Conformation of the γ-Carboxyglutamic Acid-Rich Region of Prothrombin: Conformation-Specific Antibodies as Structural Probes

Stabilization of the tertiary structure of bovine prothrombin (PT) by Ca(II) has been evaluated immunochemically using conformation-specific antibodies directed against limited regions of the protein. Antisera raised against native PT or des-γ-carboxyPT (AbPT) were fractionated by sequential immunabsorption using affinity chromatography. Three antibody subpopulations, specific for different determinants on the protein, were isolated and evaluated using RIA: (1) Anti-(12—44), directed against the Ola-rich region 12-44, requires Ca(II) or other metals to interact strongly with PT. Anti-(12-44) cross-reacts poorly with AbPT, factor IX, factor X, and human PT; (2) Anti-PT-Ca(II) is specific for the binary PT-Ca(II) complex and does not interact with PT. Anti-PT-Ca(II) is directed against multiple antigenic determinants in the fragment 1 region; (3) Anti-AbPT is specific for AbPT and does not cross-react with PT. We conclude that occupancy of the high affinity metal binding site by metal ions is associated with a significant change in the conformation of the 12-44 region and other aspects of the N-terminal portion of PT. These antigenic surfaces are not present in AbPT. Conversely, some determinants on AbPT are not observed on PT. These results are consistent with our current model of the role of Gla in metal-dependent intramolecular and intermolecular bridging in the vitarnin-K-dependent proteins.

Interactive computer surface graphics approach to study of the active site of bovine trypsin

A descriptive medium for the presentation of protein structure has been developed and used to evaluate the structure of the active site of bovine trypsin (EC 3.4.21.4). This technique, involving advanced computer graphics technology, permits the facile display of a representation of the molecular surface of proteins of known structure and employs color to code the structural or chemical features of this surface. Benzamidine derivatives were inserted into the benzamidine-binding site of trypsin and the binary inhibitor-trypsin complex was evaluated by using the computer-generated structure. On the basis of qualitative assessments of the contribution of electrostatic and hydrophobic forces to the binding energy associated with complex formation, we made predictions concerning the effects of interaction of benzamidine substituents and amino acid side chains upon the binding energy associated with inhibitor-protein binding. The computer display of the molecular surfaces of the binary complex of substituted benzamidines and trypsin permitted unique insight into the identity and chemical properties of the atoms that participate at the interface of the molecular surfaces of the inhibitor and the protein. The computer-generated molecular surface display can potentially be combined with quantitative definition of the physical forces involved in the interaction of molecular surfaces. This technology should facilitate the study of the structure-activity relationship of substrates, inhibitors, and drugs that bind to proteins of known three-dimensional structure.

Circulating inhibitors of blood coagulation associated with procainamide-induced lupus erythematosus

We studied a patient being treated with procainamide in whom we observed a high antinuclear antibody titer and prolonged activated partial thromboplastin (PTT), prothrombin (PT), and Stypven times (ST). Serum antibody concentrations against single-stranded DNA were elevated while those aginst native DNA were not elevated, suggesting the procainamide-induced lupus syndrome. Dilution of the patient's plasma with normal plasma failed to correct the PTT and PT, indicating the presence of an inhibitor(s) to blood coagulation. The anticoagulant activity was associated with the IgG fraction of the patient's serum. Addition of purified or partially purified human factors IX, X, VIII, VII, XIa, prekallikrein, high molecular weight kininogen, or phospholipids to the patient's plasma failed to correct the PTT, PT, or ST; however, purified human factor XII and prothrombin corrected the PTT and ST, respectively. These results indicate that production of antibodies directed against antigenic determinants on coagulation proteins can be a manifestation of procainamide-induced lupus erythematosus.

Syndrome of acquired factor X deficiency and systemic amyloidosis; in vivo studies of the metabolic fate of factor X

To determine the metabolic fate of factor X in primary amyloidosis associated with factor X deficiency, we examined the pathways of its catabolism in a man with this syndrome. Intravenous infusion of human or bovine 131I-labeled factor X established a triphasic plasma clearance pattern for factor X. About 85 per cent of the factor X disappeared, with a disappearance half-time of less than 30 seconds. A second and third phase showed a T1/2 of 90 minutes and nine hours respectively. 131I-labeles factor X in plasma did not appear to be rapidly modified or degraded. Relatively minor quantities of 131I were cleared into the urine. We observed a diffuse distribution of radioactivity over the body surface, with a concentration in the hepatic and splenic regions. These studies demonstrate than factor X deficiency associated with systemic amyloidosis is due to binding of factor X to body tissue, probably within the circulatory system.

Spectral changes in bovine factor X associated with activation by the venom coagulant protein of Vipera russelli

Bovine Factor X is a zymogen involved in blood coagulation that is converted to activated Factor X in the presence of Ca(II) by the coagulant protein of Russell's viper venom. To monitor structural transitions in Factor X during conversion to activated Factor X, the ultraviolet absorption, fluorescence emission, and circular dichroism spectra of activated Factor X and Factor X were compared. The ultraviolet absorption difference spectrum in the aromatic region comparing activated Factor X and Factor X indicates minima at 292.5, 285, and 278 nm and a small maximum at 305 nm; these differences are due to tryptophan and tyrosine perturbations. The activation of Factor X at 25 degrees in the presence of 8.3 mM CaCl2 with the use of Factor X:venom coagulant protein in molar rations of 1500:1 yielded a time-dependent increase in this spectrum which was linear for about 60 min and which temporally paralleled the development of activated Factor X activity. The binding of Ca(II) to factor X or activated Factor X is associated with a red-shifted tryptophan difference spectrum; however, this perturbation appears to make only a small contribution to the total perturbation observed during Factor X activation. Solvent perturbation studies in 20% glycerol suggest that an average of 3.1 tryptophan residues and 9.0 tyrosine residues are exposed to solvent in Factor X in 8.3 mM CaCl2 at pH 7.4; an additional 0.5 tryptophan residue and tyrosine reside become exposed to solvent during activation of Factor X in 8.3 mM CaCl2. The activation of Factor X by the venom coagulant protein is associated with a small red shift in the intrinsic tryptophan fluorescence emission spectrum. Far- and near-ultraviolet circular dichroism spectroscopy detected no difference between Factor X and activated Factor X. In summary, the activation of Factor X to activated Factor X appears associated with exposure of tryptophan and tyrosine side chains previously buried within the protein and with minimal changes in the secondary structur. These results suggest that conversion of Factor X to activated Factor X involves functionally important, but structurally subtle, changes in the three-dimentional structure.

Substitution of lanthanide ions for calcium ions in the activation of bovine prothrombin by activated factor X. High affinity metal-binding sites of prothrombin and the derivatives of prothrombin activation

The substitution of lanthanide ions for Ca(II) in the Ca(II)-binding sites of prothrombin and the derivatives of prothrombin activation and in the metal-dependent conversion of prothrombin or prethrombin 1 to thrombin was studied at pH 6.8. Gd(III), Tb(III), La(III), Dy(III), Pr(III), Sm(III), and Ce(III) may be substituted for Ca(II) in the generation of thrombin from prothrombin or prethrombin 1 by activated factor X. The rates of thrombin generation in the presence of optimal concentrations of Gd(III) were about 25% for prothrombin and prethrombin 1 compared to the rate of thrombin generation with optimal concentrations of Ca(II). Maximal rates of thrombin generation were observed at 20 muM Gd(III) using prothrombin as substrate, compared to 10 muM Gd(III) when prethrombin 1 was employed. Using the steady state rate-dialysis method, the high affinity metal-binding sites of prothrombin and the products formed during prothrombin activation were characterized using 153Gd(III). Prothrombin has two high affinity binding sites for Gd(III) (Kd = 0.75 muM). Prethrombin 1 and prethrombin 2 each bind one Gd(III) tightly (Kd = 1.10 muM and 0.81 muM, respectively). Fragment 1, the phospholipid-binding portion of prothrombin, has two sites which bind Gd(III) tightly (Kd 0.16 muM). Fragment 2 has no high affinity metal-binding sites, but has intermediate affinity metal-binding sites (Kd greater than 1.6 muM). Thrombin has numerous high affinity binding sites (Kd less than 0.1 muM), suggesting that the conversion of prethrombin 2 to thrombin is associated with a significant change in tertiary structure. These results indicate that Gd(III) binds tightly to the metal-binding sites of these proteins and can substitute for Ca(II) in metal-dependent prothrombin activation. In the activation of prothrombin by activated factor X, these data suggest that Ca(II) is required for metal-dependent factor V and phospholipid binding and not as a cofactor in enzyme catalysis.

Isolation of antibodies specific to sickle hemoglobin by affinity chromatography using a synthetic peptide

Antibodies to hemoglobin have been studied with a radioimmunoassay which employs [14C]carbamylated (= carbamolyated) hemoglobin S. An antiserum raised against hemoglobin S, which initially discriminated poorly between hemoglobins S and A, was fractionated by absorption to a column of Sepharose to which a synthetic peptide corresponding to the first 13 amino-acid residues of the beta chain of sickle hemoglobin had been covalently bound. A subpopulation of the antiserum was eluted from this column with 4 M guanidine - HC1. These antibodies showed binding to hemoglobin S but not to hemoglobin A and this interaction could be inhibited by the synthetic peptide. These antibodies, of demonstrated fine structural specificity, may be useful in the detection of sickle hemoglobin and in the study of its structure in solution.

Application of nuclear magnetic resonance spectroscopy to proteins

The active sites of enzymes can be studied in great detail using nuclear magnetic resonance spectroscopy. The determination of pKa values of active site histidine residues in bovine pancreatic ribonuclease and the characterization of the binding of peptide hormones to carrier proteins are two such examples. The study of the active site of staphylococcal nuclease is another example and is presented in detail in this paper. The structure of 3'5'-thymidine diphosphate bound in the active site of staphylococcal nuclease has been studied by measuring the relaxation rate enhancement of substrate analog nuclei by a paramagnetic metal ion. The lanthanide ion, Gd(III), was substituted for Ca(II) in the formation of the ternary complex of nuclease: Gd(III) : 3'5'-thymidine diphosphate. Measurements were made of the transverse relaxation rates of protons and the longitudinal and transverse relaxation rates of the phosphorus nuclei of bound nucleotide. Internuclear distances between the metal ion and atoms of the 3'5'-thymidine diphosphate nucleotide were determined from these data by using the Solomon-Bloembergen equation. In general, these distances corresponded closely to those determined by previous X-ray crystallography of the thymidine diphosphate complex. These internuclear distances were also used with a computer program and graphics display to solve for metal : nucleotide geometries which were consistent with the experimental data. A geometry similar to the structure of the metal : nucleotide complex bound to nuclease determined by X-ray analysis was one of the solutions to this computer modeling process. For staphylococcal nuclease the NMR and X-ray methods yield compatible high resolution information about the structure of the active site.

Interaction of lanthanide ions with bovine factor X and their use in the affinity chromatography of the venom coagulant protein of Vipera russelli

The substitution of trivalent lanthanide ions for Ca(II) in the Ca(II)-DEPENDENT ACTIVATION OF BOVINE Factor X by the coagulant protein of Russell's viper venom was studied at pH 6.8. Factor X contains two high affinity metal binding sites which bind Gd(III), Sm(III), and Yb(III) with a Kd of about 4 X 10-7 M and four to six lower affinity metal binding sites which bind Gd(III), Sm(III) with a Kd of about 1.5 X 10-5M. In comparison, 1 mol of Factor X binds 2 mol of Ca(II) with a Kd of 3 X 10-4M and weakly binds many additional Ca(II) ions. No binding of Gd(III) to the venom protein was observed. Dy(III), Yb(III), Tb(III), Gd(III), Eu(III), La(III), AND Nd(III) cannot substitute for Ca(II) in the Ca(II)-dependent activation of Factor X by the venom protein at pH 6.8. Kinetic data consistent with the models of competitive inhibition of Ca(II) by Nd(III) yielded a Ki of 1 to 4 X 10-6M. The substitution of lanthanide ions for Ca(II) to promote protein complex formation of Factor X-metal-venom protein without the activation of Factor X facilitated the purification of the coagulant protein from crude venom by affinity chromatography. Using a column containing Factor X covalently bound to agarose which was equilibrated in 10 mM Nd(III), Tb(III), Gd(III), or La(III), the coagulant protein was purified 10-fold in 40% yield from crude venom and migrated as a single band on gel electrophoresis in sodium dodecyl sulfate. These data suggest that lanthanide ions complete with Ca(II) for the metal binding sites of Factor X and facilitate the formation of a nonproductive ternary complex of venom protein-Factor X-metal. Tb(III) fluorescence, with emission maxima at 490 and 545 nm, is enhanced 10,000-fold in the presence of Factor X. The study of the participation of an energy donor intrinsic to Factor X in energy transfer to Tb(III) may be useful in the characterization of the metal binding sites of Factor X.

The active site of staphylococcal nuclease: paramagnetic relaxation of bound nucleotide inhibitor nuclei by lanthanide ions

The structure of 3',5'-thymidine diphosphate bound in the active site of staphylococcal nuclease (EC 3.1.4.7) was studied by measuring the relaxation rate enhancement of substrate analog nuclei by a paramagnetic metal ion. The lanthanide ion, Gd(III), was substituted for Ca(II) in the formation of the ternary complex of nuclease-Gd(III)-3',5'-thymidine diphosphate. Measurements were made of the transverse relaxation rates of protons and the longitudinal and transverse relaxation rates of the phosphorus nuclei of the bound nucleotide. Internuclear distances between the metal ion and atoms of the 3',5'-thymidine diphosphate nucleotide were determined from these data by the Solomon-Bloembergen equation. In general, these distances corresponded closely to those determined by previous x-ray crystallography of the thymidine diphosphate complex.These internuclear distances were also used with a computer program and graphics display to solve for metal-nucleotide geometries, which were consistent with the experimental data. A geometry similar to the structure of the metal-nucleotide complex bound to nuclease determined by x-ray analysis was one of the solutions to this computer modeling process. For staphylococcal nuclease, the nuclear magnetic resonance and x-ray methods yield compatible high resolution information about the structure of the active site. However, differences of uncertain significance exist between the two structures.