The gamma-carboxylation recognition site is sufficient to direct vitamin K-dependent carboxylation on an adjacent glutamate-rich region of thrombin in a propeptide-thrombin chimera

The propeptides of the vitamin K-dependent proteins contain a gamma-carboxylation recognition site that is required for gamma-glutamyl carboxylation. To determine whether the propeptide is sufficient to direct carboxylation, two mutant prothrombin species were expressed and characterized with regard to posttranslational gamma-carboxylation. A double point mutant, in which serine substituted for cysteines 17 and 22 disrupted a conserved loop formed by a disulfide bond, was fully carboxylated when expressed in Chinese hamster ovary cells. A propeptide/thrombin chimeric protein, constructed by deleting the Gla, aromatic amino acid stack, and kringle domains of prothrombin, has the signal peptide and propeptide juxtaposed to a glutamate-rich COOH-terminal region of prothrombin, residues 249-530. Of the 8 glutamic acid residues contained within the first 40 residues of the NH2 terminus adjacent to the propeptide, at least seven were fully carboxylated as demonstrated by direct gamma-carboxyglutamic acid analysis of the alkaline hydrolysate and by NH2-terminal sequence analysis. These results indicate that the gamma-carboxylation recognition site within the prothrombin propeptide in a prothrombin propeptide-thrombin chimeric protein is sufficient to direct gamma-carboxylase-catalyzed carboxylation of adjacent glutamic acid residues in a glutamate-rich region of thrombin that is not normally gamma-carboxylated. Furthermore, the disulfide loop in the Gla domain of prothrombin is not required for complete carboxylation.

Interactions of neutrophils and coagulation proteins

Some of the interactions between coagulation factors and neutrophils are described. Proteins of the contact system, FXa, thrombin, and fibrinogen all bind to various sites on the neutrophil. This binding has a dual purpose: the assembly of coagulation complexes such as the prothrombinase complex and the contact system on the neutrophil membrane and influencing the various neutrophil functions including chemotaxis, aggregation, degranulation, and transendothelial migration. In addition, neutrophil elastase degrades many coagulation proteins, thus modulating both the thrombotic and the fibrinolytic systems. These interactions should be viewed in a wider context as part of the links between the coagulation and inflammation pathways.

Platelet-leukocyte-endothelial cell interaction on the blood vessel wall

Leukocytes, platelets, and endothelial cells interact at sites of vascular injury and inflammation through adhesion receptors on the cell surface. On binding of ligand to receptor, these receptors initiate intracellular signaling that leads to the modulation of several biological properties of the cells involved. These finely regulated processes involve several classes of cell adhesion molecules: integrins, immunoglobulin-like proteins, selectins, and mucin-like proteins as well as an array of soluble mediators. Interaction of these cell adhesion molecules serves to recruit circulating cells to the blood vessel endothelium or to accumulated platelets on the vessel wall and to foster cell-cell communication. The importance of these interactions to inflammation, blood coagulation, and the immune response is outlined.

Propeptide and glutamate-containing substrates bound to the vitamin K-dependent carboxylase convert its vitamin K epoxidase function from an inactive to an active state

The vitamin K-dependent gamma-glutamyl carboxylase catalyzes the posttranslational conversion of glutamic acid to gamma-carboxyglutamic acid in precursor proteins containing the gamma-carboxylation recognition site (gamma-CRS). During this reaction, glutamic acid is converted to gamma-carboxyglutamic acid while vitamin KH2 is converted to vitamin K 2,3-epoxide. Recombinant bovine carboxylase was purified free of gamma-CRS-containing propeptide and endogenous substrate in a single-step immunoaffinity procedure. We show that in the absence of gamma-CRS-containing propeptide and/or glutamate-containing substrate, carboxylase has little or no epoxidase activity. Epoxidase activity is induced by Phe-Leu-Glu-Glu-Leu (FLEEL) (9.2 pmol per min per pmol of enzyme), propeptide, residues -18 to -1 of proFactor IX (3.4 pmol per min per pmol of enzyme), FLEEL and propeptide (100 pmol per min per pmol of enzyme), and proPT28 (HVFLAPQQARSLLQRVRRANTFLEEVRK, residues -18 to +10 of human acarboxy-proprothrombin), (5.3 pmol per min per pmol of enzyme). These results indicate that in the absence of propeptide or glutamate-containing substrate, oxygenation of vitamin K by the carboxylase does not occur. Upon addition of propeptide or glutamate-containing substrate, the enzyme is converted to an active epoxidase. This regulatory mechanism prevents the generation of a highly reactive vitamin K intermediate in the absence of a substrate for carboxylation.

Structure and mechanism of action of the vitamin K-dependent gamma-glutamyl carboxylase: recent advances from mutagenesis studies

The vitamin K-dependent gamma-glutamyl carboxylase catalyzes the conversion of glutamic acid to gamma-carboxyglutamic acid in substrate proteins. The enzyme has recently been purified and the cDNA encoding the enzyme has been cloned. The availability of recombinant enzyme provides the opportunity to probe the mechanism of this unique enzyme. The binding sites for the gamma-carboxylation recognition site containing propeptide and carboxylatable glutamate residues of a vitamin K-dependent substrate protein have been localized to the amino-terminal 250 residues of the enzyme. Regions of the carboxy-terminal of the enzyme are important for conversion of vitamin K hydroquinone to vitamin K epoxide, a reaction that occurs concomitantly with carboxylation and is catalyzed by the vitamin K-dependent carboxylase. Using pure recombinant vitamin K-dependent carboxylase it has been demonstrated that catalysis of vitamin K oxygenation by the enzyme is regulated by the availability of carboxylatable substrate.

Leukocyte crosstalk at the vascular wall

P-selectin is an adhesion molecule on platelets and endothelial cells that mediates the interaction of these cells with leukocytes. In addition to its docking function in cell-cell recognition, P-selectin is involved in cell signalling and cell communication. Upon platelet activation, P-selectin undergoes rapid phosphorylation and dephosphorylation. The biological role of these phosphorylation events within platelets is unknown. P-selectin, upon contact with its receptor on monocytes, initiates the de novo biosynthesis of tissue factor and other cytokines. Tissue factor upregulation on monocytes is mediated by PSGL-1 and is independent of CD14, the LPS receptor.

Three-dimensional structure of a gamma-carboxyglutamic acid-containing conotoxin, conantokin G, from the marine snail Conus geographus: the metal-free conformer

Conantokin G is a gamma-carboxyglutamic acid-containing conotoxin from the venom of the marine cone snail Conus geographus. The 17-residue peptide, which contains five gamma-carboxyglutamic acid (Gla) residues and an amidated C-terminal asparagine amide, was synthesized chemically in a form identical to the natural conantokin G. To gain insight into the role of gamma-carboxyglutamic acid in the structure of this peptide, we determined the three-dimensional structure of conantokin G by 1H NMR and compared its structure to other conotoxins and to the gamma-carboxyglutamic acid-containing regions of the vitamin K-dependent blood-clotting proteins. Complete resonance assignments were made by two-dimensional 1H NMR spectroscopy in the absence of metal ions. NOE cross-peaks d(alphaN), d(NN), and d(betaN) provided interproton distance information, and vicinal spin-spin coupling constants 3J(HN alpha) were used to calculate phi torsion angles. Distance geometry and simulated annealing methods were used to derive 20 convergent structures from a set of 227 interproton distance restraints and 13 torsion angle measurements. The backbone rmsd to the geometric average for 20 final structures is 0.8 +/- 0.1 A. Conantokin G consists of a structured region commencing at Gla 3 and extending through arginine 13. This structure includes a partial loop centered around Gla 3 and Gla 4, a distorted type I turn between glutamine 6 and glutamine 9, and two type I turns involving Gla 10, leucine 11, and isoleucine 12 and arginine 13. Together, these two turns define approximately 1.6 turns of a distorted 3(10) helix. The observed structure possesses structural elements similar to those seen in the disulfide-linked conotoxins.

The propeptide of the vitamin K-dependent carboxylase substrate accelerates formation of the gamma-glutamyl carbanion intermediate

Vitamin K-dependent carboxylase catalyzes the post-translational gamma-carboxylation of 9-12 glutamyl residues of several blood coagulation proteins. Carboxylase purified from Chinese hamster ovary (CHO) cells as a recombinant FLAG-carboxylase fusion protein [Sugiura, I., et al. (1996) J. Biol. Chem. 271, 17837-17844] was utilized with pentapeptide substrate FL[3H-R,S]EAL with high specific radioactivity to probe the timing of glutamyl Cgamma-3H cleavage relative to Cgamma-COO- bond formation by 14CO2 incorporation rates. Studies were conducted over a range of NaH14CO3 concentrations to assess uncoupling of gamma-glutamyl carbanion formation and over a range of concentrations of ProPT18, the 18-residue peptide corresponding to the -18 to -1 propeptide region of prothrombin known to affect the catalytic efficiency of carboxylase. At saturation, ProPT18 accelerates Cgamma-3H cleavage 11-13-fold and Cgamma-14CO2- formation 6-7-fold, converting a Cgamma-3H cleavage/Cgamma-14CO2- formation ratio of 1.2-1.4 in the absence of ProPT18 to 2.3-2.8 in its presence, a relative increase in and uncoupling of Cgamma-3H cleavage from C-C bond formation. When the HCO3- concentration was varied, the V/K3H+/V/K14CO2 ratios rose as HCO3- fractional saturation dropped to a ratio of 9.3-10.8/l at low bicarbonate, indicating an uncoupling of nine out of ten gamma-glutamyl carbanion formations from carboxylative capture, consistent with prior reports on microsomal enzyme [Larson, A. E., et al. (1981) J. Biol. Chem. 256, 11032-11035]. These results with pentapeptide substrate FLEAL validate reversible gamma-glutamyl carbanion formation by pure carboxylase and indicate the ProPT18 increase in catalytic efficiency is in selective lowering of an energy barrier preceding the gamma-glutamyl carbanion intermediate.

Refinement of the NMR solution structure of the gamma-carboxyglutamic acid domain of coagulation factor IX using molecular dynamics simulation with initial Ca2+ positions determined by a genetic algorithm

A genetic algorithm (GA) successfully identified the calcium positions in the crystal structure of bovine prothrombin fragment 1 bound with calcium ions (bf1/Ca). The same protocol was then used to determine the calcium positions in a closely related fragment, the Gla domain of coagulation factor IX, the structure of which had previously been determined by NMR spectroscopy in the presence of calcium ions. The most frequently occurring low-energy structure found by GA was used as the starting structure for a molecular dynamics refinement. The molecular dynamics simulation was performed using explicit water and the Particle-Mesh Ewald method to accommodate the long-range electrostatic forces. While the overall conformation of the NMR structure was preserved, significant refinement is apparent when comparing the simulation average structure with its NMR precursor in terms of the N-terminal (Tyr1-N) network, the total number of hydrogen bonds, the calcium ion coordinations, and the compactness of the structure. It is likely that the placement of calcium ions in the protein is critical for refinement. The calcium ions apparently induce structural changes during the course of the simulation that result in a more compact structure.

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

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

Structure and function of the epidermal growth factor domain of P-selectin

P-selectin is a multidomain adhesion protein on the surface of activated platelets and endothelial cells that functions in the recruitment of leukocytes to the site of inflammation. The amino-terminal lectin and EGF domains constitute the ligand recognition unit. We have produced a synthetic 40-residue P-selectin EGF domain (P-sel:EGF) to examine the structure and function of this domain independent of P-selectin. The peptide was folded in vitro and exhibited the same disulfide bonding pattern as other EGF-like domains. P-sel:EGF did not inhibit P-selectin-mediated cellular adhesion assays, indicating that the lectin domain is also required. We undertook the study of the P-selectin EGF by 1H NMR to determine its structure independent of the lectin domain and to compare its structure to that of E-selectin determined crystallographically [Graves et al. (1994) Nature 367, 532]. Although the binding of P-selectin to its carbohydrate ligand is calcium dependent, and some EGF domains have calcium binding sites, addition of calcium had no effect on the NMR spectrum or on the pH-induced changes. Nearly complete resonance assignments were made from 2D 1H NMR spectra at pH 6.0. Two sections of antiparallel beta-sheet were identified on the basis of the pattern of long-range NOEs, 3JHN alpha coupling constants, and slowly exchanging amides. The solution structure of the peptide backbone was determined using distance geometry and simulated annealing calculations. The backbone RMSD to the geometric average for 19 final structures is 0.64 +/- 0.17 A. The resulting fold closely resembles that of other EGF-like peptides, including the E-selectin EGF domain (RMSD approximately 1.08 A). However, compared to the E-selectin EGF structure which also contains the lectin domain, some residues from 1-11 are less ordered, and novel contacts occur between the amino terminus and the core beta-sheet. Despite marked structural homology of the selectin polypeptide backbones, the selectin EGF surfaces show unique distributions of charged residues, a feature that likely correlates to the functional differences.

Biosynthesis of prothrombin: intracellular localization of the vitamin K-dependent carboxylase and the sites of gamma-carboxylation

Prothrombin is a vitamin K-dependent blood coagulation protein that undergoes posttranslational gamma-carboxylation and propeptide cleavage during biosynthesis. The propeptide contains the gamma-carboxylation recognition site that directs gamma-carboxylation. To identify the intracellular sites of carboxylation and propeptide cleavage, we monitored the synthesis of prothrombin in Chinese hamster ovary cells stably transfected with the prothrombin cDNA by immunofluorescent staining. The vitamin K-dependent carboxylase was located in the endoplasmic reticulum and Golgi complex. Antibodies specific to prothrombin processing intermediates were used for immunocytolocalization. Anti-des-gamma-carboxyprothrombin antibodies stained only the endoplasmic reticulum whereas antiproprothrombin antibodies (specific for the propeptide) and antiprothrombin:Mg(II) antibodies (which bind the carboxylated forms of proprothrombin and prothrombin) stained both the endoplasmic reticulum and the Golgi complex. Antiprothrombin:Ca(II)-specific antibodies (which bind only to the carboxylated form of prothrombin lacking the propeptide) stained only the Golgi complex and secretory vesicles, and colocalized with antimannosidase II and anti-p200 in the juxtanuclear Golgi complex. These results indicate that uncarboxylated proprothrombin undergoes complete gamma-carboxylation in the endoplasmic reticulum and that gamma-carboxylation precedes propeptide cleavage during prothrombin biosynthesis.

Profactor IX propeptide and glutamate substrate binding sites on the vitamin K-dependent carboxylase identified by site-directed mutagenesis

The vitamin K-dependent carboxylase, a constituent of the endoplasmic reticulum membrane, catalyzes the conversion of reduced vitamin K to vitamin K epoxide and the concomitant conversion of glutamic acid to gamma-carboxyglutamic acid. To study structure-function relationships in the enzyme, seventeen clusters of charged residues of the bovine gamma-glutamyl carboxylase were substituted with alanines using site-specific mutagenesis. Wild-type and mutant carboxylase species were expressed in Chinese hamster ovary cells with an immunodetectable octapeptide inserted at their amino-terminal ends. Out of 17 mutant carboxylase species that contain a total of 41 charged residue to alanine substitutions, K217A/K218A (CBX217/218), R234A/H235A (CBX234/235), R359A/H360A/K361A (CBX359/360/361), R406A/H408A (CBX406/408), and R513A/K515A (CBX513/515) had impaired carboxylase activity compared with the wild-type enzyme. The vitamin K epoxidase activities of these mutants were reduced in parallel with the carboxylase activities. CBX217/218 appears to be inactive. High propeptide concentrations were required for stimulation of carboxylation of FLEEL by CBX234/235, CBX406/408, and CBX513/515, suggesting defects in the propeptide binding site. CBX359/360/361 showed normal affinity for the propeptide, FLEEL, proPT28, and vitamin K hydroquinone but exhibited a low catalytic rate for carboxylation. These results suggest that residue 217, residue 218, or both are either critical for catalysis or for maintaining the structure of a catalytically active enzyme. Regions around residues 234, 406, and 513 define in part the propeptide binding site, while the regions around residue 359 are involved in catalysis.

Identification of the phospholipid binding site in the vitamin K-dependent blood coagulation protein factor IX

The blood coagulation and regulatory proteins that contain gamma-carboxyglutamic acid are a part of a unique class of membrane binding proteins that require calcium for their interaction with cell membranes. Following protein biosynthesis, glutamic acids on these proteins are converted to gamma-carboxyglutamic acid (Gla) in a reaction that requires vitamin K as a cofactor. The vitamin K-dependent proteins undergo a conformational transition upon metal ion binding, but only calcium ions mediate protein-phospholipid interaction. To identify the site on Factor IX that is required for phospholipid binding, we have determined the three-dimensional structure of the Factor IX Gla domain bound to magnesium ions by NMR spectroscopy. By comparison of this structure to that of the Gla domain bound to calcium ions, we localize the membrane binding site to a highly ordered structure including residues 1-11 of the Gla domain. In the presence of Ca2+, Factor IX Gla domain peptides that contain the photoactivatable amino acid p-benzoyl-L-phenylalanine at positions 6 or 9 cross-link to phospholipid following irradiation, while peptides lacking this amino acid analog or with this analog at position 46 did not cross-link. These results indicate that the NH2 terminus of the Gla domain, specifically including leucine 6 and phenylalanine 9 in the hydrophobic patch, is the contact surface on Factor IX that interacts with the phospholipid bilayer.

Mouse P-selectin glycoprotein ligand-1: molecular cloning, chromosomal localization, and expression of a functional P-selectin receptor

A mouse homolog of P-selectin glycoprotein ligand-1 (PSGL-1), a P-selectin receptor on myeloid cells, has been cloned using the human cDNA sequence to probe a cDNA library prepared from the mouse WEHI-3 monocytic cell line and a genomic DNA library prepared from 129/SvJ mouse tissue. The gene flanking the entire open reading frame of 397 amino acids is composed of a single exon. Mouse and human PSGL-1 show an overall similarity of 67% and an identity of 50% and contain a similar domain organization. However, there are 10 threonine/serine-rich decameric repeats in mouse PSGL-1 as compared with 15 threonine-rich repeats in human PSGL-1. When the mouse PSGL-1 cDNA is coexpressed with an alpha 1,3/1,4 fucosyltransferase cDNA in COS cells, a functional protein is expressed on the COS cell surface mediating binding to human P-selectin. The mouse PSGL-1 gene, Selpl, was mapped to a position on mouse chromosome 5 (Chr 5). Northern blot analyses of mouse tissues showed moderate expression of a PSGL-1 mRNA species in most tissues including heart, kidney, liver, muscle, ovary, and stomach and high levels of expression in blood, bone marrow, brain, adipose tissue, spleen, and thymus. Whereas certain mouse myeloid cell lines including PU5-1.8, WEHI-3B, and 32DC13 express high levels of PSGL-1 mRNA, only WEHI-3B and 32DC13 bind to P-selectin; this interaction is blocked by anti-PSGL-1 antibody. WEHI-3B cells bind significantly better to P-selectin than to E-selectin. Although comparable P-selectin binding is observed in 32DC13 cells, these cells bind better to E-selectin. Binding of 32DC13 cells to E-selectin is not blocked by anti-PSGL-1 antibody. Treatment of WEHI-3B cells with trypsin or neuraminidase abolished their ability to interact with P-selectin. These results indicate that mouse PSGL-1 has structural and functional homology to human PSGL-1 but is characterized by differences in the composition and number of the decameric repeats. PSGL-1 on mouse myeloid cells is critical for high-affinity binding to P-selectin but not E-selectin.

P-selectin induction of tissue factor biosynthesis and expression

P-selectin on activated platelets and stimulated endothelial cells mediates cell adhesion with monocytes and neutrophils. Since activated platelets induce tissue factor on monocytes, we examined the effect of P-selectin on this activity. Chinese hamster ovary cells expressing P-selectin stimulated tissue factor activity in purified monocytes whereas untransfected Chinese hamster ovary cells and CHO cells expressing E-selectin did not. Anti-P-selectin antibodies inhibited these effects. Incubation of CHO:P-selectin with monocytes stimulates tissue factor mRNA development and the expression of tissue factor antigen on the monocyte surface. These results indicate that P-selectin upregulates the expression of tissue factor on monocytes.

Structure of the calcium ion-bound gamma-carboxyglutamic acid-rich domain of factor IX

We have determined the Ca(II)-bound structure of factor IX, residues 1-47, by nuclear magnetic resonance (NMR) spectroscopy. The amino-terminal 47 residues include the gamma-carboxyglutamic acid-rich and aromatic amino acid stack domains, and this region is responsible for Ca(II)-dependent phospholipid binding in factor IX. Protons in the 1-47 amino acid sequence were assigned using standard two-dimensional homonuclear NMR experiments. A total of 851 distance restraints and 57 torsion angle restraints were used to generate 17 final structures by distance geometry and simulated annealing methods. The backbone RMSD to the geometric average is 0.6 +/- 0.1 A. The Ca(II)-bound structure is substantially more ordered with increased helical content compared to the apo-factor IX (1-47) structure. The global fold is similar to the crystal structure of the Ca(II)-bound Gla domain of prothrombin fragment I from residues 12 to 47 (RMSD approximately 1.3 A), but the backbone conformation differs in the first 11 residues, particularly between residues 3 and 6. The amino-terminal nine Gla residues are oriented to the interior of the protein and suggest an internal Ca(II) binding pocket. The carboxyl-terminal three Gla residues are exposed to solvent. The majority of hydrophobic residues are required to stabilize a globular core in the carboxyl-terminal three-quarters of the molecule. However, a hydrophobic surface patch in the amino-terminal region may represent a phospholipid binding site in factor IX.

Histidine phosphorylation of P-selectin upon stimulation of human platelets: a novel pathway for activation-dependent signal transduction

Transient phosphorylation of histidine characterizes the two-component systems in prokaryotes that control important physiological functions, but analogous events have not been implicated in signal transduction in mammalian cells. To explore histidine phosphorylation during activation of human cells, stimulated platelets were analyzed for the formation of protein phosphohistidine in a model system employing P-selectin. P-selectin, a leukocyte adhesion molecule, undergoes rapid phosphorylation and selective dephosphorylation of tyrosine, serine, and threonine. We now establish that phosphorylation following platelet activation with thrombin or collagen generates phosphohistidine at histidines on the cytoplasmic tail of P-selectin. With thrombin stimulation, the kinetics of phosphohistidine appearance and disappearance of P-selectin are very rapid. Platelets exhibit a novel ligand-induced signaling pathway to generate phosphohistidine. These results provide direct biochemical evidence for the induction of rapid and reversible histidine phosphorylation in mammalian cells upon cell activation and represent a novel paradigm for mammalian cell signaling.

Structure of the metal-free gamma-carboxyglutamic acid-rich membrane binding region of factor IX by two-dimensional NMR spectroscopy

The gamma-carboxyglutamic acid-rich domain of blood coagulation Factor IX is required for the binding of the protein to phospholipid membranes. To investigate the three-dimensional structure of this domain, a synthetic peptide corresponding to residues 1-47 of Factor IX was studied by 1H NMR spectroscopy. In the absence of metal ions, the proton chemical shift dispersion in the one-dimensional NMR spectrum indicated that the peptide contains regular structural elements. Upon the addition of Ca(II) or Mg(II), large chemical shift changes were observed in the amide proton and methyl proton regions of the spectrum, consistent with the conformational transitions that metal ions are known to induce in native Factor IX. The apopeptide was studied by two-dimensional NMR spectroscopy at 500 MHz to determine its solution structure. Protons were assigned using total correlation spectroscopy, nuclear Overhauser effect spectroscopy, and double quantum-filtered correlation spectroscopy experiments. Intensities of cross-peaks in the nuclear Overhauser effect spectrum were used to generate a set of interproton distance restraints. The structure of the apopeptide was then calculated using distance geometry methods. There are three structural elements in the apopeptide that are linked by a flexible polypeptide backbone. These elements include a short amino-terminal tetrapeptide loop (amino acids 6-9), the disulfide-containing hexapeptide loop (amino acids 18-23), and a carboxyl-terminal alpha helix (amino acids 37-46). Amide hydrogen exchange kinetics indicate that the majority of the peptide is solvent accessible, except in the carboxyl-terminal element. The structured regions in the apopeptide are insufficient to support phospholipid binding, indicating the importance of additional structural features in the Ca(II)-stabilized conformer.

Mutagenesis of vitamin K-dependent carboxylase demonstrates a carboxyl terminus-mediated interaction with vitamin K hydroquinone

The gamma-glutamyl carboxylase and vitamin K epoxidase activities of a series of mutants of bovine vitamin K-dependent carboxylase with progressively larger COOH-terminal deletions have been analyzed. The recombinant wild-type (residues 1-758) and mutant protein carboxylases, Cbx 711, Cbx 676, and Cbx 572, representing residues 1-711, 1-676, and 1-572, respectively, were expressed in baculovirus-infected Sf9 cells. Wild-type carboxylase had a Km for the substrate Phe-Leu-Glu-Glu-Leu (FLEEL) of 0.87 mM; the carboxylation of FLEEL was stimulated 2.5-fold by proPT18, the propeptide of prothrombin. Its Km for vitamin K hydroquinone was 23 microM and the specific epoxidase activity of the carboxylase was 938 pmol vitamin KO/30 min/pmol of carboxylase. Cbx 711, which was also stimulated by proPT18, had a Km for FLEEL, a Km for vitamin K hydroquinone, and a specific epoxidase activity that was comparable to the wild-type carboxylase. In contrast Cbx 572 lacked both carboxylase and epoxidase activities. Although Cbx 676 had a normal carboxylase active site in terms of the Km for FLEEL and its stimulation by proPT18, the Km for vitamin K hydroquinone was 540 microM, and the specific epoxidase activity was 97 pmol KO/30 min/pmol of Cbx 676. The catalytic efficiencies of Cbx 676 for glutamate carboxylation and vitamin K epoxidation were decreased 15- and 400-fold, respectively, from wild-type enzyme reflecting the requirement for formation of an activated vitamin K species for carboxylation to occur. These data indicate that the truncation of COOH-terminal segments of the carboxylase had no effect on FLEEL or propeptide recognition, but in the case of Cbx 676, selectively affected the interaction with vitamin K hydroquinone and the generation of epoxidase activity. These data suggest that a vitamin K epoxidase activity domain may reside near the COOH terminus while the carboxylase active site domain resides toward the NH2 terminus.

The second kringle domain of prothrombin promotes factor Va-mediated prothrombin activation by prothrombinase

The incorporation of factor Xa into the prothrombinase complex, factor Xa-factor Va-phospholipid-Ca(II), results in an approximately 10(5)-fold higher rate of substrate activation than that of the enzyme alone. To examine the role of the prothrombin kringle domains in the interaction with prothrombinase we have employed site-directed mutagenesis to produce prothrombin species that lack either the first kringle domain, PT/delta K1, or the second kringle domain, PT/delta K2. Previously, we have shown that these proteins are fully carboxylated and that they bind to phospholipid vesicles. In this investigation we demonstrate that cleavage at Arg271-Thr272 and Arg320-Ile321 peptide bonds occurs upon activation with prothrombinase to yield normal thrombin from both PT/delta K1 and PT/delta K2. In the absence of factor Va, the Km(app) for the activation of PT/delta K1, PT/delta K2, or plasma-derived prothrombin by factor Xa-phospholipid-Ca(II) are equivalent. The Km(app) for the activation of PT/delta K2 by prothrombinase is approximately 4-5-fold higher than that obtained for plasma-derived prothrombin or PT/delta K1. These data demonstrate that the prothrombin kringle domains do not contribute significantly to the binding affinity of the substrate-enzyme interaction. In the absence of factor Va, equivalent kcat values were obtained for all of the prothrombin species when they were activated by factor Xa-Ca(II)-phospholipid. In contrast, a 7-fold lower kcat value was obtained for the activation of PT/delta K2 by prothrombinase as compared with that obtained for plasma prothrombin or PT/delta K1. Collectively, these data suggest that determinants within the second prothrombin kringle domain interact with factor Va to elicit a significant acceleration in the catalytic rate of substrate turnover. Indeed, in contrast to plasma-derived prothrombin, no direct binding of PT/delta K2 to factor Va to form the PT/delta K2-factor Va complex could be demonstrated by 90 degrees light scattering.