Zymogens and cofactors of blood coagulation

Factor VIII-driven changes in activated factor IX explored by hydrogen-deuterium exchange mass spectrometry

The assembly of the enzyme-activated factor IX (FIXa) with its cofactor, activated factor VIII (FVIIIa) is a crucial event in the coagulation cascade. The absence or dysfunction of either enzyme or cofactor severely compromises hemostasis and causes hemophilia. FIXa is a notoriously inefficient enzyme that needs FVIIIa to drive its hemostatic potential, by a mechanism that has remained largely elusive to date. In this study, we employed hydrogen-deuterium exchange-mass spectrometry (HDX-MS) to investigate how FIXa responds to assembly with FVIIIa in the presence of phospholipids. This revealed a complex pattern of changes that partially overlaps with those changes that occur upon occupation of the substrate-binding site by an active site-directed inhibitor. Among the changes driven by both cofactor and substrate, HDX-MS highlighted several surface loops that have been implicated in allosteric networks in related coagulation enzymes. Inspection of FVIIIa-specific changes indicated that 3 helices are involved in FIXa-FVIIIa assembly. These are part of a basic interface that is also known as exosite II. Mutagenesis of basic residues herein, followed by functional studies, identified this interface as an extended FVIIIa-interactive patch. HDX-MS was also applied to recombinant FIXa variants that are associated with severe hemophilia B. This revealed that single amino acid substitutions can silence the extended network of FVIIIa-driven allosteric changes. We conclude that HDX-MS has the potential to visualize the functional impact of disease-associated mutations on enzyme-cofactor complexes in the hemostatic system.

The role of beta-2-glycoprotein I in health and disease associating structure with function: More than just APS

Beta-2-Glycoprotein I (β2GPI) plays a number of essential roles throughout the body. β2GPI, C-reactive protein and thrombomodulin are the only three proteins that possess the dual capability to up and down regulate the complement and coagulation systems depending upon external stimulus. Clinically, β2GPI is the primary antigen in the autoimmune condition antiphospholipid syndrome (APS), which is typically characterised by pregnancy morbidity and vascular thrombosis. This protein is also capable of adopting at least two distinct structural forms, but it has been argued that several other intermediate forms may exist. Thus, β2GPI is a unique protein with a key role in haemostasis, homeostasis and immunity. In this review, we examine the genetics, structure and function of β2GPI in the body and how these factors may influence its contribution to disease pathogenesis. We also consider the clinical implications of β2GPI in the diagnosis of APS and as a potentially novel therapeutic target.

Cytotoxicity associated with prolonged room temperature storage of serum and proposed methods for reduction of cytotoxicity

Canine serum preserved at room temperature (25°C) for longer than 24h is known to exhibit significant cytotoxicity. This phenomenon is one of the major reasons for the failure of virus neutralization tests. In this study, a method for reducing this cytotoxicity was investigated by applying several treatments to dog, cat and human serum prior to room temperature storage. Additionally, the identity of the cytotoxic factor generated during room temperature storage was investigated. Heat-inactivation at 56°C or 65°C and the addition of protease inhibitor prior to storage were found to be effective for reducing cytotoxicity in the serum. Furthermore, heat-inactivation at 65°C reduced the cytotoxicity that was induced under room temperature storage. Several protein factors in serum were suspected to play a role in the observed cytotoxicity. According to this study, the membrane-attack-complex in serum was not involved in the cytotoxicity. This study provides useful information for development and improvement of cell culture and virus neutralization tests.

Complex assemblies of factors IX and X regulate the initiation, maintenance, and shutdown of blood coagulation

Blood hemostasis is accomplished by a complex network of (anti-)coagulatory and fibrinolytic processes. These physiological processes are implemented by the assembly of multiprotein complexes involving both humoral and cellular components. Coagulation factor X, and particularly, factor IX, exemplify the dramatic enhancement that is obtained by the synergistic interaction of cell surface, inorganic and protein cofactors, protease, and substrate. With a focus on structure-function relationship, we review the current knowledge of activity modulation principles in the coagulation proteases factors IX and X and indicate future challenges for hemostasis research. This chapter is organized by describing the principles of hierarchical activation of blood coagulation proteases, including endogenous and exogenous protease activators, cofactor binding, substrate specificities, and protein inhibitors. We conclude by outlining pharmaceutical opportunities for unmet needs in hemophilia and thrombosis.

Factor Va increases the affinity of factor Xa for prothrombin: a binding study using a novel photoactivable thiol-specific fluorescent probe

The multiprotein complex of factor Xa, factor Va, and prothrombin efficiently generates the blood-clotting agent, thrombin. Here, the formation of the factor Xa*prothrombin complex and the effects of factor Va on this complex were examined using a photoactivable thiol-specific fluorescent probe (LWB), which was synthesized and incorporated into the active site of factor Xa. The use of fluorescent LWB illustrated that factor Xa has an increased affinity for prothrombin in the presence of factor Va. Further exposure of these components to UV light resulted in a specific photocrosslinking of LWB-factor Xa to prothrombin, suggesting a physical association between these proteins. These data demonstrate that LWB can successfully function both as a spectroscopic probe and as a photocrosslinking reagent for studying protein-protein interactions.

A total fibrinogen deficiency is compatible with the development of pulmonary fibrosis in mice

In addition to their well-known roles in hemostasis, fibrinogen (Fg) and fibrin (Fn) have been implicated in a number of other physiological and pathophysiological events. One of these involves the fibroproliferative response after acute lung injury, which is the focus of the current study. Mice with a total Fg deficiency (FG(-/-)) were generated by breeding heterozygous (FG(+/-)) pairs, each of which contained an allele with a targeted deletion of its Fg-gamma-chain gene. The resulting FG(-/-) animals did not possess detectable plasma Fg. FG(-/-) mice were then used to assess the roles of Fg and Fn in a bleomycin-induced acute lung injury model. Intratracheal administration of bleomycin in wild-type and FG(-/-) mice resulted in equivalent deposition of interstitial collagen and fibrotic lesions at days 7 and 14 after administration. This indicates that Fg and/or Fn are not essential for the development of bleomycin-induced pulmonary fibrosis.

Biochemical defects of mutant nudel alleles causing early developmental arrest or dorsalization of the Drosophila embryo

The nudel gene of Drosophila is maternally required both for structural integrity of the egg and for dorsoventral patterning of the embryo. It encodes a structurally modular protein that is secreted by ovarian follicle cells. Genetic and molecular studies have suggested that the Nudel protein is also functionally modular, with a serine protease domain that is specifically required for ventral development. Here we describe biochemical and immunolocalization studies that provide insight into the molecular basis for the distinct phenotypes produced by nudel mutations and for the interactions between these alleles. Mutations causing loss of embryonic dorsoventral polarity result in a failure to activate the protease domain of Nudel. Our analyses support previous findings that catalytic activity of the protease domain is required for dorsoventral patterning and that the Nudel protease is auto-activated and reveal an important role for a region adjacent to the protease domain in Nudel protease function. Mutations causing egg fragility and early embryonic arrest result in a significant decrease in extracellular Nudel protein, due to defects in post-translational processing, stability, or secretion. On the basis of these and other studies of serine proteases, we suggest potential mechanisms for the complementary and antagonistic interactions between the nudel alleles.

Specific molecular interaction sites on factor VII involved in factor X activation

Factor VII, a serine-protease zymogen, and tissue factor, the cellular receptor/coenzyme, are the protein components of the macromolecular complex which initiates the extrinsic pathway of the coagulation cascade. Previous studies were directed to the identification of functional sites on factor VII which mediate factor X activation, employing a series of potentially inhibitory synthetic peptides representing the primary structure of factor VII and antibodies to selected peptides. The involvement of at least four high-affinity interactive regions [factor VII (44-50), (196-229), (285-305) and (376-396) peptides] on the surface of factor VII was clearly demonstrated. The minimal sequences for the expression of inhibitory activity of these four molecular recognition domains on factor VII were identified using short and overlapping peptides. The short factor VII-(206-218)-peptide (most inhibitory peptide in the sequence 196-229 on factor VII) inhibited the binding of factor VII to the tissue-factor-expressing J82 cell line. Furthermore, radiolabeled [Tyr201] factor VII-(199-221)-peptide, with a tyrosine substituted for the normal tryptophan residue, was specifically bound to J82 cells, and also the binding of the radiolabeled peptide to this cell line was specifically inhibited by a monoclonal antibody to tissue factor, confirming that the interaction site for tissue factor on factor VII is present within the peptide sequence 196-229. Kinetic analyses suggested that the regions represented by factor VII-(285-305)- and factor VII-(376-396)-peptides are involved in factor X recognition and the chemical cross-linking of the radiolabeled peptides resulted in specific binding to factor X, confirming that these two regions on factor VII represent the substrate-recognition site. Furthermore, these radiolabeled peptides specifically interact with the heavy chain of factor X, suggesting that the complementary binding region for the substrate-recognition site on factor VII are present on the heavy chain of factor X.

Two forms of factor C from the amoebocytes of Carcinoscorpius rotundicauda: purification and characterisation

The two apparent form of the endotoxin-sensitive Factor C which were found to exist in the amoebocytes of horseshoe crabs have been separately purified to homogeneity from the lysate of the South-East Asian species, Carcinoscorpius rotundicauda. Both forms are serine proteinase zymogens having an apparent molecular mass of 132 kDa. By reducing SDS-PAGE, one was shown to consist of a single polypeptide while the other has a heavy chain (80 kDa) and a light chain (52 kDa) bridged by disulfide linkage(s). Both zymogen forms have endotoxin (lipopolysaccharide) receptors to which endotoxin binds to activate their catalytic sites. However, single-chain Factor C appears to have higher-affinity endotoxin-binding sites which are competitively but reversibly occupied by DMSO when the latter was added during its purification. Another salient difference between the two forms of Factor C is exhibited in their manner of activation by endotoxin. While double-chain Factor C appears similar to that of Tachypleus tridentatus, single-chain Factor C did not undergo any proteolytic cleavage upon activation. This conformational transition of zymogen activation suggests an alternative reversible pathway of endotoxin activation for the single-chain Factor C.

Evidence from total internal reflection fluorescence microscopy for calcium-independent binding of prothrombin to negatively charged planar phospholipid membranes

Measurements to test for a proposed Ca2+-independent interaction of prothrombin with membranes containing acidic phospholipids are described. Fluorescein-labeled bovine prothrombin and its amino- and carboxy-terminal peptides, prothrombin fragment 1 and prethrombin 1, were added at various concentrations in the presence or absence of Ca2+ to the aqueous space bathing substrate-supported planar membranes composed of 1-palmitoyl-2-oleoyl-3-sn-phosphatidylcholine (POPC), POPC/bovine brain phosphatidylserine (bovPS) (70:30 mol/mol), or POPC/1,2-dioleoyl-3-sn-phosphatidylglycerol (DOPG) (70:30 mol/mol). Total internal reflection fluorescence microscopy (TIRFM) at the membrane-solution interface showed a significant enhancement by acidic lipids of prothrombin and prothrombin fragment 1 binding in the presence of 5 mM Ca2+, with apparent dissociation constants of 0.4 and 1 microM, respectively. TIRFM measurements indicated that bovPS and DOPG also significantly enhanced the binding of fluorescein-labeled prothrombin to the planar membranes in the absence of Ca2+, with apparent dissociation constants (13-30 microM) at least an order of magnitude larger than the Ca(2+)-dependent constant for prothrombin binding. Association of prethrombin 1 but not prothrombin fragment 1 with membranes in the absence of Ca2+ was enhanced by the presence of bovPS in the membranes, which suggests that the Ca(2+)-independent binding site(s) is (are) in the prethrombin 1 but not the fragment 1 portion of prothrombin.

Tb(III)-ion-binding-induced conformational changes in platelet factor XIII

Ca(II) ions are crucial during proteolytic conversion of Factor XIII zymogen into the active enzyme Factor XIIIa. Factor XIII proteolyzed by thrombin or trypsin in the presence of 5 mM-EDTA resulted in rapid inactivation of transglutaminase activity. Factor XIIIa formed by thrombin or trypsin in the presence of 40 microM-Tb(III) ions, however, was indistinguishable from Factor XIIIa formed in the presence of 2-5 mM-Ca(II) ions with respect to molecular mass and transglutaminase activity. Thrombin treatment of Factor XIII in the presence of 1-5 microM-Tb(III) ions resulted in three fragments (76 kDa, 51 kDa and 19 kDa) with simultaneous loss of transglutaminase activity. Tb(III) ions at concentrations greater than 40 microM made platelet Factor XIII resistant to proteolysis by either thrombin or trypsin. Other lanthanide(III) ions [Ln(III) ions] tested [Ce(III), La(III) and Gd(III) ions] functioned similarly to Tb(III) ions during proteolytic activation of Factor XIII. Ln(III) ions (10-100 microM) were unable to replace the Ca(II) ions required for transglutaminase activity of Factor XIIIa. Tb(III) ions also inhibited in a non-competitive manner the transglutaminase activity of Factor XIIIa (Ki 71 microM) even when measured in the presence of 200-fold molar excess of Ca(II) ions. Factor XIII selectively bound to a Tb(III)-chelate affinity column, and could not be eluted by 100 mM-CaCl2. Binding of Tb(III) ions to Factor XIII was demonstrated by fluorescence emission due to Forster energy transfer. A 10(4)-fold molar excess of CaCl2, but not NaCl, partially quenched Tb(III) fluorescence. Low concentrations (5-20 microM) of Tb(III) ions also inhibited the binding of Factor XIII to des-A-fibrinogen by about 43%, whereas higher concentrations (40-100 microM) promoted binding. Conformational changes in Factor XIII consequent to the binding of Tb(III) ions could be responsible for the observed effects on protein structure and function.

Irreversible degradation of histidine-96 of prothrombin fragment 1 during protein acetylation: another unusually reactive site in the kringle

Acetylation of prothrombin fragment 1 in acetate-borate buffer at pH 8.5 resulted in the appearance of increased light absorbance at about 250 nm. Protease digestions resulted in isolation of a single peptide (residues 94-99) with intense absorbance at about 250 nm (estimated extinction coefficient of 5000 M-1 cm-1). Amino acid analysis showed the expected composition except for the absence of His-96. Instead, an unidentified amino acid which had a ninhydrin product with absorption properties similar to those of proline eluted near aspartate. When sequenced, this peptide (YP?KPE containing epsilon-amino-acetyllysine) lacked histidine at the third position but gave a high yield of a PTH derivative that eluted near PTH-Gly from the HPLC column. Fast atom bombardment mass spectrometry of the derivatized 94-99 peptide showed a mass that was 74 units higher than expected. The histidine degradation product was identified as a di-N-acetylated side chain with an opened imidazole ring and loss of C2 of the ring. While a similar degradation pattern has previously been reported during acylation of histidine, the high chemical reactivity exhibited by His-96 was unusual. For example, under conditions sufficient for quantitative derivatization of His-96, His-105 of fragment 1 was not derivatized to a detectable level. Furthermore, His-96 in fragment 1 was at least an order of magnitude more susceptible to degradation than His-96 in the isolated 94-99 peptide. His-96 is therefore one of several neighboring amino acids of the kringle portion of fragment 1 that displays highly unusual chemistry (see also Asn-101 [Welsch, D.J., & Nelsestuen, G. L. (1988) Biochemistry 27 4946-4952] and Lys-97 [Pollock, J.S., Zapata, G.A., Weber, D.J., Berkowitz, P., Deerfield, D.W., II, Olson, D.L., Koehler, K.A., Pedersen, L.G., & Hiskey, R.G. (1988) in Current Advances in Vitamin K Research (Suttie, J.W., Ed.) pp 325-334, Elsevier Science, New York]).(ABSTRACT TRUNCATED AT 250 WORDS)

The binding of divalent metal ions to platelet factor XIII modulates its proteolysis by trypsin and thrombin

We investigated the effect of divalent metal ions on the proteolytic cleavage and activation of platelet Factor XIII by thrombin and trypsin. In the absence of metal ions (5 mM EDTA), trypsin and thrombin rapidly degraded platelet Factor XIII (80 kDa) to low-molecular-mass peptides (50-19 kDa) with simultaneous loss of transglutaminase activity. Divalent metal ions protected Factor XIII from proteolytic inactivation with an order of efficacy of Ca2+ greater than Zn2+ greater than Mg2+ greater than Mn2+. Calcium (2 mM) increased by 10- to 1000-fold the trypsin and thrombin concentrations required to degrade Factor XIII to a 19-kDa peptide. Factor XIIIa formed by thrombin in the presence of 5 mM EDTA had one-half the specific activity of Factor XIIIa formed in the presence of calcium. Factor XIII was cleaved by trypsin in the presence of 5 mM Ca2+ to a 51 +/- 3-kDa fragment that had 60% of the original Factor XIIIa activity. A similar tryptic peptide formed in the presence of 5 mM EDTA did not have transglutaminase activity. In the presence of 5 mM Mg2+, thrombin cleaved Factor XIII to a major 51 +/- 3-kDa fragment that had 60% of the Factor XIIIa activity. Mn2+ (0.1-5 mM) limited trypsin and thrombin proteolysis. The resulting digest containing a population of Factor XIII fragments (50-14 kDa) expressed 50-60% transglutaminase activity of Factor XIIIa. Factor XIII was fully activated by both trypsin and thrombin in the presence of 5 mM Zn2+, resulting in two fragments of 76 and 72 kDa. We conclude that the binding of divalent metal ions to platelet Factor XIII induces conformational changes in the protein that alter its susceptibility to proteolysis and influence the expression of transglutaminase activity.

Initiation of coagulation by tissue factor

Tissue factor (TF) is an integral membrane glycoprotein which functions as an initiator of coagulation. Furthermore, it is probably the principal biological initiator of this essential hemostatic process. This article reviews the studies which form the basis for these assertions. The work on TF is traced from the 19th century discovery of the thromboplastic activity of tissues to the recent purification of the protein from bovine and human tissues and the isolation cDNA clones coding from human TF. The features of TF structure and function which tailor it to the role of initiator of the coagulation cascade are considered. For example, cell-surface TF and factor VII, the plasma serine proteases zymogen, form a proteolytic complex without prior proteolysis of either component. In addition, a kinetic model for the molecular mechanism of TF-initiated clotting is reviewed. The factors which control the expression of TF procoagulant activity by cultured cells are examined in light of the hypothesized role of TF in normal hemostasis. Also, the potential pathological consequences of aberrant TF expression, i.e., thrombosis and hemorrhage, are explored.

Expression of human factor IX and its subfragments in Escherichia coli and generation of antibodies to the subfragments

A cDNA clone encoding the entire human blood clotting factor IX (amino acids -3 to 415 has been placed under control of transcription and translation signals from bacteriophage T7 and expressed in Escherichia coli. The full-length cDNA and 13 different subfragments (which together cover the entire coding sequence of mature factor IX plus amino acids -40 to -19 of the prepro leader sequence) have each been joined to the coding sequence for the major capsid protein of T7 after the 326th codon and expressed as fusion proteins. All of the fusion proteins were insoluble, which facilitated their purification. A goat polyclonal antiserum against human factor IX reacted to different extents with the different fusion proteins, and rabbit polyclonal antibodies raised against the purified fusion proteins recognize the factor IX molecule, as demonstrated by immunoblotting techniques. Antibodies against at least one of the fusion proteins can also inhibit the biological activity of purified factor IX in a one-stage partial thromboplastin time bioassay. We expect these fusion proteins and the antibodies against them to be useful in studying the structure and function of factor IX.

Isolation of cDNA clones coding for human tissue factor: primary structure of the protein and cDNA

Tissue factor is a membrane-bound procoagulant protein that activates the extrinsic pathway of blood coagulation in the presence of factor VII and calcium. lambda Phage containing the tissue factor gene were isolated from a human placental cDNA library. The amino acid sequence deduced from the nucleotide sequence of the cDNAs indicates that tissue factor is synthesized as a higher molecular weight precursor with a leader sequence of 32 amino acids, while the mature protein is a single polypeptide chain composed of 263 residues. The derived primary structure of tissue factor has been confirmed by comparison to protein and peptide sequence data. The sequence of the mature protein suggests that there are three distinct domains: extracellular, residues 1-219; hydrophobic, residues 220-242; and cytoplasmic, residues 243-263. Three potential N-linked carbohydrate attachment sites occur in the extracellular domain. The amino acid sequence of tissue factor shows no significant homology with the vitamin K-dependent serine proteases, coagulation cofactors, or any other protein in the National Biomedical Research Foundation sequence data bank (Washington, DC).

Fibrin assembly after fibrinopeptide A release in model systems and human plasma studied with magnetic birefringence

Magnetically induced birefringence was used to monitor fibrin polymerization after the release of the small negatively charged A fibrinopeptides from human fibrinogen by the action of the snake-venom-derived enzymes reptilase and ancrod. A range of conditions was investigated. Fibrin polymerization in solutions of purified fibrinogen shows a distinct break near the gelation point. On addition of Ca2+ or albumin the lag period is shortened, fibre thickness is increased and the break in assembly almost vanishes, probably because both of these additives promote lateral aggregation. There are minor differences in the kinetics, depending on the venom enzyme used. The kinetics of fibrin assembly in model systems containing either Ca2+ or albumin and in human plasma with a largely dormant coagulation cascade are very similar. Therefore in the latter condition there is no significant alteration in the assembly process due to interaction between fibrin or the venom enzymes and any of the plasma proteins. When the cascade is activated, the polymerization progress curves have a character that resembles a combination of the reactions observed when the venom enzymes and endogenously generated thrombin separately induce coagulation, except for a region near gelation where, paradoxically, polymerization appears to be slower on activation. The low-angle neutron-diffraction patterns from oriented gels made with thrombin or reptilase are identical. Therefore at low resolution the packing of the monomers within fibres is the same when fibrinopeptide A only or both fibrinopeptides A and B are removed.

Recognition site directing vitamin K-dependent gamma-carboxylation resides on the propeptide of factor IX

Posttranslational processing of vitamin K-dependent proteins includes gamma-carboxylation of specific glutamic acid residues to form gamma-carboxyglutamic acids. To determine whether carboxylation is directed by the propeptide sequence, homologous among the precursors of these proteins, alterations were made in the Factor IX propeptide cDNA. The extent of gamma-carboxylation of recombinant Factor IX was assessed using conformation-specific antibodies directed against the gamma-carboxyglutamic acid-dependent, metal-stabilized structure. Deletion of the propeptide (residues -18 to -1) abolished carboxylation, but not secretion, of Factor IX. Substitution of alanine for phenylalanine -16 or glutamic acid for alanine -10 also impaired carboxylation. These results indicate that the Factor IX propeptide participates in defining a recognition site that designates an adjacent glutamic acid-rich domain for gamma-carboxylation. The association of the propeptide with the gamma-carboxylation recognition site provides the first demonstration of a specific function served by a propeptide in posttranslational protein processing.

Fibrin assembly in human plasma and fibrinogen/albumin mixtures

Magnetic birefringence is used to monitor the kinetics of thrombin-catalyzed fibrin polymerization in model systems of increasing complexity (i.e., fibrinogen solutions, fibrinogen/albumin mixtures, and plasma anticoagulated with citrate) and in plasma containing free calcium which is the physiological condition. The introduction of albumin into fibrinogen solutions shortens the lag period and enhances fiber thickness. The polymerization progress curves are sigmoidal at zero or low albumin concentrations, but at physiological and higher concentrations, they become hyperbola-like from the end of the lag period. High albumin concentration has thus induced a change in the assembly kinetics. The progress curves from plasma in which the cascade is dormant are also hyperbola-like although they round off more quickly because of antithrombin activity. In plasma containing free calcium, thrombin is endogenously produced, and the progress curves are nearly linear; hence, the assembly kinetics are very different from those of the model systems. The curves are not influenced by calcium-dependent cross-linking involving factor XIIIa. The progress curves are also linear when polymerization is induced with Russell's viper venom, which by directly activating factor X circumvents earlier steps in the cascade. This implies that linear polymerization is caused by events posterior to factor X activation and are thus likely to be largely dependent on the functioning of the prothrombinase complex. Addition of thrombin to plasma containing free calcium reduces the lag period. At low exogenous thrombin levels, the polymerization rate is increased, and the progress curves remain linear. However, at higher levels, the curves become more complicated and, paradoxically, full polymerization takes longer.(ABSTRACT TRUNCATED AT 250 WORDS)

A domain of membrane-bound blood coagulation factor Va is located far from the phospholipid surface. A fluorescence energy transfer measurement

The larger subunit of blood coagulation factor Va was covalently labeled with iodoacetamido derivatives of fluorescein and rhodamine without loss of functional activity, as measured by either the one-stage clotting assay or the ability to accelerate prothrombin activation in a purified system. The spectral properties of the dyes were not altered by the presence or absence of the smaller subunit of factor Va, Ca2+, prothrombin, factor Xa, or phosphatidylcholine/phosphatidylserine (PC/PS, 4:1) vesicles. When fluorescein-labeled protein (factor VaF) was titrated with PC/PS vesicles containing either octadecylrhodamine or 5-(N-hexadecanoylamino)eosin, fluorescence energy transfer was observed between the protein-bound donor dyes and the acceptor dyes at the outer surface of the phospholipid bilayer. The extent of energy transfer correlated directly with the extent of protein binding to the vesicles monitored by light scattering. The distance of closest approach between the fluorescein on factor Va and the bilayer surface averaged 90 A for the two different acceptors. Association of factor VaF with factor Xa on the phospholipid surface reduced this separation by 7 A, but association with prothrombin did not alter the distance between the labeled domain on factor VaF and the surface. The efficiency of diffusion-enhanced energy transfer between rhodamine-labeled factor Va and terbium dipicolinate entrapped inside PC/PS vesicles was less than 0.01, consistent with the location of the dye far above the inner surface of the vesicle. Thus, a domain of membrane-bound factor Va is located a minimum of 90 A above the phospholipid surface.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular basis of hemophilia B: a defective enzyme due to an unprocessed propeptide is caused by a point mutation in the factor IX precursor

A mutant factor IX, designated factor IXCambridge, was isolated from a patient with hemophilia B. This protein includes an 18-residue propeptide attached to the NH2 terminus of factor IX. A point mutation at residue -1, from an arginine to a serine, precludes cleavage of the propeptide by a processing protease and interferes with gamma-carboxylation of the factor IX, indicating the importance of the leader sequence in substrate recognition by the vitamin K-dependent carboxylase. This represents an example of an enzyme defect due to the presence of a point mutation in a precursor protein (preproenzyme) that is the cause of a human hereditary disease. This defect will serve as a prototype for understanding the molecular basis of some forms of hemophilia and other hereditary enzyme deficiencies.

Calcium-dependent changes in properties of human prothrombin: a study using high-performance size-exclusion chromatography and gel-permeation chromatography

High-performance size-exclusion chromatography using a TSK 3000 SW column and aqueous gel filtration with Sephacryl S-200 SF have been used to characterize the effects of calcium ions on the hydrodynamic properties of human prothrombin and prethrombin 1. The results suggest that the effective hydrodynamic radius of prothrombin is less in the presence than in the absence of calcium ions. In addition, when using the TSK-3000 SW column, Ca2+-dependent formation of a hydrophobic site in the fragment 1 region of prothrombin results in an apparent further decrease in hydrodynamic radius.

Effect of divalent cations on the limited proteolysis of prothrombin by thrombin

The inhibitory influence of divalent cations on the ability of bovine alpha-thrombin to hydrolyze prothrombin showed the trend Mn2+ much greater than Ca2+ greater than or equal to Mg2+ greater than Sr2+ much greater than Ba2+. This effect was not due to an inhibition of thrombin's catalytic activity as measured by hydrolysis of a specific synthetic substrate, H-D-Phe-pipecolyl-Arg-p-nitroanilide (D-PhePipArgNA). The presence of divalent cations did not inhibit thrombic proteolysis of gamma-carboxyglutamic acid (Gla)-domainless prothrombin. Prothrombin and Gla-domainless prothrombin were used as competitive inhibitors in the thrombic hydrolysis of D-PhePipArgNA. The apparent Ki value calculated for prothrombin was 18 microM. When either Ca2+ or Mn2+ were present, there was no inhibition. The apparent Ki value determined for Gla-domainless prothrombin was 28 microM in either the absence or presence of Ca2+. Addition of divalent cations to prothrombin, but not to Gla-domainless prothrombin, resulted in an altered protein conformation as measured by high-performance size-exclusion chromatography and ultraviolet difference spectroscopy. These results suggest that a conformational change secondary to the interaction of divalent cations with the Gla-containing domain of prothrombin is required for cation-dependent inhibition of thrombin hydrolysis.

The procoagulant effect of zinc on fibrin clot formation

The influence of Zn+2 on fibrin clot formation was investigated by measuring its effect on the clotting times of fibrinogen exposed to thrombin. It was observed with either human or bovine thrombin that 0.01-0.1 mM Zn+2 induced significant reductions of clotting times in a concentration-dependent manner. The procoagulant effect of Zn+2 occurred in the presence of Ca+2 but was inhibited by metal chelating agents. Higher levels of Zn+2 (greater than 0.2 mM final concentration) were required to accelerate thrombin-induced clot formation in the presence of citrate or oxalate. Similarly with oxalated human plasma, greater than 0.2 mM Zn+2 decreased the clotting time. Cations such as Mg+2 and Mn+2 caused little change in clotting times. As an extension of these findings, we examined the effect of Zn+2 on the inhibition of thrombin by antithrombin-III (AT-III). The presence of as little as 0.006 mM Zn+2 in an incubating mixture of thrombin and AT-III severely reduced the inhibitory activity of AT-III towards thrombin. It was observed that the relative intrinsic fluorescence emission of human thrombin decreased upon exposure to Zn+2 but was unaffected by Mg+2 or Mn+2. It is suggested that Zn+2 can form a complex with thrombin, which results in altered reactivity towards fibrinogen and decreased inhibition by AT-III.

The binding of Gla-containing proteins to phospholipids

It is demonstrated here that osteocalcin, the Gla-containing protein from bone, is unable to interfere with the binding of the blood coagulation factors to phospholipid vesicles. Therefore, it seems that besides the Gla residues other structural features of the coagulation factors are required for their effective binding to phospholipid surfaces.

Factor VIII: structure and function in blood clotting

Factor VIII (antihemophilic factor) is the protein that is deficient or defective in patients with classical hemophilia and Von Willebrand syndrome. Factor VIII in plasma is thought to be associated in a complex with the highest molecular weight multimers of another glycoprotein, Von Willebrand protein. Highly purified human factor VIII appears to have an Mr of between 200,000 and 300,000 and to consist of several polypeptide chains. The concentration of factor VIII in plasma is around 100-200 ng/ml, equivalent to around 1 nM. The purified proteins retain one or more of the known properties of factor VIII, including the acceleration of factor IXa-mediated activation of factor X, ability to be activated by thrombin and factor Xa, inactivation by activated protein C, and by human antibodies to factor VIII. Among the known clotting factors, factors VIII and V are exceptional in not possessing enzymatic activity. Factors IXa and VIII and X appear to form a functional complex, all of which need to be present and active simultaneously for optimal activation of factor X. The mechanism by which factor VIII promotes activation of factor X by factor IXa is not known, but the major effect is to increase the rate of the reaction. Following treatment of factor VIII with thrombin, a new and smaller polypeptide Mr around 70,000 +/- 5,000 is produced. Factors IXa and Xa also have been reported to activate factor VIII. It is not known whether limited proteolytic cleavage is required absolutely for the expression of factor VIII activity or if it only increases an activity already expressed by the uncleaved protein. Factor VIII is inactivated by thrombin and by activated protein C. Thus, factor VIII can be modulated by at least four of the serine proteases in the clotting system. A major goal for future research is to increase our understanding of the role in blood clotting played by factor VIII, and to apply this information to clinical problems which result from inherited abnormalities of factor VIII.