Multiple modes of association in bovine prothrombin and its proteolysis products

Intramolecular domain-domain interactions and intermolecular self-association in bovine prothrombin. A potentiometric and laser light-scattering study

The interaction of bovine prothrombin with Ca2+ and Mg2+ ions was investigated by following H+ release as a function of metal ion concentration at pH 6 and pH 7.4 at high and low ionic strength. Prothrombin Ca2+ and Mg2+ binding is characterized by high- and low-affinity sites. M2+ binding at these sites is associated with intramolecular conformational changes and also with intermolecular self-association. The pH dependence of H+ release by M2+ is bell shaped and consistent with controlling pKa values of 4.8 and 6.5. At pH 6 and low ionic strength, both Ca2+ and Mg2+ titrations following H+ release clearly show independent low- and high-affinity binding sites. Laser light scattering reveals that at pH 7.4 and low ionic strength, and at pH 6.0 and high ionic strength, the prothrombin molecular weight is between 73 and 98 kD. At pH 7.4 and high ionic strength, prothrombin is monomeric in the absence of metal ions, but appears to dimerize in the presence of M2+. At pH 6.0 and low ionic strength prothrombin exists as a dimer in the absence of metal ions and is tetrameric in the presence of Ca2+ and remains dimeric in the presence of Mg2+. These results and those for metal ion-dependent H+ release indicate that H+ release occurs concomitantly with association processes involving prothrombin.

Direct enthalpy measurements of factor X and prothrombin association with small and large unilamellar vesicles

Isothermal titration calorimetry was used to determine the enthalpy for the calcium-dependent protein conformation change and subsequent interaction of blood clotting factor X and prothrombin with phospholipid vesicles. The effect of vesicle size was also determined. The protein conformation change was accompanied by -12 +/- 1 and -7 to -15 kcal/mol for factor X and prothrombin, respectively. The range of values for prothrombin arose from use of different protein preparations and may be due to non-ideal behavior of this protein when calcium was added. The apparent enthalpy of association (delta H(assoc)) of both factor X and prothrombin with phosphatidylserine (PS)/phosphatidylcholine (PC) large unilamellar vesicles (LUVs, 120 nm diameter) was shown to be near 0 kcal/mol. In comparison, delta H(assoc) for interaction with PS/PC small unilamellar vesicles (SUVs, 40 nm diameter) was -9 +/- 3 and -7 +/- 2 kcal/mol for factor X and prothrombin, respectively. This difference appeared complementary to delta H(assoc) for calcium binding to these vesicles. That is, the interaction of calcium was athermic with SUVs and exothermic with LUVs. While such properties might suggest a considerable difference in the manner of calcium binding to LUVs versus SUVs, little difference in the quantity of calcium bound to SUVs and LUVs was detected by equilibrium dialysis. In any event, the results indicate that protein binding to LUVs was primarily entropy driven whereas binding to SUVs was primarily enthalpy driven. The exothermic process for calcium-dependent factor X or prothrombin binding to SUVs may result from protein-induced changes in the phospholipid packing/calcium interaction, possibly related to changes in how calcium is bound to the phospholipid.

Ca(2+)-dependent interactions between Gla and EGF domains in human coagulation factor IX

The Ca(2+)-induced interaction between the Gla and EGF domains of human factor IX was investigated by means of three fragments: 6-kDa Gla, 19-kDa (EGF)2, and 25-kDa Gla-(EGF)2. Size-exclusion chromatography and spectroscopic measurements revealed that the Gla-EGF interaction is rather strong; it can be reconstituted by mixing the 6-kDa and 19-kDa fragments which form a stable 1:1 heterocomplex in the presence of Ca2+. By itself, the 6-kDa Gla self-associates in these conditions. The Gla-EGF interaction can be disrupted in 5 M urea where the compact structure of both domains is preserved. Binding of Ca2+ to 19-kDa (EGF)2 occurred with a Kd of 71 microM in the absence and 108 microM in the presence of 5 M urea and stabilized the first EGF domain, increasing its Tm by 12 degrees C. Addition of Ca2+ to the 6-kDa and 25-kDa fragments produced biphasic changes in their fluorescence; the intensity increased slightly at low Ca2+ concentration and then decreased in a monotonic manner. In 5 M urea, only the decrease occurred, with apparent Kds of 0.33 and 0.30 mM for 6-kDa Gla and 25-kDa Gla-(EGF)2, respectively. Thus, in 5 M urea in the presence of Ca2+, the isolated Gla domain has a compact structure and Ca2+ binding properties similar to those in the 25-kDa fragment. In the absence of urea, the Gla domain interacts either with itself, when isolated, or with the first EGF domain when present, as in the 6-kDa/19-kDa heterocomplex, in the 25-kDa fragment and presumably intact factor IX.

Localization of the murine Hmg1 gene, encoding an HMG-box protein, to mouse chromosome 2

In conclusion, using concatenated AhRE sequences and the recognition site probe methodology, we have cloned the murine Hmg1 cDNA and determined an additional 141 bp of 5' noncoding sequence (GenBank Accession No. S50213; entry name MUSHMG1A). The gene product represents an HMG-box transcription factor that recognizes DNA shape- and sequence-specific elements; this is perhaps the reason that this cDNA was isolated with concatomeric oligonucleotides. We have mapped the Hmg1 gene to mouse Chr 2, between regions homologous with human Chr 2q and 11p11-q12.

Preferential ligand binding to multi-state acceptor systems: comparisons of the calcium-binding and dimerization characteristics of prothrombin and fragment 1

Consideration is given to the interactions of a ligand with self-associating acceptor systems for which preferential binding is an ambiguous term in that ligand-mediated self-association does not necessarily imply a greater binding constant for polymeric acceptor--even in instances where binding sites are preserved in the self-association process. This dilemma is shown to arise in situations involving the binding of ligand to monomeric and polymeric forms of an acceptor that also coexist in equilibrium with inactive isomeric states. For example, the ten-fold increase in the measured dimerization constant for prothrombin Fragment 1 in the presence of a saturating concentration of Ca2+ ion may well reflect the existence of a 12% greater binding constant for the interaction of metal ion with dimeric acceptor. However, that result, as well as the detailed form of the sigmoidal binding curve, are also reasonably described by another extreme model in which the monomeric and dimeric forms of the acceptor possess equal affinities for Ca2+ ion. Likewise, the fact that the same experimental dimerization constant applies to prothrombin and its Ca(2+)-saturated complex does not preclude the possibility that the active form of dimeric zymogen exhibits a 12% greater affinity for metal ion. Numerical simulations have established that characterization of the dimerization behaviour as a function of free ligand concentration should allow greater discrimination between such models of the interplay between calcium binding and self-association of prothrombin and Fragment 1. Finally, by illustrating the likelihood that the disparity in self-association behaviour of prothrombin and Fragment 1 merely reflects minor differences in the relative magnitudes of isomerization constants and/or binding constants for monomeric and dimeric states of the two acceptors, the present investigation serves to allay concern about the validity of employing the proteolytic fragment as a model of the intact zymogen.

Amino-terminal alanine functions in a calcium-specific process essential for membrane binding by prothrombin fragment 1

Two acetylation sites on prothrombin fragment 1 (amino-terminal 156 amino acid residues of bovine prothrombin) are essential for the tight calcium and membrane binding functions of the protein; calcium protects both of these sites from acetylation [Welsch, D. J., Pletcher, C. H., & Nelsestuen, G. L. (1988) Biochemistry (first of three papers in this issue)]. The epsilon-amino groups of the lysine residues (positions 3, 11, 44, 57, and 97) were not critical to protein function and were acetylated in the calcium-protected protein. The most reactive of the two essential acetylation sites was identified as amino-terminal alanine. To identify this site, fragment 1 was first acetylated in the presence of calcium to derivatize the nonessential sites. Removal of calcium and partial acetylation with radioactive reagent produced a single major radioactive peptide. Isolation and characterization of this peptide showed that the radioactivity was associated with amino-terminal alanine. In addition, sequence analysis of calcium-protected protein showed the presence of underivatized amino-terminal alanine. Surprisingly, covalent modification with a trinitrophenyl group did not alter membrane binding activity. Thus, the positive charge on the amino terminus did not appear critical to its function. Acetylation of amino-terminal alanine without acetylation of the second essential site produced a fragment 1 derivative which had a high requirement for calcium and which had lost most membrane binding function. However, this protein had only slightly altered affinity for magnesium ion. In agreement with this metal ion selectivity, protection of amino-terminal alanine was calcium specific, and magnesium ion did not protect this site from acetylation.(ABSTRACT TRUNCATED AT 250 WORDS)

Structure and order of the protein and carbohydrate domains of prothrombin fragment 1

The three-dimensional structure of prothrombin fragment 1 has been determined by X-ray crystallography at 3.8 A resolution. The fragment is composed of a number of structural units, some of which are ordered while others are disordered. The ordered part of the structure includes a compact kringle unit, a helical domain and a carbohydrate chain. The kringle structure is organized around a close pair of buried disulfide bridges. One of its carbohydrate chains, that attached to Asn 101, is fully ordered, but the carbohydrate chain attached to Asn 77 appears to be disordered. The calcium binding unit is composed of a disordered part containing all ten gamma-carboxyglutamic acid residues and an ordered part forming the helical domain. The highly conserved residues Phe 41, Trp 42 and Tyr 45, which form a hydrophobic cluster on the first helix, interact around a crystallographic two-fold axis with the equivalent residues in another molecule to form a dimer in the crystal.

Homo- and heterodimer formation with prothrombin and prothrombin fragment 1 in the presence of calcium ions

The purpose of the current study is to present further evidence for prothrombin self-association as assessed by chemical crosslinking. When the self-association (evaluated by covalent crosslinking with dithiobis(succinimidylpropionate) of prothrombin or fragment 1 was evaluated at the same molar concentration of protein, similar rates of dimer formation were observed for either protein. When prothrombin and fragment 1 were incubated together with the crosslinking reagent and calcium ions, a heterodimer consisting of prothrombin and fragment 1 was observed in addition to prothrombin dimer and fragment 1 dimer. Similar experiments with prethrombin 1 showed neither significant self-association nor effect on prothrombin self-association. Comparison of the formation of prothrombin fragment 1 heterodimer formation with the effect of fragment 1 on prothrombin activation by factor Xa suggests that the anticoagulant activity of fragment 1 is not solely a result of the formation of a heterodimer between prothrombin and fragment 1.

Properties of chemically modified protein S: effect of the conversion of gamma-carboxyglutamic acid to gamma-methyleneglutamic acid on functional properties

Protein S, the protein cofactor for activated protein C in the proteolytic inactivation of factor Va, was chemically modified with a mixture of morpholine and formaldehyde. This treatment resulted in the conversion of the gamma-carboxyglutamic acid (Gla) residues of this vitamin K dependent protein to gamma-methyleneglutamic acid. With a 10,000-fold molar excess of morpholine and formaldehyde over protein S it was found that between 10 and 11 Gla residues could be modified. The degree of modification was proportional to the concentration of the modifying reagents used. The modification of as few as two residues resulted in the 70% loss of activity. Calcium inhibited the modification of several residues. In the presence of 3.2 mM calcium ion, a derivative with 2.5 residues modified was prepared that appeared to have full activity. Modification of protein S resulted in the alteration of a number of its properties. The quenching of intrinsic fluorescence by calcium decreased. The quenching effect of terbium ions was also decreased. However, the modified protein and the native protein were equivalent when protein-dependent terbium fluorescence was measured. When modified, protein S would no longer bind to phospholipid vesicles. Finally, the ability of protein S to self-associate was decreased by modification. These findings suggest that the gamma-carboxyglutamic acid residues of protein S may play several roles in the maintenance of structure.

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.

Origin of a fluorescence increase accompanying the limited proteolysis of fluorescein-labeled human prothrombin by Factor Xa

In a search for a probe which would report its proteolysis to thrombin, the human blood coagulation zymogen prothrombin was covalently labeled with fluorescein. Fluorescein isothiocyanate (FITC) and dichlorotriazinylaminofluorescein (DCTAF) both introduced approximately 1 molecule of dye, but labeling occurred at different locations, as FITC had no effect on clotting activity whereas DCTAF caused 95% inactivation. At pH 9.0 DCTAF, but not FITC, could induce labeling up to 4 mol/mol. All derivatives were activated normally by prothrombinase (the activating complex of Factor Xa, Factor V(a), Ca2+ and phospholipids), as indicated by the pattern of bands on SDS gel electrophoresis and an unaltered yield of activity toward a chromogenic substrate for thrombin. Upon undergoing this limited proteolysis, the most heavily labeled derivative showed a 40% increase in fluorescence of the fluorescein at 520 nm (lambda ex 480 nm). In contrast, the fluorescence of lightly labeled forms was more intense but increased by only 0-5% upon activation. The data suggest that the lower fluorescence of the most labeled form is due to an intramolecular quenching effect between the dye molecules on individual polypeptide chains that is partly relieved when activation occurs.

Calcium effects on prothrombin and its reaction with bifunctional alkylating reagents

Monodisperse bovine prothrombin was prepared and its molecular states under several conditions examined. The protein showed no tendency to self-associate in the absence of calcium. Calcium (4 mM) caused small increases in the apparent molecular weight of the protein which may or may not represent protein dimerization with very low affinity. The allowed conclusion was that calcium-induced prothrombin dimerization is minimal up to protein concentrations of many mg/ml. Calcium-induced protein shape changes did not measurably alter the protein diffusion constant. A bifunctional alkylating reagent did produce extensive calcium-dependent prothrombin crosslinking. Prothrombin dimers formed by the crosslinking agent were not a measure of the state of native prothrombin.

Gamma-carboxyglutamic acid

Gamma-carboxyglutamic acid is an amino acid with a dicarboxylic acid side chain. This amino acid, with unique metal binding properties, confers metal binding character to the proteins into which it is incorporated. This amino acid has been discovered in blood coagulation proteins (prothrombin, Factor X, Factor IX, and Factor VII), plasma proteins of unknown function (Protein C, Protein S, and Protein Z), and proteins from calcified tissue (osteocalcin and bone-Gla protein). It has also been observed in renal calculi, atherosclerotic plaque, and the egg chorioallantoic membrane, among other tissues. Gamma-carboxyglutamic acid is synthesized by the post-translational modification of glutamic acid residues. This reaction, catalyzed by a hepatic carboxylase, requires reduced vitamin K, oxygen, and carbon dioxide. The function of gamma-carboxyglutamic acid is uncertain. In prothrombin gamma-carboxyglutamic acid residues bound to metal ions participate as an intramolecular non-covalent bridge to maintain protein conformation. Additionally, these amino acids participate in the calcium-dependent molecular assembly of proteins on membrane surfaces through intermolecular bridges involving gamma-carboxyglutamic acid and metal ions.

Calcium ion-protein interactions in prothrombin activation

1. The protein concentration dependence observed in the calcium binding to fragment 1 indicates that calcium-mediated dimerization is responsible for the cooperative calcium binding behavior usually observed. "Unusual" fragment 1, which exhibits negative cooperativity (the type of binding behavior expected for ions interacting with a charged protein) at high concentration, also exhibit altered self-association behavior. 2. The calcium-induced spectral perturbations that are observed by fluorescence and ultraviolet difference spectroscopy are influenced by calcium-mediated dimerization. Similar spectral perturbations may also be induced by other divalent, trivalent, and monovalent ions, as well as changes in pH. Because this is a multi-site system, only limited interpretation of the spectral data is possible without calcium binding data. 3. Although strong side chain CD signals make estimation of fragment 1 secondary structure ambiguous, the CD data do indicate small changes in structure during calcium binding. Similar changes are observed upon addition of monovalent ions at high concentration or after lowering the pH. No coupling between changes in conformation and the cooperative calcium binding behavior has yet been observed to exist.

Prothrombin--membrane interaction. Effects of ionic strength, pH, and temperature

The effects of ionic strength, pH, and temperature on three separate aspects of prothrombin-phospholipid membrane binding were studied. The three parameters include a calcium-dependent protein transition, a calcium-membrane interation, and, finally, the binding of calcium-saturated protein to a calcium-saturated phospholipid membrane. The results are consistent with calcium binding to carbonyl groups in the protein and to phosphate in the phospholipids. These interactions show the expected pH profiles and sensitivity to ionic strength. Temperature effects indicate a small negative enthalpy change for each process. The binding of calcium-saturated protein to calcium-saturated membrane shows very little variation between pH 6 and pH 9, is accompanied by no detected enthalpy change, and is relatively insensitive to ionic strength. These latter results indicate that ionic calcium bridging between the protein and membrane is not important. A chelation model for prothrombin-membrane binding is proposed where the two interacting species have no net charge; ligands on the protein complete the coordination sphere of membrane-bound calcium and vice versa.

Zymogens and cofactors of blood coagulation

Blood coagulation is a system in which a series of zymogens of serine proteases are sequentially activated. In this regard, there is little fundamental difference between coagulation and the activation of the homologous pancreatic zymogens. There are, however, several aspects unique to coagulation which are discussed in detail. These are (1) the requirement for a high-molecular-weight protein or lipoprotein cofactor for optimal reaction rates, (2) the requirement for membranes or a membrane-like surface which further distinguishes this system; (3) a metal ion requirement for most reactions (in contrast to the pancreatic serine proteases) relating to the content of the newly described amino acid gamma-carboxyglutamic acid in the four vitamin K-dependent proteins, regarding which recent data relating to the metal binding sites on prothrombin are discussed in detail; and (4) the uniqueness of the initiating reactions in comparison to those which activate the pancreatic zymogens, insofar as no enzyme corresponding to enterokinase has been identified. The implications of this phenomenon are analyzed with particular attention to the potential role of the endogenous activity of certain zymogens in initiating coagulation. The article deals finally with the specific problems attendant on analyzing a system in which many serine proteases lacking absolute specificity are generated and regulated.