Role of the antithrombin-binding pentasaccharide in heparin acceleration of antithrombin-proteinase reactions. Resolution of the antithrombin conformational change contribution to heparin rate enhancement

The synthetic antithrombin-binding heparin pentasaccharide and a full-length heparin of approximately 26 saccharides containing this specific sequence have been compared with respect to their interactions with antithrombin and their ability to promote inhibition and substrate reactions of antithrombin with thrombin and factor Xa. The aim of these studies was to elucidate the pentasaccharide contribution to heparin's accelerating effect on antithrombin-proteinase reactions. Pentasaccharide and full-length heparins bound antithrombin with comparable high affinities (KD values of 36 +/- 11 and 10 +/- 3 nM, respectively, at I 0.15) and induced highly similar protein fluorescence, ultraviolet and circular dichroism changes in the inhibitor. Stopped-flow fluorescence kinetic studies of the heparin binding interactions at I 0.15 were consistent with a two-step binding process for both heparins, involving an initial weak encounter complex interaction formed with similar affinities (KD 20-30 microM), followed by an inhibitor conformational change with indistinguishable forward rate constants of 520-700 s-1 but dissimilar reverse rate constants of approximately 1 s-1 for the pentasaccharide and approximately 0.2 s-1 for the full-length heparin. Second order rate constants for antithrombin reactions with thrombin and factor Xa were maximally enhanced by the pentasaccharide only 1.7-fold for thrombin, but a substantial 270-fold for factor Xa, in an ionic strength-independent manner at saturating oligosaccharide. In contrast, the full-length heparin produced large ionic strength-dependent enhancements in second order rate constants for both antithrombin reactions of 4,300-fold for thrombin and 580-fold for factor Xa at I 0.15. These enhancements were resolvable into a nonionic component ascribable to the pentasaccharide and an ionic component responsible for the additional rate increase of the larger heparin. Stoichiometric titrations of thrombin and factor Xa inactivation by antithrombin, as well as sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the products of these reactions, indicated that pentasaccharide and full-length heparins similarly promoted the formation of proteolytically modified inhibitor during the inactivation of factor Xa by antithrombin, whereas only the full-length heparin was effective in promoting this substrate reaction of antithrombin during the reaction with thrombin.(ABSTRACT TRUNCATED AT 400 WORDS)

Saturation mutagenesis of the plasminogen activator inhibitor-1 reactive center

Plasminogen activator inhibitor-1 (PAI-1) is a specific inhibitor of the serine proteases tissue-type plasminogen activator (tPA) and urokinase-type plasminogen activator (uPA). To systematically investigate the roles of the reactive center P1 and P1' residues in PAI-1 function, saturation mutagenesis was utilized to construct a library of PAI-1 variants. Examination of 177 unique recombinant proteins indicated that a basic residue was required at P1 for significant inhibitory activity toward uPA, whereas all substitutions except proline were tolerated at P1'. P1Lys variants exhibited lower inhibition rate constants and greater sensitivity to P1' substitutions than P1Arg variants. Alterations at either P1 or P1' generally had a larger effect on the inhibition of tPA. A number of variants that were relatively specific for either uPA or tPA were identified. P1Lys-P1'Ala reacted 40-fold more rapidly with uPA than tPA, whereas P1Lys-P1'Trp showed a 6.5-fold preference for tPA. P1-P1' variants containing additional mutations near the reactive center demonstrated only minor changes in activity, suggesting that specific amino acids in this region do not contribute significantly to PAI-1 function. These findings have important implications for the role of reactive center residues in determining serine protease inhibitor (serpin) function and target specificity.

Parallel procoagulant and anticoagulant pathways for high molecular weight kininogen coagulant function

High molecular weight kininogen (HK) or its procoagulant light-chain but not the heavy chain potentiated the heparin enhancement of antithrombin III inactivation of plasma kallikrein and factor XIa from 10-50-fold to approximately 1000-fold at I 0.15, pH 7.4, 25 degrees C. This potentiation resulted in antithrombin becoming a predominant inhibitor of kallikrein and factor XIa in heparinized normal but not HK-deficient plasmas. The heparin chain-length and salt dependence of this potentiation suggested an anticoagulant action of HK analogous to its procoagulant action.

Transient kinetics of heparin-catalyzed protease inactivation by antithrombin III. Characterization of assembly, product formation, and heparin dissociation steps in the factor Xa reaction

The kinetics of alpha-factor Xa inhibition by antithrombin III (AT) were studied in the absence and presence of heparin (H) with high affinity for antithrombin by stopped-flow fluorometry at I 0.3, pH 7.4 and 25 degrees C, using the fluorescence probe p-aminobenzamidine (P) and intrinsic protein fluorescence to monitor the reactions. Active site binding of p-aminobenzamidine to factor Xa was characterized by a 200-fold enhancement and 4-nm blue shift of the probe fluorescence emission spectrum (lambda max 372 nm), 29-nm red shift of the excitation spectrum (lambda max 322 nm), and dissociation constant (KD) of about 80 microM. Under pseudo-first order conditions [( AT]0, [H]0, [P]0 much greater than [Xa]0), the observed factor Xa inactivation rate constant (kobs) measured by p-aminobenzamidine displacement or residual enzymatic activity increased linearly with the "effective" antithrombin concentration (i.e. corrected for probe competition) up to 300 microM in the absence of heparin, indicating a simple bimolecular process with a rate constant of 2.1 x 10(3) M-1 s-1. In the presence of heparin, a similar linear dependence of kobs on effective AT.H complex concentration was found up to 25 microM whether the reaction was followed by probe displacement or the quenching of AT.H complex protein fluorescence due to heparin dissociation, consistent with a bimolecular reaction between AT.H complex and free factor Xa with a 300-fold enhanced rate constant of 7 x 10(5) M-1 s-1. Above 25 microM AT.H complex, an increasing dead time displacement of p-aminobenzamidine and a downward deviation of kobs from the initial linear dependence on AT.H complex concentration were found, reflecting the saturation of an intermediate Xa.AT.H complex with a KD of 200 microM and a limiting rate of Xa-AT product complex formation of 140 s-1. Kinetic studies at catalytic heparin concentrations yielded a kcat/Km for factor Xa at saturating antithrombin of 7 x 10(5) M-1 s-1 in agreement with the bimolecular rate constant obtained in single heparin turnover experiments. These results demonstrate that 1) the accelerating effect of heparin on the AT/Xa reaction is at least partly due to heparin promoting the ordered assembly of antithrombin and factor Xa in an intermediate ternary complex and that 2) heparin catalytic turnover is limited by the rate of conversion of the ternary complex intermediate to the product Xa-AT complex with heparin dissociation occurring either concomitant with this step or in a subsequent faster step.

Binding of heparin to human high molecular weight kininogen

The binding of heparin to high molecular weight kininogen (H-kininogen) was analyzed by the effect of kininogen in decreasing the heparin-induced enhancement of the rate of inactivation of thrombin by antithrombin. The conditions were arranged so that the heparin-catalyzed antithrombin-thrombin reaction, monitored in the presence of the reversible thrombin inhibitor p-aminobenzamidine, followed pseudo-first-order kinetics and the observed rate constant (kappa obsd) varied linearly with the heparin concentration. In the absence of metal ions, H-kininogen minimally affected kappa obsd, measured at a constant concentration of heparin with high affinity for antithrombin (30 nM), at I = 0.15, pH 7.4 and 25 degrees C. However, at a saturating concentration of Zn2+ (10 microM), kappa obsd was reduced to 50% at approximately 20 nM H-kininogen and to that of the uncatalyzed reaction at greater than or equal to approximately 0.2 microM H-kininogen. Conversely, at a saturating concentration of H-kininogen (0.5 microM), kappa obsd was decreased to 50% at approximately 0.6 microM Zn2+ and to the kappa obsd of the uncatalyzed reaction at greater than or equal to 10 microM Zn2+. Other metal ions were effective in the order Zn2+ approximately Ni2+ greater than Cu2+ approximately Co2+ approximately Cd2+. The single-chain and two-chain forms of H-kininogen and the H-kininogen light chain reduced the heparin enhancement in the presence of Zn2+ to the same extent, whereas low molecular weight kininogen had no influence.(ABSTRACT TRUNCATED AT 250 WORDS)

Acceleration of surface-dependent autocatalytic activation of blood coagulation factor XII by divalent metal ions

The effect of divalent metal ions on the rate of dextran sulfate dependent autocatalytic activation of human blood coagulation factor XII was studied at pH 7.4 and 25 degrees C. Zn2+ and Cu2+, but not Co2+, increased the rate of factor XII activation induced by dextran sulfate with optimum effects at approximately 5 and 1 microM, respectively, while Ca2+ acceleration required much higher concentrations (millimolar). Further investigation of the effect of Zn2+ on factor XII activation demonstrated a complete dependence on the presence of dextran sulfate, lack of inhibition by soybean trypsin inhibitor, the appearance of alpha-XIIa as the primary reaction product, and reaction kinetics characteristic of an autocatalytic process. These results were consistent with Zn2+ affecting only the rate of surface-mediated factor XII autoactivation. The initial turnover velocity of dextran sulfate induced factor XII autoactivation increased linearly with factor XII concentration in the absence of Zn2+ up to 0.9 microM factor XII but showed saturation behavior over this same concentration range in the presence of 5 microM Zn2+, indicating that Zn2+ increased the reaction rate primarily by lowering the apparent Km. Comparison of the kinetics of autoactivation at mu = 0.15 and 0.24 revealed that the enhancement in the apparent kcat/Km brought about by Zn2+ increased from 19-fold to 520-fold, respectively, due to a differential dependence of the Zn2+-stimulated and unstimulated reactions on ionic strength.(ABSTRACT TRUNCATED AT 250 WORDS)

Transient kinetics of heparin-catalyzed protease inactivation by antithrombin III. The reaction step limiting heparin turnover in thrombin neutralization

The intrinsic protein fluorescence quenching which accompanies the heparin-accelerated inhibition of thrombin (T) by antithrombin III (AT) was resolved into a heparin-independent component associated with formation of the product T-AT complex and a component associated with an AT conformational change linked to heparin dissociation. To determine whether dissociation of heparin from the product T-AT complex limits the rate at which heparin can turn over catalytically, the kinetics of protein fluorescence quenching during the reaction of thrombin with AT X heparin complex (AT X H) were investigated by stopped-flow fluorimetry under pseudo-first order conditions ([AT X H]o much greater than [T]o). Both fluorescence components were quenched in a single exponential reaction with a hyperbolic dependence of the first order rate constant (kobs) on [AT X H]o. An indistinguishable hyperbolic dependence of kobs on [AT X H]o was measured by displacement of p-aminobenzamidine from the T active site, with both signals extrapolating to a limiting rate constant of 5 s-1. These results indicate that heparin dissociation occurs concomitant with T-AT complex formation at the limiting 5 s-1 rate constant. In reasonable agreement with this value, a kcat of 2.3 s-1 was determined for heparin turnover at catalytic concentrations. We conclude that formation of the T-AT complex is the primary rate-limiting step in heparin catalytic turnover and that this reaction is accompanied by a change in conformation of the AT component resulting in facile heparin dissociation.

Protein-protein interactions in contact activation of blood coagulation. Binding of high molecular weight kininogen and the 5-(iodoacetamido) fluorescein-labeled kininogen light chain to prekallikrein, kallikrein, and the separated kallikrein heavy and light chains

Binding of the 5-(iodoacetamido)fluorescein (IAF)-labeled high molecular weight (HMW) kininogen light chain to prekallikrein and D-Phe-Phe-Arg-CH2Cl-inactivated kallikrein was monitored by a 0.040 +/- 0.002 increase in fluorescence anisotropy. Indistinguishable average dissociation constants and stoichiometries of 14 +/- 3 nM and 1.1 +/- 0.1 mol of prekallikrein/mol of IAF-light chain and 17 +/- 3 nM and 0.9 +/- 0.1 mol of kallikrein/mol of IAF-light chain were determined for these interactions at pH 7.4, mu 0.14 and 22 degrees C. Prekallikrein which had been reduced and alkylated in 6 M guanidine HCl lost the ability to increase the fluorescence anisotropy of the IAF-kininogen light chain, suggesting that the native tertiary structure was required for tight binding. The kallikrein heavy and light chains were separated on the basis of the affinity of the heavy chain for HMW-kininogen-Sepharose, after mild reduction and alkylation of kallikrein under nondenaturing conditions. Under these conditions, alkylation with iodo [14C]acetamide demonstrated that only limited chemical modification had occurred. Binding of the IAF-kininogen light chain to the isolated alkylated kallikrein heavy chain, when compared to prekallikrein and kallikrein, was characterized by an indistinguishable increase in fluorescence anisotropy, average dissociation constant of 14 +/- 3 nM, and stoichiometry of 1.2 +/- 0.1 mol of kallikrein heavy chain/mol of IAF-light chain. In contrast, no binding of the D-Phe-Phe-Arg-CH2Cl-inactivated kallikrein light chain was detected at concentrations up to 500 nM. Furthermore, 300 nM kallikrein light chain did not affect IAF-kininogen light chain binding to prekallikrein, kallikrein, or the kallikrein heavy chain. The binding of monomeric single chain HMW-kininogen to prekallikrein, kallikrein, and the kallikrein heavy and light chains was studied using the IAF-kininogen light chain as a probe. Analysis of the competitive binding of HMW-kininogen gave average dissociation constants and stoichiometries of 12 +/- 2 nM and 1.2 +/- 0.1 mol of prekallikrein/mol of HMW-kininogen, 15 +/- 2 nM and 1.3 +/- 0.1 mol of kallikrein/mol of HMW-kininogen, 14 +/- 3 nM and 1.4 +/- 0.2 mol of kallikrein heavy chain/mol of HMW-kininogen, and no detectable effect of 300 nM kallikrein light chain on these interactions. We conclude that a specific, nonenzymatic interaction between sites located exclusively on the light chain of HMW-kininogen and the heavy chain of kallikrein or prekallikrein is responsible for the formation of 1:1 noncovalent complexes between these proteins.

Anion effects on the liver alcohol dehydrogenase reaction

The effects of various anions on the rate constant for dissociation of NADH from a binary complex with horse liver alcohol dehydrogenase were evaluated. Phosphate, sulfate, and fluoride had no effect, while nitrate and the other halide ions caused a three- to fourfold increase in the rate constant for NADH dissociation. These results indicate that a ternary enzyme-NADH-anion complex is formed, and from the anion concentration dependence the relative affinities are iodide greater than nitrate and bromide greater than chloride. At high salt concentrations, above 0.2 M, the rate constants for NADH dissociation decreased, which was attributed to a decrease in the activity coefficient of the reactants or "salting in." The rate constant for NADH dissociation from ternary complex with imidazole, which crystallizes in an orthorhombic form rather than triclinic, was also substantially enhanced by anions. This provides an indication that the enhancement is independent of the conformational state of the enzyme complex. Thus, the most likely explanation for the observed enhancement of NADH dissociation is anion interference with binding of the coenzyme pyrophosphate group, which does not occur with larger anions such as phosphate or sulfate. Since NADH dissociation partially limits the turnover of the enzyme, the effect of nitrate on steady-state turnover was determined. A twofold increase was observed at optimal levels of nitrate, at both substrate inhibitory and noninhibitory concentrations of ethanol.

Protein-protein interactions in contact activation of blood coagulation. Characterization of fluorescein-labeled human high molecular weight kininogen-light chain as a probe

Limited proteolysis of high molecular weight kininogen by kallikrein resulted in the generation of an inactive heavy chain of Mr = 64,000 and active light chains of Mr = 64,000 and 51,000 when analyzed by sodium dodecyl sulfate (SDS)-gel electrophoresis under reducing conditions. Starting with kininogen from outdated plasma, a light chain with an apparent molecular weight of 51,000 on 7.5% SDS gels was purified and characterized. Molecular weights of 28,900 +/- 1,100 and 30,500 +/- 1,600 were obtained by gel filtration of the reduced and alkylated protein in 6 M guanidine HCl and equilibrium sedimentation under nondenaturing conditions in the air-driven ultracentrifuge, respectively. The light chain stained positively with periodic acid-Schiff reagent on SDS gels indicating that covalently attached carbohydrate may be responsible for the anomalously high molecular weight estimated by SDS-gel electrophoresis. A single light chain thiol group reacted with 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) in the presence and absence of 6 M guanidine HCl. Specific fluorescent labeling of the thiol group with 5-(iodoacetamido)fluorescein (IAF) occurred without loss of clotting activity. Addition of purified human plasma prekallikrein to the IAF-light chain resulted in a maximum increase in fluorescence anisotropy of 0.041 +/- 0.001 and no change in the fluorescence intensity. Fluorescence anisotropy measurements of the equilibrium binding of prekallikrein to the IAF-light chain yielded an average Kd of 17.3 +/- 2.5 nM and stoichiometry of 1.07 +/- 0.07 mol of prekallikrein/mol of IAF-light chain. Measurements of the interaction of prekallikrein with iodoacetamide-alkylated light chain using the IAF-light chain as a probe gave an average Kd of 16 +/- 4 nM and stoichiometry of 1.0 +/- 0.2 indicating indistinguishable affinities for prekallikrein.

Transient kinetic studies of substrate inhibition in the horse liver alcohol dehydrogenase reaction

The rate-limiting step of ethanol oxidation by alcohol dehydrogenase (E) at substrate inhibitory conditions (greater than 500 mM ethanol) is shown to be the dissociation rate of NADH from the abortive E-ethanol-NADH complex. The dissociation rate constant of NADH decreased hyperbolically from 5.2 to 1.4 s-1 in the presence of ethanol causing a decrease in the Kd of NADH binding from 0.3 microM for the binary complex to 0.1 microM for the abortive complex. Correspondingly, ethanol binding to E-NADH (Kd = 37 mM) was tighter than to enzyme (Kd = 109 mM). The binding rate of NAD+ (7 X 10(5) M-1s-1) to enzyme was not affected by the presence of ethanol, further substantiating that substrate inhibition is totally due to a decrease in the dissociation rate constant of NADH from the abortive complex. Substrate inhibition was also observed with the coenzyme analog, APAD+, but a single transient was not found to be rate limiting. Nevertheless, the presence of substrate inhibition with APAD+ is ascribed to a decrease in the dissociation rate of APADH from 120 to 22 s-1 for the abortive complex. Studies to discern the additional limiting transient(s) in turnover with APAD+ and NAD+ were unsuccessful but showed that any isomerization of the enzyme-reduced coenzyme-aldehyde complex is not rate limiting. Chloride increases the rate of ethanol oxidation by hyperbolically increasing the dissociation rate constant of NADH from enzyme and the abortive complex to 12 and 2.8 s-1, respectively. The chloride effect is attributed to the binding of chloride to these complexes, destabilizing the binding of NADH while not affecting the binding of ethanol.

p-Aminobenzamidine as a fluorescent probe for the active site of serine proteases

p-Aminobenzamidine is weakly fluorescent in neutral aqueous buffer, with excitation and emission maxima at 293 and 376 nm, respectively. Binding to trypsin results in a blue shift of the emission peak to 362 nm, and 50-fold fluorescence enhancement, while binding to thrombin causes a shift to 368 nm and a 230-fold fluorescence enhancement. This phenomenon is due to hydrophobic interactions, as evidenced by the similar properties observed when p-aminobenzamidine is dissolved in solvents of decreasing polarity. The absorbance spectrum of p-aminobenzamidine is red-shifted by formation of a complex with proteases, with the major difference peak appearing at 317 nm and 323 nm for trypsin and thrombin, respectively. The difference extinction coefficients were 6000 M-1 cm-1 for trypsin complex and 13,300 M-1 cm-1 for thrombin complex at the peak wavelengths. Titration of trypsin and thrombin with the probe indicated one binding site per molecule, with dissociation constants equal to the kinetically determined inhibition constants. The KD values for trypsin and thrombin were 6.1 and 65 microM, respectively. An important potential use of this probe is in studies of inhibitor and substrate binding by rapid reaction kinetic techniques. Using this probe to study the interaction of thrombin with antithrombin III yielded a bimolecular rate constant of 8.0 x 10(3) M-1 s-1, which compares favorably with the value of 8.7 x 10(3) M-1 s-1 obtained from discontinuous assays of the rate of thrombin neutralization.