influenced by several factors including glycosaminoglycans, as recently demonstrated by our group. The aim of the present study was to investigate the effect of glycosaminoglycans (30 to 250 micro/ml) on kallikrein activity on plasminogen and factor XII and on the inhibition of kallikrein by the plasma proteins C1-inhibitor and antithrombin. Almost all available glycosaminoglycans (heparin, heparan sulfate, bovine and tuna dermatan sulfate, chondroitin 4- and 6-sulfates) reduced (1.2 to 3.0 times) the catalytic efficiency of kallikrein (in a nanomolar range) on the hydrolysis of plasminogen (0.3 to 1.8 microM) and increased (1.9 to 7.7 times) the enzyme efficiency in factor XII (0.1 to 10 microM) activation. On the other hand, heparin, heparan sulfate, and bovine and tuna dermatan sulfate improved (1.2 to 3.4 times) kallikrein inhibition by antithrombin (1.4 microM), while chondroitin 4- and 6-sulfates reduced it (1.3 times). Heparin and heparan sulfate increased (1.4 times) the enzyme inhibition by the C1-inhibitor (150 nM).

Purification of a modified form of bovine antithrombin III as an HIV-1 CD8+ T-cell antiviral factor

CD8(+) T-cells secrete soluble factor(s) capable of inhibiting both R5- and X4-tropic strains of human immunodeficiency virus type 1 (HIV-1). CCR5 chemokine ligands, released from activated CD8(+) T-cells, contribute to the antiviral activity of these cells. These CC-chemokines, however, do not account for all CD8(+) T-cell antiviral factor(s) (CAF) released from these cells, particularly because the elusive CAF can inhibit the replication of X4 HIV-1 strains that use CXCR4 and not CCR5 as a coreceptor. Here we demonstrate that activated CD8(+) T-cells of HIV-1-seropositive individuals modify serum bovine antithrombin III into an HIV-1 inhibitory factor capable of suppressing the replication of X4 HIV-1. These data indicate that antithrombin III may play a role in the progression of HIV-1 disease.

The effects of reactive site location on the inhibitory properties of the serpin alpha(1)-antichymotrypsin

The large size of the serpin reactive site loop (RSL) suggests that the role of the RSL in protease inhibition is more complex than that of presenting the reactive site (P1 residue) to the protease. This study examines the effect on inhibition of relocating the reactive site (Leu-358) of the serpin alpha(1)-antichymotrypsin either one residue closer (P2) or further (P1') from the base of the RSL (Glu-342). alpha(1)-Antichymotrypsin variants were produced by mutation within the P4-P2' region; the sequence ITLLSA was changed to ITLSSA to relocate the reactive site to P2 (Leu-357) and to ITITLS to relocate it to P1' (Leu-359). Inhibition of the chymotrypsin-like proteases human chymase and chymotrypsin and the non-target protease human neutrophil elastase (HNE) were analyzed. The P2 variant inhibited chymase and chymotrypsin but not HNE. Relative to P1, interaction at P2 was characterized by greater complex stability, lower inhibition rate constants, and increased stoichiometry of inhibition values. In contrast, the P1' variant inhibited HNE (stoichiometry of inhibition = 4) but not chymase or chymotrypsin. However, inhibition of HNE was by interaction with Ile-357, the P2 residue. The P1' site was recognized by all proteases as a cleavage site. Covalent-complexes resistant to SDS-PAGE were observed in all inhibitory reactions, consistent with the trapping of the protease as a serpin-acyl protease complex. The complete loss in inhibitory activity associated with lengthening the Glu-342-reactive site distance by a single residue and the enhanced stability of complexes associated with shortening this distance by a single residue are compatible with the distorted-protease model of inhibition requiring full insertion of the RSL into the body of the serpin and translocation of the linked protease to the pole opposite from that of encounter.

Heparin-protein interactions

Heparin, a sulfated polysaccharide belonging to the family of glycosaminoglycans, has numerous important biological activities, associated with its interaction with diverse proteins. Heparin is widely used as an anticoagulant drug based on its ability to accelerate the rate at which antithrombin inhibits serine proteases in the blood coagulation cascade. Heparin and the structurally related heparan sulfate are complex linear polymers comprised of a mixture of chains of different length, having variable sequences. Heparan sulfate is ubiquitously distributed on the surfaces of animal cells and in the extracellular matrix. It also mediates various physiologic and pathophysiologic processes. Difficulties in evaluating the role of heparin and heparan sulfate in vivo may be partly ascribed to ignorance of the detailed structure and sequence of these polysaccharides. In addition, the understanding of carbohydrate-protein interactions has lagged behind that of the more thoroughly studied protein-protein and protein-nucleic acid interactions. The recent extensive studies on the structural, kinetic, and thermodynamic aspects of the protein binding of heparin and heparan sulfate have led to an improved understanding of heparin-protein interactions. A high degree of specificity could be identified in many of these interactions. An understanding of these interactions at the molecular level is of fundamental importance in the design of new highly specific therapeutic agents. This review focuses on aspects of heparin structure and conformation, which are important for its interactions with proteins. It also describes the interaction of heparin and heparan sulfate with selected families of heparin-binding proteins.

Heparin-Enhanced Zymographic Detection of Matrilysin and Collagenases

Unlike the gelatinases (MMP-2 and -9), matrilysin (MMP-7) and collagenases (MMP-1 and -13) are difficult to detect at low levels in conventional casein or gelatin zymography. In this report, heparin was used to enhance the zymographic assays for MMP-7, -1, and -13. With the addition of heparin to the enzyme sample, MMP-7 can be detected at a level of 30 pg in transferrin zymography and MMP-1 and -13 can be detected at a level of 0.2 ng in gelatin zymography. Carboxymethylated transferrin is used instead of casein as a substrate for assaying rat MMP-7. This substrate does not require a prerun step or substrate cross-linking to give uniform staining and clear band formation. It is necessary for heparin to run to the same region of the gel as the enzyme to produce its enhancing effect. For MMP-7 movement of heparin and enzyme is almost equal; for the collagenases it is necessary to add heparin to each well after the electrophoretic run is underway. Possible mechanisms of activity enhancement are discussed.

Separation of native and latent forms of human antithrombin by hydrophobic interaction high-performance liquid chromatography

Hydrophobic interaction high-performance liquid chromatography (HIC-HPLC) was utilized for the separation of native human antithrombin (AT) and a partially denaturated form of AT, known as the latent form (L-AT). The AT used in this study is commercially available (Atenativ, Pharmacia & Upjohn, Sweden) and contains albumin as the main stabilizer. The AT was reconstituted and heat treated in order to generate L-AT. This latent form of AT has been shown to exhibit a strong antiangiogenic activity and also to suppress tumor growth. The HPLC system included a TSK Phenyl 5PW column and a segmented gradient, 4.5-0 mol/L sodium chloride. Antithrombin was eluted at about 13 min, and L-AT, at 30 min, corresponding to about 4.2 and 1.6 mol/L sodium chloride, respectively. A reference sample gave 42% L-AT when analyzed by the HIC method and 41% L-AT when analyzed by the heparin affinity chromatography method. The resolution between AT and L-AT was higher with the HIC method than with the heparin affinity method. Incubation of Atenativ at 45 degrees C for 15 h gave about 18% L-AT and was shown by native polyacrylamide gel electrophoresis to contain only monomeric AT. A good resolution between AT and L-AT, but not between albumin and L-AT, was also achieved by a linear gradient of 2-0 mol/L ammonium sulfate, in 25 mmol/L Tris/HCl, pH 8.0.

Identification of a major heparin-binding site in kallistatin

Kallistatin is a heparin-binding serine proteinase inhibitor (serpin), which specifically inhibits human tissue kallikrein by forming a covalent complex. The inhibitory activity of kallistatin is blocked upon its binding to heparin. In this study we attempted to locate the heparin-binding site of kallistatin using synthetic peptides derived from its surface regions and by site-directed mutagenesis of basic residues in these surface regions. Two synthetic peptides, containing clusters of positive-charged residues, one derived from the F helix and the other from the region encompassing the H helix and C2 sheet of kallistatin, were used to assess their heparin binding activity. Competition assay analysis showed that the peptide derived from the H helix and C2 sheet displayed higher and specific heparin binding activity. The basic residues in both regions were substituted to generate three kallistatin double mutants K187A/K188A (mutations in the F helix) and K307A/R308A and K312A/K313A (mutations in the region between the H helix and C2 sheet), using a kallistatin P1Arg variant as a scaffold. Analysis of these mutants by heparin-affinity chromatography showed that the heparin binding capacity of the variant K187A/K188A was not altered, whereas the binding capacity of K307A/R308A and K312A/K313A mutants was markedly reduced. Titration analysis with heparin showed that the K312A/K313A mutant has the highest dissociation constant. Like kallistatin, the binding activity of K187A/K188A to tissue kallikrein was blocked by heparin, whereas K307A/R308A and K312A/K313A retained significant binding and inhibitory activities in the presence of heparin. These results indicate that the basic residues, particularly Lys(312)-Lys(313), in the region between the H helix and C2 sheet of kallistatin, comprise a major heparin-binding site responsible for its heparin-suppressed tissue kallikrein binding.

A positively charged loop on the surface of kallistatin functions to enhance tissue kallikrein inhibition by acting as a secondary binding site for kallikrein

Kallistatin is a serine proteinase inhibitor (serpin) that specifically inhibits tissue kallikrein. The inhibitory activity of kallistatin is abolished upon heparin binding. The loop between the H helix and C2 sheet of kallistatin containing clusters of basic amino acid residues has been identified as a heparin-binding site. In this study, we investigated the role of the basic residues in this region in tissue kallikrein inhibition. Kallistatin mutants containing double Ala substitutions for these basic residues displayed a 70-80% reduction of association rate constants, indicating the importance of these basic residues in tissue kallikrein inhibition. A synthetic peptide derived from the sequence between the H helix and C2 sheet of kallistatin was shown to suppress the kallistatin-kallikrein interaction through competition for tissue kallikrein binding. To further evaluate the function of this loop, we used alpha1-antitrypsin, a non-heparin-binding serpin and slow tissue kallikrein inhibitor as a scaffold to engineer kallikrein inhibitors. An alpha1-antitrypsin chimera harboring the P3-P2' residues and a sequence homologous to the positively charged region between the H helix and C2 sheet of kallistatin acquired heparin-suppressed inhibitory activity toward tissue kallikrein and exhibited an inhibitory activity 20-fold higher than that of the other chimera, which contained only kallistatin's P3-P2' sequence, and 2300-fold higher than that of wild-type alpha1-antitrypsin. The alpha1-antitrypsin chimera with inhibitory characteristics similar to those of kallistatin demonstrates that the loop between the H helix and C2 sheet of kallistatin is crucial in tissue kallikrein inhibition, and this functional loop can be used as a module to enhance the inhibitory activity of a serpin toward tissue kallikrein. In conclusion, our results indicate that a positively charged loop between the H helix and C2 sheet of a serpin can accelerate the association of a serpin with tissue kallikrein by acting as a secondary binding site.

Roles of the P1, P2, and P3 residues in determining inhibitory specificity of kallistatin toward human tissue kallikrein

Kallistatin is a serpin with a unique P1 Phe, which confers an excellent inhibitory specificity toward tissue kallikrein. In this study, we investigated the P3-P2-P1 residues (residues 386-388) of human kallistatin in determining inhibitory specificity toward human tissue kallikrein by site-directed mutagenesis and molecular modeling. Human kallistatin mutants with 19 different amino acid substitutions at each P1, P2, or P3 residue were created and purified to compare their kallikrein binding activity. Complex formation assay showed that P1 Arg, P1 Phe (wild type), P1 Lys, P1 Tyr, P1 Met, and P1 Leu display significant binding activity with tissue kallikrein among the P1 variants. Kinetic analysis showed the inhibitory activities of the P1 mutants toward tissue kallikrein in the order of P1 Arg > P1 Phe > P1 Lys >/= P1 Tyr > P1 Leu >/= P1 Met. P1 Phe displays a better selectivity for human tissue kallikrein than P1 Arg, since P1 Arg also inhibits several other serine proteinases. Heparin distinguishes the inhibitory specificity of kallistatin toward kallikrein versus chymotrypsin. For the P2 and P3 variants, the mutants with hydrophobic and bulky amino acids at P2 and basic amino acids at P3 display better binding activity with tissue kallikrein. The inhibitory activities of these mutants toward tissue kallikrein are in the order of P2 Phe (wild type) > P2 Leu > P2 Trp > P2 Met and P3 Arg > P3 Lys (wild type). Molecular modeling of the reactive center loop of kallistatin bound to the reactive crevice of tissue kallikrein indicated that the P2 residue required a long and bulky hydrophobic side chain to reach and fill the hydrophobic S2 cleft generated by Tyr(99) and Trp(219) of tissue kallikrein. Basic amino acids at P3 could stabilize complex formation by forming electrostatic interaction with Asp(98J) and hydrogen bond with Gln(174) of tissue kallikrein. Our results indicate that tissue kallikrein is a specific target proteinase for kallistatin.

Antithrombotic and prothrombotic activities of the vascular endothelium

Vascular endothelium plays a key role in the control of haemostasis and thrombosis. The main reactions involved in the regulation of platelet reactivity, blood coagulation and fibrinolysis take place at the luminal surface of endothelial cells. Following exposure to certain pathological stimuli, remarkable functional changes of the endothelial cells occur, including downregulation of antithrombotic mechanisms and upregulation of prothrombotic activities. Based on the recent knowledge of vascular endothelial function, a better understanding of the pathogenesis of atherothrombosis is expected.

Identification of basic residues in the heparin-binding exosite of factor Xa critical for heparin and factor Va binding

We recently demonstrated that a template mechanism makes a significant contribution to the heparin-accelerated inactivation of factor Xa (FXa) by antithrombin at physiologic Ca(2+), suggesting that FXa has a potential heparin-binding site. Structural data indicate that 7 of the 11 basic residues of the heparin-binding exosite of thrombin are conserved at similar three-dimensional locations in FXa. These residues, Arg(93), Lys(96), Arg(125), Arg(165), Lys(169), Lys(236), and Arg(240) were substituted with Ala in separate constructs in Gla domainless forms. It was found that all derivatives cleave Spectrozyme FXa with similar catalytic efficiencies. Antithrombin inactivated FXa derivatives with a similar second-order association rate constant (k(2)) in both the absence and presence of pentasaccharide. In the presence of heparin, however, k(2) with certain mutants were impaired up to 25-fold. Moreover, these mutants bound to heparin-Sepharose with lower affinities. Heparin concentration dependence of the inactivation revealed that only the template portion of the cofactor effect of heparin was affected by the mutagenesis. The order of importance of these residues for binding heparin was as follows: Arg(240) > Lys(236) > Lys(169) > Arg(165) > Lys(96) > Arg(93) >/= Arg(125). Interestingly, further study suggested that certain basic residues of this site, particularly Arg(165) and Lys(169), play key roles in factor Va and/or prothrombin recognition by FXa in prothrombinase.

Antiangiogenic activity of the cleaved conformation of the serpin antithrombin

Antithrombin, a member of the serpin family, functions as an inhibitor of thrombin and other enzymes. Cleavage of the carboxyl-terminal loop of antithrombin induces a conformational change in the molecule. Here it is shown that the cleaved conformation of antithrombin has potent antiangiogenic and antitumor activity in mouse models. The latent form of intact antithrombin, which is similar in conformation to the cleaved molecule, also inhibited angiogenesis and tumor growth. These data provide further evidence that the clotting and fibrinolytic pathways are directly involved in the regulation of angiogenesis.

Human tissue inhibitor of metalloproteinases 3 interacts with both the N- and C-terminal domains of gelatinases A and B. Regulation by polyanions

We compared the association constants of tissue inhibitor of metalloproteinases (TIMP)-3 with various matrix metalloproteinases with those for TIMP-1 and TIMP-2 using a continuous assay. TIMP-3 behaved more like TIMP-2 than TIMP-1, showing rapid association with gelatinases A and B. Experiments with the N-terminal domain of gelatinase A, the isolated C-terminal domain, or an inactive progelatinase A mutant showed that the hemopexin domain of gelatinase A makes an important contribution to the interaction with TIMP-3. The exchange of portions of the gelatinase A hemopexin domain with that of stromelysin revealed that residues 568-631 of gelatinase A were required for rapid association with TIMP-3. The N-terminal domain of gelatinase B alone also showed slower association with TIMP-3, again implying significant C-domain interactions. The isolation of complexes between TIMP-3 and progelatinases A and B on gelatin-agarose demonstrated that TIMP-3 binds to both proenzymes. We analyzed the effect of various polyanions on the inhibitory activity of TIMP-3 in our soluble assay. The association rate was increased by dextran sulfate, heparin, and heparan sulfate, but not by dermatan sulfate or hyaluronic acid. Because TIMP-3 is sequestered in the extracellular matrix, the presence of certain heparan sulfate proteoglycans could enhance its inhibitory capacity.

Characterization of a heparin binding site on the heavy chain of factor XI

The glycosaminoglycan heparin enhances several reactions involving coagulation factor XI (FXI) including activation of FXI by factor XIIa, thrombin, and autoactivation; and inactivation of activated FXI (FXIa) by serine protease inhibitors. We examined the effect of heparin on inhibition of FXIa by the inhibitors C1-inhibitor (C1-INH) and antithrombin III (ATIII). Second order rate constants for inhibition in the absence of heparin were 1.57 x 10(3) and 0.91 x 10(3) M-1 s-1 for C1-INH and ATIII, respectively. Therapeutic heparin concentrations (0.1-1.0 units/ml) enhanced inhibition by ATIII 20-55-fold compared with 0.1-7.0-fold for C1-INH. For both inhibitors, the effect of heparin over a wide range of concentrations (10(-1) to 10(5) units/ml) produced bell-shaped curves, demonstrating that inhibition occurs by a template mechanism requiring both inhibitor and protease to bind to heparin. This implies that FXI/XIa contains structural elements that interact with heparin. Human FXI contains a sequence of amino acids (R250-I-K-K-S-K) in the apple 3 domain of the heavy chain that binds heparin (Ho, D., Badellino, K., Baglia, F., and Walsh, P. (1998) J. Biol. Chem. 273, 16382-16390). To determine the importance of this sequence to heparin-mediated reactions, recombinant FXI molecules with alanine substitutions for basic amino acids were expressed in 293 fibroblasts, and tested in heparin-dependent assays. Inhibition of FXIa by ATIII in the presence of heparin was decreased 4-fold by alanine substitution at Lys253 (A253), with smaller effects noted for mutants A255 and A252. FXI undergoes autoactivation to FXIa in the presence of heparin. The rate of autoactivation was decreased substantially for A253 with modest decreases for A255 and A252. Substituting all four charged residues in the sequence resulted in a profound decrease in autoactivation, significantly greater than for any single substitution. Relative affinity for heparin was tested by determining the concentration of NaCl required to elute FXIa from heparin-Sepharose. Wild type FXIa eluted from the column at 320 mM NaCl, whereas FXIa with multiple substitutions (A252-254 or A250-255) eluted at 230 mM NaCl. All proteins with single substitutions in charged amino acids eluted at intermediate NaCl concentrations. The data indicate that FXI/XIa must bind to heparin for optimal inhibition by ATIII and for autoactivation. Lys253 is the most important amino acid involved in binding, and Lys255 and Lys252 also have roles in interactions with heparin.

Site-directed mutagenesis of boar proacrosin reveals residues involved in binding of zona pellucida glycoproteins

Proacrosin, the zymogen form of the serine protease beta-acrosin, is thought to function as a secondary binding molecule between mammalian gametes during fertilization (Jansen et al., 1995: Int J Dev Biol 39, 501-510). The interaction involves strong ionic bonds between positively charged amino acids on proacrosin and negatively charged polysulphate groups on zona pellucida glycoproteins. In this investigation, we identified the basic residues on proacrosin that are important for this binding. Site-directed mutagenesis shows that two groups of amino acids comprising His47, Arg50, and Arg51 together with Arg250, Lys252, and Arg253 are crucial because their deletion or replacement severely reduces affinity for zona glycoproteins. Molecular models of proacrosin reveal that these residues are located along one face of the protein on two exposed surface loops that project over and around the catalytic site. These findings support the hypothesis that polysulphate binding sites on proacrosin are formed by a restricted number of basic amino acids on the surface of the protein, presenting a specific orientation that is complementary to negatively charged sulphate groups on zona glycoproteins. Identification and elucidation of the stereochemistry of these charged moieties will aid design of new kinds of nonsteroidal antifertility agents.

Calcium enhances heparin catalysis of the antithrombin-factor Xa reaction by a template mechanism. Evidence that calcium alleviates Gla domain antagonism of heparin binding to factor Xa

It is believed that heparin accelerates factor Xa (FXa) inactivation by antithrombin (AT) by conformationally activating the inhibitor rather than by bridging AT and FXa in a ternary complex (template effect). This is derived from kinetic studies done in the absence of Ca2+ or in the presence of EDTA. To test the possibility that the anionic Gla domain of FXa, when not neutralized by Ca2+ ions, prevents heparin binding to FXa, the heparin and pentasaccharide dependence of FXa inactivation by AT in both the absence (100 microM EDTA) and presence of Ca2+ (2.5 mM) was studied using wild-type FXa and a FXa derivative that lacks the Gla domain (GDFXa). AT inactivated both FXa derivatives similarly in both the absence and presence of Ca2+ (k2 = 1.7-2.5 x 10(3) M-1 s-1). The active AT-binding pentasaccharide also accelerated the inactivation rates of both derivatives similarly in both the absence and presence of Ca2+ (k2 = 5.7-8.0 x 10(5) M-1 s-1). However, in the presence of an optimum concentration of heparin ( approximately 50 nM) the inactivation rate constant of FXa in the presence of Ca2+ (k2 = 4.4 x 10(7) M-1 s-1) was 13-fold higher than the rate constant in the absence of Ca2+ (k2 = 3.5 x 10(6) M-1 s-1). Heparin acceleration of GDFXa inactivation by AT was rapid and insensitive to the presence or absence of Ca2+ (k2 = 5.1-5.9 x 10(7) M-1 s-1). The additional cofactor effect of heparin with all FXa derivatives was a bell-shaped curve, which disappeared if the ionic strength of the reaction was increased to approximately 0.4. These results suggest that although the major effect of heparin in acceleration of FXa inactivation is through a heparin-induced conformational change in the reactive site loop of AT, the template effect of heparin, nevertheless, contributes significantly to rapid FXa inactivation at physiological Ca2+.

A kinetic analysis of the interaction of human recombinant tissue factor pathway inhibitor with factor Xa utilizing and immunoassay and the effect of antithrombin III/heparin on the complex formation

We have recently shown that a complex formation of tissue factor pathway inhibitor (TFPI) and factor Xa (Xa) promotes a clearance of proteoglycans-associated TFPI. In the current studies, the interaction between human recombinant TFPI (h-rTFPI) and Xa were kinetically analyzed by utilizing both a protease inhibitor, p-(amidophenyl) methanesulfonyl fluoride hydrochloride, and a specific enzyme-linked immunosorbent assay for the complex of h-rTFPI with Xa. We further investigated the effect of antithrombin III on the complex formation between h-rTFPI and Xa. We found that the h-rTFPI/Xa complex formed in a time-dependent manner: the second-order rate constant (K1) for the complex formation was calculated to be 0.86x10(6) M(-1)s(-1). The addition of antithrombin III to the h-rTFPI solution modestly reduced the rate of the complex formation between h-rTFPI and Xa. Heparin strikingly enhanced antithrombin III's inhibition of Xa and resulted in complete abrogation of the complex formation between h-rTFPI and Xa in the absence or presence of acidic phospholipids. Furthermore, antithrombin III induced dissociation of the preformed h-rTFPI/Xa complex in the presence of heparin. These results suggest that in the presence of heparin, antithrombin III interferes with the catabolism of TFPI mediated via Xa.

Limulus intracellular coagulation inhibitor type 3. Purification, characterization, cDNA cloning, and tissue localization

We reported that limulus intracellular coagulation inhibitor type-1 (LICI-1) (Miura, Y., Kawabata, S., and Iwanaga, S. (1994) J. Biol. Chem. 269, 542-547) and LICI type-2 (LICI-2) (Miura, Y., Kawabata, S. , Wakamiya, Y., Nakamura, T., and Iwanaga, S. (1995) J. Biol. Chem. 270, 558-565) found in the hemocyte lysate belong to the serpin family. The LICI-1 specifically inhibits limulus lipopolysaccharide-sensitive serine protease, factor C (k1 = 2.5 x 10(6) M-1 s-1), whereas LICI-2 inhibits preferentially limulus clotting enzyme (k1 = 4.3 x 10(5) M-1 s-1). In our ongoing studies on limulus serpin, we found another inhibitor, named LICI type-3 (LICI-3), which strongly inhibits (1,3)-beta-D-glucan-sensitive serine protease, factor G (k1 = 3.9 x 10(5) M-1 s-1). Thus, the limulus hemolymph coagulation cascade is effectively regulated by at least the three endogenous serpins. LICI-3, newly identified in hemocytes, is a single chain glycoprotein with an apparent Mr = 53,000, the largest one among known limulus serpins. A cDNA sequence for LICI-3 coded a mature protein of 392 amino acids, of which 68 residues were confirmed by peptide sequencing. LICI-3 showed significant sequence similarity to LICI-1 (45.8% identity) and LICI-2 (33.7% identity). LICI-3 contained a putative reactive site, -Arg-Ser-, distinct from that of LICI-2 (-Lys-Ser-) but the same as that of LICI-1. Expression of LICI-3 mRNA was detected only in hemocytes, and not in heart, brain, stomach, intestine, coxal gland, and skeletal muscle. Immunoblotting of the hemocyte-derived large and small granules with antiserum against LICI-3 suggested that it is stored specifically in large granules, as in the case of LICI-1 and LICI-2, and is released in response to external stimuli.

Modulation of contact system proteases by glycosaminoglycans. Selective enhancement of the inhibition of factor XIa

We investigated the influence of dextran sulfate, heparin, heparan sulfate, and dermatan sulfate on the inhibition of FXIa (where FXIa is activated factor XI, for example), FXIIa, and kallikrein by C1 inhibitor, alpha1-antitrypsin, alpha2-antiplasmin, and antithrombin III. The second-order rate constants for the inhibition of FXIa by C1 inhibitor, alpha1-antitrypsin, alpha2-antiplasmin, and antithrombin III, in the absence of glycosaminoglycans, were 1.8, 0.1, 0.43, and 0.32 x 10(3) M-1 s-1, respectively. The rate constants of the inactivation of FXIa by C1 inhibitor and by antithrombin III increased up to 117-fold in the presence of glycosaminoglycans. These data predicted that considering the plasma concentration of the inhibitors, C1 inhibitor would be the main inhibitor of FXIa in plasma in the presence of glycosaminoglycans. Results of experiments in which the formation of complexes between serine protease inhibitors and FXIa was studied in plasma agreed with this prediction. Glycosaminoglycans did not enhance the inhibition of alpha-FXIIa, beta-FXIIa, or kallikrein by C1 inhibitor. Thus, physiological glycosaminoglycans selectively enhance inhibition of FXIa without affecting the activity of FXIIa and kallikrein, suggesting that glycosaminoglycans may modulate the biological effects of contact activation, by inhibiting intrinsic coagulation without affecting the fibrinolytic potential of FXIIa/kallikrein.

Importance of specific amino acids in protein binding sites for heparin and heparan sulfate

Heparin and heparan sulfate bind a variety of proteins and peptides to regulate many biological activities. Past studies have examined a limited number of established heparin binding sites and have focused on basic amino acids when modeling binding site structural motifs. This study examines the prevalence of individual amino acids in peptides binding to heparin or heparan sulfate. A 7-mer random peptide library was synthesized using the 20 common amino acids. This 7-mer library was affinity separated using both heparin and heparan sulfate-Sepharose. Bound peptide populations were eluted with a salt step gradient (pH 7) and analysed for amino acid composition. Peptides released from heparin-Sepharose by 0.3 M NaCl were enriched in arginine, lysine, glycine and serine; and depleted in methionine and phenylalanine. In contrast, peptides released from heparan sulfate-Sepharose were enriched in arginine, glycine, serine, and proline (at 0.15 M NaCl). These peptides were depleted in histidine, isoleucine, methionine (not detectable) and phenylalanine. In the heparin binding sites of proteins, which have been published, the enriched amino acids were arginine, lysine and tyrosine. Depleted amino acids include aspartic acid, glutamic acid, glutamine, alanine, glycine, phenylalanine, serine, threonine and valine. This study demonstrates that heparin and heparan sulfate bind different populations of peptide sequences. The differences in amino acid composition indicate that the positive charge density and spacing requirements differ for peptides binding these two glycosaminoglycans.

Mechanism of acceleration of antithrombin-proteinase reactions by low affinity heparin. Role of the antithrombin binding pentasaccharide in heparin rate enhancement

The role of the sequence-specific pentasaccharide region of high affinity heparin (HAH) in heparin acceleration of antithrombin-proteinase reactions was elucidated by determining the accelerating mechanism of low affinity heparin (LAH) lacking this sequence. LAH was shown to be free of HAH (< 0.001%) from the lack of exchange of added fluorescein-labeled HAH into LAH after separating the polysaccharides by antithrombin-agarose chromatography. Fluorescence titrations showed that LAH bound to antithrombin with a 1000-fold weaker affinity (KD 19 +/- 6 microM) and 5-6-fold smaller fluorescence enhancement (8 +/- 3%) than HAH. LAH accelerated the antithrombin-thrombin reaction with a bell-shaped dependence on heparin concentration resembling that of HAH, but with the bell-shaped curve shifted to approximately 100-fold higher polysaccharide concentrations and with a approximately 100-fold reduced maximal accelerating effect. Rapid kinetic studies indicated these differences arose from a reverse order of assembly of an intermediate heparin-thrombin-antithrombin ternary complex and diminished ability of LAH to bridge antithrombin and thrombin in this complex, as compared to HAH. By contrast, LAH and HAH both accelerated the antithrombin-factor Xa reaction with a simple saturable dependence on heparin or inhibitor concentrations which paralleled the formation of an antithrombin-heparin binary complex. The maximal accelerations of the two heparins in this case correlated with the inhibitor fluorescence enhancements induced by the polysaccharides, consistent with the accelerations arising from conformational activation of antithrombin. 1H NMR difference spectroscopy of antithrombin complexes with LAH and HAH and competitive binding studies were consistent with LAH accelerating activity being mediated by binding to the same site on the inhibitor as HAH. These results demonstrate that LAH accelerates antithrombin-proteinase reactions by bridging and conformational activation mechanisms similar to those of HAH, with the reduced magnitude of LAH accelerations resulting both from a decreased antithrombin affinity and the inability to induce a full activating conformational change in the inhibitor.

Heparin receptors in two murine mammary adenocarcinomas with different metastatic ability: relationship with growth inhibition

Binding of heparin to primary cultured cells of two murine mammary adenocarcinomas with low (M3) and high (MM3) lung, metastatic capacity was determined. Heparin binding was rapid, specific and saturable. MM3 cells grown for 24 h in fetal calf serum (FCS)-free medium exhibited a higher number of binding sites for 3H-heparin [(11 +/- 1) x 10(5) sites per cell than M3 cells [(6.9 +/- 0.6) x 10(5) sites per cell]. However, when M3 cells were grown in the presence of 2% FCS, they showed less heparin binding sites [(3.5 +/- 0.4) x 10(5) sites per cell]. In contrast, dissociation constants were very similar for MM3 and M3 cells grown with or without FCS (Kd = 2-4 x 10(-9) M). Furthermore, heparin inhibited MM3 and M3 cell growth both in the absence or presence of FCS. Competition studies showed that chemically modified heparins lacking antiproliferative effect (O-desulfated; O/N-desulfated N-acetylated and N-desulfated heparins) were not able to inhibit 3H-heparin binding. N-desulfated N-acetylated heparin, which had partial antiproliferative effect, partially inhibited 3H-heparin binding, while heparin with a high antiproliferative activity inhibited more than 90% 3H-heparin binding. The antiproliferative effect of heparin and chemically modified heparins seems to be related to their binding ability to the cell membrane.

Lysine-heparin interactions in antithrombin. Properties of K125M and K290M,K294M,K297M variants

Lysine residues in two different regions of antithrombin have been proposed to be involved in heparin binding and heparin-mediated acceleration of proteinase inhibition. Lysine 125 has been implicated as an essential heparin binding residue from chemical modification studies [Peterson, C. B., Noyes, C. M., Pecon, J. M., Church, F. C., & Blackburn, M. N. (1987) J. Biol. Chem. 262, 8061-8065] whereas lysines 290, 294, and 297 have been proposed from model building studies to constitute the heparin binding site [Villanueva, G. B. (1984) J. Biol. Chem. 259, 2531-2536]. To evaluate both of these proposals, we have prepared two variant human antithrombins, K125M and K290M,K294M,K297M, in which these lysines have been changed by site-directed mutagenesis to methionines. The K290M,K294M,K297M variant had properties very similar to those of wild-type recombinant antithrombin in affinity for heparin, and in rates of inhibition of thrombin and factor Xa. In contrast, K125M antithrombin had reduced affinity for both heparin pentasaccharide and full-length heparin, corresponding to delta delta Gs of 3.1 and 2.0 kcal mol-1, respectively. However, this variant was still able to inhibit both thrombin and factor Xa. Whereas the rate of thrombin inhibition was similar to that of wild-type antithrombin, the rate of factor Xa inhibition was enhanced between 2- and 3-fold, suggesting a role for lysine 125 in the allosteric coupling between the heparin binding site and the reactive center region. At saturation with either heparin pentasaccharide or full-length high-affinity heparin, the rates of inhibition of both proteinases were similar to those of wild-type antithrombin for both the K125M and K290M,K294M,K297M variants.(ABSTRACT TRUNCATED AT 250 WORDS)

Targeting the glycosaminoglycan-binding sites on proteins

Sulfated glycosaminoglycans bind to a wide variety of proteins, and in so doing have important roles in diverse biological processes. Selective mimics or inhibitors of protein-glycosaminoglycan interactions could have broad application in biology and medicine.

The molecular genetics of familial venous thrombosis

This study of naturally occurring mutations predisposing to venous thrombosis has led to a number of important advances in our understanding of protein structure and function relationships and the molecular basis of gene mutation. It has also potentiated the accurate and reliable presymptomatic and antenatal detection of predisposing gene lesions. Perhaps the major challenge facing us is the probabilistic nature of thromboembolism; only a certain proportion of patients with recognized gene defects predisposing to thrombosis will actually suffer from thrombotic episodes. Environmental insults of various kinds, and perhaps epistatic effects resulting from the influence of other loci, are likely to be contributory factors and will help to determine whether a thrombotic event occurs in individuals already compromised by a defect in a gene whose malfunction is known to predispose to thrombosis. Since molecular genetic techniques allow us to dissect the allelioheterogeneity of the different deficiency states by characterizing the wide spectrum of gene mutations giving rise to thrombosis, it may eventually prove possible to relate specific gene lesions to the probability of thromboembolism as well as to the severity and frequency of thrombotic episodes. The multifactorial nature of thrombosis demands a multidisciplinary approach to the analysis of its causation, early detection, treatment and prevention. The application of the new and powerful techniques of molecular genetics promises to make a substantial contribution to all aspects of thrombosis research.

A highly purified antithrombin III concentrate prepared from human plasma fraction IV-1 by affinity chromatography

We describe an improved method for large-scale purification of antithrombin III (AT-III) from human plasma involving heparin affinity chromatography of redissolved fraction IV-1 paste, viral inactivation by heating, followed by a second heparin affinity column. The characteristics of a new heparin affinity resin and the ability to extrapolate process behavior from small-scale (20 ml) to large-scale (40 liter) columns are described. This supports the use of the small-scale column for process optimization and validation studies in compliance with current regulatory requirements for biological products. The process has been characterized by analytical techniques including sodium dodecyl sulfate (SDS), reducing SDS, and nondenaturing polyacrylamide gel electrophoresis; laser desorption time-of-flight mass spectroscopy, and electrospray mass spectroscopy. These results demonstrate that greater than 95% of the protein in the final products is AT-III, which is greater than 95% active as defined by thrombin inhibition.

Structural aspects of serpin inhibition

The essential roles of proteins of the serpin family in many physiological processes, along with new discoveries of their unique folding properties, have attracted intense interest in recent years. Many serpins display unusual mobile behavior attributed to rearrangements of alpha-helical or beta-sheet domains, whereby large scale transitions accompany a variety of functions, including inactivation. This unusual behavior was first recognized with the X-ray structure of modified alpha 1-proteinase inhibitor. Subsequent experiments, including new X-ray structures, have revealed a surprising variety of conformations which are functionally important but only partially understood. We review here experimental evidence for conformations relevant to the serpin inhibitory mechanism.

Heparin binding domain peptides of antithrombin III: analysis by isothermal titration calorimetry and circular dichroism spectroscopy

The serine proteinase inhibitor antithrombin III (ATIII) is a key regulatory protein of intrinsic blood coagulation. ATIII attains its full biological activity only upon binding polysulfated oligosaccharides, such as heparin. A series of synthetic peptides have been prepared based on the proposed heparin binding regions of ATIII and their ability to bind heparin has been assessed by CD spectrometry, by isothermal titration calorimetry, and by the ability of the peptides to compete with ATIII for binding heparin in a factor Xa procoagulant enzyme assay. Peptide F123-G148, which encompasses both the purported high-affinity pentasaccharide binding region and an adjacent, C-terminally directed segment of ATIII, was found to bind heparin with good affinity, but amino-terminal truncations of this sequence, including L130-G148 and K136-G148 displayed attenuated heparin binding activities. In fact, K136-G148 appears to encompass only a low-affinity heparin binding site. In contrast, peptides based solely on the high-affinity binding site (K121-A134) displayed much higher affinities for heparin. By CD spectrometry, these high-affinity peptides are chiefly random coil in nature, but low microM concentrations of heparin induce significant alpha-helix conformation. K121-A134 also effectively competes with ATIII for binding heparin. Thus, through the use of synthetic peptides that encompass part, if not all, of the heparin binding site(s) within ATIII, we have further elucidated the structure-function relations of heparin-ATIII interactions.

Evolutionary relationships among the serpins

The serpins are a widely distributed group of serine proteinase inhibitors found in plants, birds, mammals and viruses. Despite the great evolutionary divergence of these organisms, their serpins are highly conserved, both in sequence and structurally. Amino acid sequences were aligned by a combination of automatic algorithms and by consideration of conserved structural elements in those serpins for which crystal structures exist. The program HOMED was used which allowed the alignment of amino acids to be simultaneously converted into the equivalently aligned nucleotide sequences. The aligned amino acids were used as the basis for superposition of the four known three-dimensional structures for which coordinates are available and compared with an optimal three-dimensional superposition in order to estimate the reliability of the sequence alignment. Phylogenetic relationships implied by these nucleotide sequence alignments were determined by the method of maximum parsimony. The proposed gene tree suggested that as much diversity existed between the plant serpin and mammalian serpins as was present among mammalian serpins and provided further evidence that the architecture of serpin molecules is highly constrained.