The existence of independent domain structures in human Lys77-plasminogen

Solution structural model of the complex of the binding regions of human plasminogen with its M-protein receptor from Streptococcus pyogenes

VEK50 is a truncated peptide from a Streptococcal pyogenes surface human plasminogen (hPg) binding M-protein (PAM). VEK50 contains the full A-domain of PAM, which is responsible for its low nanomolar binding to hPg. The interaction of VEK50 with kringle 2, the PAM-binding domain in hPg (K2hPg), has been studied by high-resolution NMR spectroscopy. The data show that each VEK50 monomer in solution contains two tight binding sites for K2hPg, one each in the a1- (RH1; R17H18) and a2- (RH2; R30H31) repeats within the A-domain of VEK50. Two mutant forms of VEK50, viz., VEK50[RH1/AA] (VEK50ΔRH1) and VEK50[RH2/AA] (VEK50ΔRH2), were designed by replacing each RH with AA, thus eliminating one of the K2hPg binding sites within VEK50, and allowing separate study of each binding site. Using 13C- and 15N-labeled peptides, NMR-derived solution structures of VEK50 in its complex with K2hPg were solved. We conclude that the A-domain of PAM can accommodate two molecules of K2hPg docked within a short distance of each other, and the strength of the binding is slightly different for each site. The solution structure of the VEK50/K2hPg, complex, which is a reductionist model of the PAM/hPg complex, provides insights for the binding mechanism of PAM to a host protein, a process that is critical to S. pyogenes virulence.

Structure, function, and genetics of lipoprotein (a)

Lipoprotein (a) [Lp(a)] has attracted the interest of researchers and physicians due to its intriguing properties, including an intragenic multiallelic copy number variation in the LPA gene and the strong association with coronary heart disease (CHD). This review summarizes present knowledge of the structure, function, and genetics of Lp(a) with emphasis on the molecular and population genetics of the Lp(a)/LPA trait, as well as aspects of genetic epidemiology. It highlights the role of genetics in establishing Lp(a) as a risk factor for CHD, but also discusses uncertainties, controversies, and lack of knowledge on several aspects of the genetic Lp(a) trait, not least its function.

A high affinity interaction of plasminogen with fibrin is not essential for efficient activation by tissue-type plasminogen activator

Fibrin (Fn) enhances plasminogen (Pg) activation by tissue-type plasminogen activator (tPA) by serving as a template onto which Pg and tPA assemble. To explore the contribution of the Pg/Fn interaction to Fn cofactor activity, Pg variants were generated and their affinities for Fn were determined using surface plasmon resonance (SPR). Glu-Pg, Lys-Pg (des(1-77)), and Mini-Pg (lacking kringles 1-4) bound Fn with K(d) values of 3.1, 0.21, and 24.5 μm, respectively, whereas Micro-Pg (lacking all kringles) did not bind. The kinetics of activation of the Pg variants by tPA were then examined in the absence or presence of Fn. Whereas Fn had no effect on Micro-Pg activation, the catalytic efficiencies of Glu-Pg, Lys-Pg, and Mini-Pg activation in the presence of Fn were 300- to 600-fold higher than in its absence. The retention of Fn cofactor activity with Mini-Pg, which has low affinity for Fn, suggests that Mini-Pg binds the tPA-Fn complex more tightly than tPA alone. To explore this possibility, SPR was used to examine the interaction of Mini-Pg with Fn in the absence or presence of tPA. There was 50% more Mini-Pg binding to Fn in the presence of tPA than in its absence, suggesting that formation of the tPA-Fn complex exposes a cryptic site that binds Mini-Pg. Thus, our data (a) indicate that high affinity binding of Pg to Fn is not essential for Fn cofactor activity, and (b) suggest that kringle 5 localizes and stabilizes Pg within the tPA-Fn complex and contributes to its efficient activation.

Functional hierarchy of plasminogen kringles 1 and 4 in fibrinolysis and plasmin-induced cell detachment and apoptosis

Plasmin(ogen) kringles 1 and 4 are involved in anchorage of plasmin(ogen) to fibrin and cells, an essential step in fibrinolysis and pericellular proteolysis. Their contribution to these processes was investigated by selective neutralization of their lysine-binding function. Blocking the kringle 1 lysine-binding site with monoclonal antibody 34D3 fully abolished binding and activation of Glu-plasminogen and prevented both fibrinolysis and plasmin-induced cell detachment-induced apoptosis. In contrast, blocking the kringle 4 lysine-binding site with monoclonal antibody A10.2 did not impair its activation although it partially inhibited plasmin(ogen) binding, fibrinolysis and cell detachment. This remarkable, biologically relevant, distinctive response was not observed for plasmin or Lys-plasminogen; each antibody inhibited their binding and activation of Lys-plasminogen to a limited extent, and full inhibition of fibrinolysis required simultaneous neutralization of both kringles. Thus, in Lys-plasminogen and plasmin, kringles 1 and 4 act as independent and complementary domains, both able to support binding and activation. We conclude that Glu-/Lys-plasminogen and plasmin conformations are associated with transitions in the lysine-binding function of kringles 1 and 4 that modulate fibrinolysis and pericellular proteolysis and may be of biological relevance during athero-thrombosis and inflammatory states. These findings constitute the first biological link between plasmin(ogen) transitions and functions.

The kringle domain of tissue-type plasminogen activator inhibits in vivo tumor growth

The two-kringle domain of tissue-type plasminogen activator (t-PA) has previously been shown to contain anti-angiogenesis activity. In this study, we explored the potential in vivo anti-tumor effects of the recombinant kringle domain (TK1-2) of human t-PA. Anti-tumor effects of purified Pichia-driven TK1-2 were examined in nude mice models by subcutaneous implantation of human lung (A-549) and colon (DLD-1, HCT-116) cancer cell lines. Mice bearing the tumors were injected with PBS or purified TK1-2 (30 mg/kg) i.p. every day for 22 days. TK1-2 treatment suppressed the A-549, DLD-1, and HCT-116 tumor growth by 85.3%, 52.4%, and 62.5%, respectively. Immunohistological examination of the tumor tissues showed that TK1-2 treatment decreased the vessel density and also the expression of angiogenesis-related factors including angiogenin, VEGF, alpha-SMA, vWF, and TNF-alpha, and increased the apoptotic fraction of cells. TK1-2 neither inhibited in vitro growth of these cancer cells nor affected t-PA-mediated fibrin clot lysis. These results suggest that TK1-2 inhibits the tumor growth by suppression of angiogenesis without interfering with fibrinolysis.

Inhibition of endothelial cell proliferation by the recombinant kringle domain of tissue-type plasminogen activator

Tissue-type plasminogen activator (tPA) is a multidomain serine protease that converts the zymogen plasminogen to plasmin. tPA contains two kringle domains which display considerable sequence identity with those of angiostatin, an angiogenesis inhibitor. TK1-2, a recombinant kringle domain composed of t-PA kringles 1 and 2 (Ala(90)-Thr(263)), was produced by both bacterial and yeast expression systems. In vitro, TK1-2 inhibited endothelial cell proliferation stimulated by basic fibroblast growth factor, vascular endothelial growth factor, and epidermal growth factor. It did not inhibit proliferation of non-endothelial cells. TK1-2 also inhibited in vivo angiogenesis in the chick embryo chorioallantoic membrane model. These results suggest that the recombinant kringle domain of t-PA is a selective inhibitor of endothelial cell growth and identifies this molecule as a novel anti-angiogenic agent.

The two-domain NK1 fragment of plasminogen: folding, ligand binding, and thermal stability profile

The two-domain fragment N+K1 (rNK1) [Glu(1)-Glu(163)] of human plasminogen was expressed in E. coli as a hexahistidine-tagged fusion protein and chromatographically purified. The (1)H NMR spectrum supports proper folding of the K1 component within the refolded rNK1 construct (rNK1/K1). The functional properties of rNK1/K1 were investigated via intrinsic fluorescence titration with kringle-specific omega-aminocarboxylic acid ligands. The affinities closely match those previously measured for the isolated K1, which indicates that the N-domain does not significantly affect the interaction of ligands with the lysine binding site of K1. Far-UV CD spectra recorded for the N-domain suggest conformational plasticity and flexibility for the module. Two classes of spectra, referred to as types A and B, were identified with the type A spectrum reflecting a higher secondary structure content than that estimated for the type B spectrum. Subtracting the CD spectrum of rK1 from that of rNK1 yields a spectrum (Delta) which reflects the conformation of the N-domain within the rNK1 construct (rNK1/N). Delta resembles the type A spectrum, suggesting that rNK1/N adopts a relatively more ordered conformation, stabilized by the adjacent rNK1/K1 domain. In contrast, thermal unfolding curves determined via CD indicate that the rNK1/N slightly lowers the melting temperature (T(m)) of rNK1/K1. Independence of the two domains within rNK1 was tested by monitoring the thermal unfolding of rNK1/K1 when in the presence of the kringle-specific ligand AMCHA, which left the rNK1/N T(m) essentially unaffected, while increasing that of the rNK1/K1 by approximately 10 degrees C.

Domain interactions between streptokinase and human plasminogen

Plasmin (Pm), the main fibrinolytic protease in the plasma, is derived from its zymogen plasminogen (Plg) by cleavage of a peptide bond at Arg(561)-Val(562). Streptokinase (SK), a widely used thrombolytic agent, is an efficient activator of human Plg. Both are multiple-domain proteins that form a tight 1:1 complex. The Plg moiety gains catalytic activity, without peptide bond cleavage, allowing the complex to activate other Plg molecules to Pm by conventional proteolysis. We report here studies on the interactions between individual domains of the two proteins and their roles in Plg activation. Individually, all three SK domains activated native Plg. While the SK alpha domain was the most active, its activity was uniquely dependent on the presence of Pm. The SK gamma domain also induced the formation of an active site in Plg(R561A), a mutant that resists proteolytic activation. The alpha and gamma domains together yielded synergistic activity, both in Plg activation and in Plg(R561A) active site formation. However, the synergistic activity of the latter was dependent on the correct N-terminal isoleucine in the alpha domain. Binding studies using surface plasmon resonance indicated that all three domains of SK interact with the Plg catalytic domain and that the beta domain additionally interacts with Plg kringle 5. These results suggest mechanistic steps in SK-mediated Plg activation. In the case of free Plg, complex formation is initiated by the rapid and obligatory interaction between the SK beta domain and Plg kringle 5. After binding of all SK domains to the catalytic domain of Plg, the SK alpha and gamma domains cooperatively induce the formation of an active site within the Plg moiety of the activator complex. Substrate Plg is then recognized by the activator complex through interactions predominately mediated by the SK alpha domain.

Streptokinase binds preferentially to the extended conformation of plasminogen through lysine binding site and catalytic domain interactions

Binding of streptokinase (SK) to plasminogen (Pg) activates the zymogen conformationally and initiates its conversion into the fibrinolytic proteinase, plasmin (Pm). Equilibrium binding studies of SK interactions with a homologous series of catalytic site-labeled fluorescent Pg and Pm analogues were performed to resolve the contributions of lysine binding site interactions, associated changes between extended and compact conformations of Pg, and activation of the proteinase domain to the affinity for SK. SK bound to fluorescein-labeled [Glu]Pg(1) and [Lys]Pg(1) with dissociation constants of 624 +/- 112 and 38 +/- 5 nM, respectively, whereas labeled [Lys]Pm(1) bound with a 57000-fold tighter dissociation constant of 11 +/- 2 pM. Saturation of lysine binding sites with 6-aminohexanoic acid had no effect on SK binding to labeled [Glu]Pg(1), but weakened binding to labeled [Lys]Pg(1) and [Lys]Pm(1) 31- and 20-fold, respectively. At low Cl(-) concentrations, where [Glu]Pg assumes the extended conformation without occupation of lysine binding sites, a 23-fold increase in the affinity of SK for labeled [Glu]Pg(1) was observed, which was quantitatively accounted for by expression of new lysine binding site interactions. The results support the conclusion that the SK affinity for the fluorescent Pg and Pm analogues is enhanced 13-16-fold by conversion of labeled [Glu]Pg to the extended conformation of the [Lys]Pg derivative as a result of lysine binding site interactions, and is enhanced 3100-3500-fold further by the increased affinity of SK for the activated proteinase domain. The results imply that binding of SK to [Glu]Pg results in transition of [Glu]Pg to an extended conformation in an early event in the SK activation mechanism.

Epsilon amino caproic acid inhibits streptokinase-plasminogen activator complex formation and substrate binding through kringle-dependent mechanisms

Lysine side chains induce conformational changes in plasminogen (Pg) that regulate the process of fibrinolysis or blood clot dissolution. A lysine side-chain mimic, epsilon amino caproic acid (EACA), enhances the activation of Pg by urinary-type and tissue-type Pg activators but inhibits Pg activation induced by streptokinase (SK). Our studies of the mechanism of this inhibition revealed that EACA (IC(50) 10 microM) also potently blocked amidolytic activity by SK and Pg at doses nearly 10000-fold lower than that required to inhibit the amidolytic activity of plasmin. Different Pg fragments were used to assess the role of the kringles in mediating the inhibitory effects of EACA: mini-Pg which lacks kringles 1-4 of Glu-Pg and micro-Pg which lacks all kringles and contains only the catalytic domain. SK bound with similar affinities to Glu-Pg (K(A) = 2.3 x 10(9) M(-1)) and to mini-Pg (K(A) = 3.8 x 10(9) M(-)(1)) but with significantly lower affinity to micro-Pg (K(A) = 6 x 10(7) M(-)(1)). EACA potently inhibited the binding of Glu-Pg to SK (K(i) = 5.7 microM), but was less potent (K(i) = 81.1 microM) for inhibiting the binding of mini-Pg to SK and had no significant inhibitory effects on the binding of micro-Pg and SK. In assays simulating substrate binding, EACA also potently inhibited the binding of Glu-Pg to the SK-Glu-Pg activator complex, but had negligible effects on micro-Pg binding. Taken together, these studies indicate that EACA inhibits Pg activation by blocking activator complex formation and substrate binding, through a kringle-dependent mechanism. Thus, in addition to interactions between SK and the protease domain, interactions between SK and the kringle domain(s) play a key role in Pg activation.

Characterization of kringle domains of angiostatin as antagonists of endothelial cell migration, an important process in angiogenesis

Angiogenesis is a complex process that involves endothelial cell proliferation, migration, basement membrane degradation, and neovessel organization. Angiostatin, consisting of four homologous triple-disulfide bridged kringle domains, has previously been shown to exhibit profound inhibition of endothelial cell proliferation in vitro and angiogenesis in vivo. It was also demonstrated that angiostatin could suppress the growth of a variety of tumors via the blocking of angiogenesis. The primary aim of our study was to characterize the kringle domains of angiostatin for their inhibitory activities of endothelial cell migration in order to elucidate their contributions to the anti-angiogenic function of angiostatin. In this report, we demonstrate for the first time that the kringles of angiostatin play different roles in inhibiting endothelial cell migration, a crucial process in angiogenesis. Kringle 4, which has only marginal anti-proliferative activity, is among the most potent fragments in inhibiting endothelial cell migration (IC50 of approximately 500 nM). In contrast, kringle 1-3, which is equivalent to angiostatin in inhibiting endothelial cell proliferation, manifests only a modest anti-migratory effect. The combination of kringle 1-3 and kringle 4 results in an anti-migratory activity comparable to that of angiostatin. When kringle 1 is removed from kringle 1-3, the resulting kringle 2-3 becomes more potent than kringle 1-3. This implies that kringle 1, although virtually ineffective in inhibiting endothelial cell migration, may influence the conformation of kringle 1-3 to alter its anti-migratory activity. We also show that disruption of the kringle structure by reducing/alkylating agents markedly attenuates the anti-migratory activity of angiostatin, demonstrating the significance of kringle conformation in maintaining the anti-angiogenic activity of angiostatin. Our data suggest that different kringle domains may contribute to the overall anti-angiogenic function of angiostatin by their distinct anti-migratory activities.

Structure and ligand binding determinants of the recombinant kringle 5 domain of human plasminogen

The X-ray crystal structure of the recombinant (r) kringle 5 domain of human plasminogen (K5HPg) has been solved by molecular replacement methods using K1HPg as a model and refined at 1.7 A resolution to an R factor of 16.6%. The asymmetric unit of K5HPg is composed of two molecules related by a noncrystallographic 2-fold rotation axis approximately parallel to the z-direction. The lysine binding site (LBS) is defined by the regions His33-Thr37, Pro54-Val58, Pro61-Tyr64, and Leu71-Tyr74 and is occupied in the apo-form by water molecules. A unique feature of the LBS of apo-K5HPg is the substitution by Leu71 for the basic amino acid, arginine, that in other kringle polypeptides forms the donor cationic center for the carboxylate group of omega-amino acid ligands. While wild-type (wt) r-K5HPg interacted weakly with these types of ligands, replacement by site-directed mutagenesis of Leu71 by arginine led to substantially increased affinity of the ligands for the LBS of K5HPg. As a result, binding of omega-amino acids to this mutant kringle (r-K5HPg[L71R]) was restored to levels displayed by the companion much stronger affinity HPg kringles, K1HPg and K4HPg. Correspondingly, alkylamine binding to r-K5HPg[L71R] was considerably attenuated from that shown by wtr-K5HPg. Thus, employing a rational design strategy based on the crystal structure of K5HPg, successful remodeling of the LBS has been accomplished, and has resulted in the conversion of a weak ligand binding kringle to one that possesses an affinity for omega-amino acids that is similar to K1HPg and K4HPg.

The importance of the hydrophobic components of the binding energies in the interaction of omega-amino acid ligands with isolated kringle polypeptide domains of human plasminogen

Three of the five kringle domains of human plasminogen (HPg), viz the first, fourth and fifth, exhibit significantly strong binding to omega-amino acids, such as epsilon-aminocaproic acid (EACA) and transaminomethylcyclohexane-1-carboxylic acid (AMCHA). In all cases, ligand stabilization is due to ion dipole attractions of its charged groups with polypeptide side chains, as well as hydrophobic clustering of the ligand methylene groups with appropriate hydrophobic residues within the kringle domain. In order to estimate the significance of the hydrophobic components of ligand stabilization, we have sought a more detailed description of these binding interactions. The standard thermodynamic binding parameters, delta G degrees, delta H degrees and delta S degrees, for association of EACA and AMCHA with isolated recombinant kringle regions of HPg have been determined at several temperatures to evaluate the changes in standard heat capacities (delta C degrees p) accompanying these interactions. In each case, the delta C degrees p values of binding were negative and in the range -36 to -91 cal mol -1 K -1, reflective of the importance of the hydrophobic components of the binding process and their probable effects on surrounding water structure.

Contributions of individual kringle domains toward maintenance of the chloride-induced tight conformation of human glutamic acid-1 plasminogen

The roles of each of the three omega-amino acid-binding kringles (K) of Glu1-Pg, viz., [K1Pg], [K4Pg], and [K5Pg], in engendering the Cl(-)-induced alteration to its tight (T) conformation and in effecting the epsilon-aminocaproic acid (EACA)-mediated change to the relaxed (R) protein conformation have been investigated by mutagenesis strategies wherein the omega-amino acid ligand-binding energies in the individual kringles in recombinant (r)-Glu1-Pg were greatly reduced. This was accomplished in the most conservative manner possible by altering a critical Asp residue in each relevant kringle to Asn. The particular mutations chosen were r-[D139N]Glu1-Pg, r-[D413N]Glu1-Pg, and r-[D518N]Glu1-Pg, in which a conserved Asp residue at a homologous sequence position in each of the three kringle domains is eliminated. These changes also lead to a great reduction of the EACA-binding strength of [K1Pg], [K4Pg], and [K5Pg], respectively. The s0(20,w) of wild-type (wt) r-Glu1-Pg in the presence of levels of Cl(-)-sufficient to fully occupy its binding sites on this protein was 5.9 S, a value reduced to 4.9 S as a result of addition of saturating concentrations of EACA to the Cl-/Glu1-Pg complex. Neither Cl- nor EACA substantially altered the s0(20,w) value of 5.2 S for r-[D139N]Glu1-Pg (4.8 S) or r-[D413N]Glu1-Pg (4.5 S). On the other hand, the s0(20,w) value of 5.2 S for r-[D518N]Glu1-Pg at saturating levels of Cl- is slightly reduced to 4.8 S upon addition of binding maximal concentrations of EACA.(ABSTRACT TRUNCATED AT 250 WORDS)

Tissue-type plasminogen activator (tPA) interacts with urokinase-type plasminogen activator (uPA) via tPA's lysine binding site. An explanation of the poor fibrin affinity of recombinant tPA/uPA chimeric molecules

Differential scanning calorimetry was used to study the domain structure and intramolecular interactions of tPA/uPA chimeras. A high temperature transition centered near 90 degrees C was observed upon melting of the tPA/uPA chimera (amino acids 1-274 of tPA and 138-411 of uPA) and its variant lacking the finger and epidermal growth factor-like modules (residues 1-3 and 87-274 of tPA and 138-411 of uPA). Since neither of the two parent plasminogen activators display such a stable structure, one may suggest that a new stabilizing intramolecular interaction occurs in the chimeras. We found that occupation of the lysine binding site of tPA by a lysine or arginine side chain from the urokinase moiety is responsible for the high temperature transition as well as for the failure of the chimeras to exhibit the expected fibrin binding properties. All uPA species, single- and two-chain high molecular weight uPA (Pro-Uk and HMW-Uk) and two-chain low molecular weight uPA (LMW-Uk), interact intermolecularly with tPA and its kringle-containing derivatives. This intermolecular interaction was strongly inhibited by epsilon-aminocaproic acid indicating that the lysine binding site of tPA is involved. The binding of uPA with the fluorescein-labeled A-chain of tPA, registered by changes in fluorescence anisotropy, was estimated to have a Kd range of 1-7 microM. The interaction of tPA with uPA determined by solid-phase assays appeared to be tighter, with a Kd range of 50-300 nM. Two synthetic peptides, with and without carboxyl-terminal lysine, corresponding to urokinase residues 144-158 and 144-157, were approximately 100-fold more potent than epsilon-aminocaproic acid with respect to inhibition of the tPA-uPA interaction, indicating that the tPA binding site on urokinase is located within this sequence, close to the activation site Lys158-Ile159. The discovered intermolecular interaction may be related to the reported synergistic effect of simultaneous administration of these two plasminogen activators.

Conformations and stabilities of human Glu1- and Lys78-plasminogen and of the fragments mini- and microplasminogen, analysed by circular dichroism and differential scanning calorimetry

The conformations and stabilities of two forms of human plasminogen, Glu1-plasminogen (Glu1-HPg, Glu1-Asn791) and Lys78-plasminogen (Lys78-HPg, Lys78-Asn791), and two enzymatically derived plasminogen fragments, miniplasminogen (mini-HPg, Val443-Asn791) and microplasminogen (micro-HPg, Lys531-Asn791) were analysed by circular dichroism and differential scanning calorimetry. The two plasminogen forms differ by the lack of 77 N-terminal amino acids in Lys78-HPg in comparison to Glu1-HPg. Mini-HPg is composed of kringle 5 and the protease domain of HPg whereas micro-HPg is built from the protease domain of HPg and a stretch of about 15 amino acids from kringle 5. Differential scanning calorimetric measurements of Glu1-HPg and Lys78-HPg reveal seven thermal transitions for both plasminogen forms. The results obtained for Lys78-HPg largely agree with recently published data (Novokhatny, V. V., Kudinov, S. A. and Privalov, P. L. J. Mol. Biol. 1984, 179, 215). Three thermal transitions corresponding to kringle 5 and to two subdomains of the C-terminal protease region were identified for mini-HPg. In micro-HPg, the two thermal transitions of the protease region were found but one of the protease subdomains was modified and its stability was much higher than in any of the other studied proteins. According to the microcalorimetric data obtained for mini-HPg and micro-HPg, transitions 5 and 6 of Glu1-HPg and Lys78-HPg were reassigned to kringle 5 and to a subdomain of the protease region, respectively, in contrast to literature data.(ABSTRACT TRUNCATED AT 250 WORDS)

Kringle solution structures via NMR: two-dimensional 1H-NMR analysis of horse plasminogen kringle 4

The kringle 4 domain of equine plasminogen (ePgn/K4), a close variant of the human homolog (hPgn/K4), contains residues, such as Trp32, which also appear in human apolipoprotein(a) kringle 4-type modules. The ePgn/K4 was investigated as a complex with epsilon-aminocaproic acid, an antifibrinolytic drug, by two-dimensional 1H-NMR spectroscopy at 500 MHz. Secondary structure elements were recognized from sequential medium and long-range dipolar (proton Overhauser) interactions, as well as from the identification of resonances originating from backbone amide protons with slow 1H-2H exchange in 2H2O. Antiparallel beta-sheets, consisting of strands 52-53, 61-65 and 71-75, were identified. Additionally, the segments 14-16 and 20-22 were found to assume characteristic interstrand antiparallel (beta-sheet-like) H-bond pairing. Four type I turns could be identified in strands 6-9, 16-19, 24-27 and 67-70. Ten structures were generated using distance geometry methods, followed by dynamic simulated annealing calculations. The root mean squares deviation of the distances was 2.79 A for all atoms and 1.81 A for backbone atoms only. Hydrogen bridges, involving side chain hydroxyl groups, were identified for Thr16 and Thr65. As observed for the hPgn/K4, the three-dimensional structure of the ePgn/K4 is mainly defined by two antiparallel beta-sheets, 14-16/20-22 and 62-66/71-75, which are oriented perpendicular to each other. Adjacent to these is a hydrophobic pocket, formed by Trp62, Tyr64, Trp72 and Phe74, whose side chains contribute a lipophilic component to the exposed lysine binding site surface. In contrast to the Trp25, Trp62 and Trp72 residues, conserved in the human and equine homologs, the spectrum of the Trp32 side chain reveals an unrestrained, solvent-exposed indole ring.

Functional independence of the kringle 4 and kringle 5 regions of human plasminogen

As part of continuing studies to evaluate whether the kringle domain regions of human plasminogen (HPg) exhibit independent conformational properties, simple model systems are required. Toward this end, we have constructed cDNA regions of HPg encoding its kringle 4 ([K4HPg]) and kringle 4-5 ([K4HPgK5HPg]) regions, expressed these gene fragments in bacterial cells, and purified the recombinant (r) products. The resulting r-[K4HPgK5HPg] was also employed to obtain the r-[K5HPg] domain of HPg by limited elastolytic digestion of this double-kringle polypeptide. The omega-amino acid ligand binding properties and thermal denaturation characteristics of r-[K4HPg], r-[K5HPg], and r-[K4HPgK5HPg] were determined, along with those for the [K5HPg] domain linked to the protease (P) region of HPg ([K5HPg]P). This allowed us to evaluate whether the conformational properties of the [K5HPg] module were influenced by the presence of its neighboring domains in HPg. The temperature midpoint of maximum heat capacity, Tm (and calorimetric enthalpy, delta H), for thermal denaturation of r-[K4HPg] was 57.8 degrees C (79.8 kcal/mol) in the absence of epsilon-aminocaproic acid (EACA) and 70.8 degrees C (93.7 kcal/mol) in the presence of that ligand. The corresponding values for isolated r-[K5HPg] were 50.4 degrees C (78.4 kcal/mol) and 61.0 degrees C (89.8 kcal/mol), respectively. These parameters for the isolated kringles were essentially unchanged when these same kringle domains were present in the r-[K4HPgK5HPg] and [K5HPg]P covalently linked pairs. Similarly, the thermodynamic characteristics (delta G, delta H, and delta S) that describe the binding energy of EACA to r-[K4HPg] at 25 degrees C were -6.3 kcal/mol, -4.5 kcal/mol, and 6.0 eu, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression and purification of kringle 4-type 2 of human apolipoprotein (a) in Escherichia coli

The most frequently occurring kringle 4 domain of human apolipoprotein (a), Kringle 4-subtype 2 (K4(2)), was expressed as a fusion protein with the maltose binding protein in Escherichia coli using the "tac" promoter. Although the fusion protein was expressed without a signal sequence, 25% was secreted into the periplasmic space; the remainder was found associated with the soluble cytosolic fraction. The fusion protein was readily isolated from whole cell lysate by amylose agarose affinity chromatography. Although a factor Xa cleavage site was engineered into the fusion protein, it was found that release of the K4(2) protein was most conveniently achieved by proteolysis with subtilisin A. The cleavage product produced in this way was shown to be intact K4(2) with only the first three amino acid residues of the leading flanking peptide missing, as judged by N-terminal sequence analysis. K4(2) was isolated from the hydrolysate by FPLC on a Mono-Q column with a yield of 170 +/- 30 micrograms/g wet cells. The resulting protein was monomeric in phosphate-buffered saline as judged by size-exclusion chromatography and appeared to be folded as shown by spectroscopic and immunological assays. The recombinant K4(2) did not bind to either lysine- or proline-Sepharose, suggesting that the ligand binding activities of lipoprotein (a) may reside in the other kringle domains of apolipoprotein (a).

Expression, purification, and characterization of the recombinant kringle 1 domain from tissue-type plasminogen activator

A novel fusion protein expression plasmid that allows ready purification and subsequent facile release of the target molecule has been constructed and employed to express in Escherichia coli and purify the tissue plasminogen activator kringle 1 domain ([K1tPA] residues C92-C173). The resulting plasmid encodes the tight lysine-binding kringle (K)1 domain of human plasminogen ([K1HPg]) followed by a peptide (PfXa) containing a factor Xa-sensitive bond, downstream of which [K1tPA] was inserted. The recombinant (r) [K1HPg]PfXa[K1tPA] fusion polypeptide was purified from various cell fractions in one step by Sepharose-lysine affinity chromatography. After cleavage with fXa, the mixture was repassaged over Sepharose-lysine, whereupon the r-[K1tPA]-containing polypeptide passed unretarded through the column. A homogeneous preparation of this material was then obtained after a simple step employing fast protein liquid chromatography. The purified r-[K1tPA], which contained the amino acid sequence SNAS[K1tPA]S, provided an amino-terminal amino acid sequence, through at least 20 amino acid residues, that was identical to that predicted from the cDNA sequence. The molecular mass of r-SNAS[K1tPA]S, determined by electrospray mass spectrometry, was 9621.9 +/- 4.0 (expected molecular mass, 9623.65). 1H-NMR spectroscopy and thermal stability studies of r-SNAS[K1tPA]S revealed that the purified material was properly folded and similar to other isolated kringle domains. Additionally, employment of this methodology revealed that only a very weak interaction between epsilon-aminocaproic acid and the isolated r-[K1tPA] domain occurred.

A structural assessment of the apo[a] protein of human lipoprotein[a]

Apolipoprotein[a], the highly glycosylated, hydrophilic apoprotein of lipoprotein[a] (Lp[a]), is generally considered to be a multimeric homologue of plasminogen, and to exhibit atherogenic/thrombogenic properties. The cDNA-inferred amino acid sequence of apo[a] indicates that apo[a], like plasminogen and some zymogens, is composed of a kringle domain and a serine protease domain. To gain insight into possible positive functions of Lp[a], we have examined the apo[a] primary structure by comparing its sequence with those of other proteins involved in coagulation and fibrinolysis, and its secondary structure by using a combination of structure prediction algorithms. The kringle domain encompasses 11 distinct types of repeating units, 9 of which contain 114 residues. These units, called kringles, are similar but not identical to each other or to PGK4. Each apo[a] kringle type was compared with kringles which have been shown to bind lysine and fibrin, and with bovine prothrombin kringle 1. Apo[a] kringles are linked by serine/threonine- and proline-rich stretches similar to regions in immunoglobulins, adhesion molecules, glycoprotein Ib-alpha subunit, and kininogen. In comparing the protease domains of apo[a] and plasmin, apo[a] contains a region between positions 4470 and 4492 where 8 substitutions, 9 deletions, and 1 insertion are apparent. Our analysis suggests that apo[a] kringle-type 10 has a high probability of binding to lysine in the same way as PGK4. In the only human apo[a] polymorph sequenced to date, position 4308 is occupied by serine, whereas the homologous position in plasmin is occupied by arginine and is an important site for proteolytic cleavage and activation. An alternative site for the proteolytic activation of human apo[a] is proposed.

Thermodynamics of ligand binding and denaturation for His64 mutants of tissue plasminogen activator kringle-2 domain

The contribution of His64 to the function and stability of tissue plasminogen activator (t-PA) kringle-2 domain (His244 in t-PA numbering) has been studied by using microcalorimetric methods to compare the ligand binding and thermal denaturation behavior of wild-type kringle-2 and mutants having His64 replaced with Tyr or Phe. This site was examined because modeling studies suggested that the His64 side chain could play an important role in ligand binding by forming an ion-pair with the carboxylate of the ligand, L-lysine. Kringle-2 domains were expressed by secretion of the 174-263 portion of t-PA in E. coli and purified as previously described for the wild-type domain. Both mutant proteins retain affinity for L-lysine, although reduced three- to four-fold relative to wild-type, demonstrating that His64 does not interact with the ligand carboxylate through an ion-pair interaction or by hydrogen bonding. The H64Y substitution does result in an altered specificity of the lysine binding site with the mutant domain having greatest affinity for a ligand of 6.8 A chain length, whereas the wild-type domain prefers an 8.8 A long ligand. For both wild-type and mutant, the binding of the optimal chain length ligand is dominated by enthalpic effects (delta H = -6,000 to -7,000 cal/mol) and T delta S accounts for less than 15% of delta G. In addition, the H64Y mutant differs from wild-type in the effect of ligand alpha-amino group modification on binding affinity. Based on examination of the x-ray structure recently determined for wild-type kringle-2, the specificity changes accompanying the H64Y substitution probably result from changes in side chain interactions in the lysine binding site. Thermal denaturation experiments show that the H64Y mutant is also more stable than the wild-type protein with the difference in stabilization free energy (delta delta G) equal to 2.7 kcal/mol at 25 degrees C and pH 3. The increased stability of the mutant appears to be related to the difference in hydrophobicity between His and Tyr.

Construction, expression, and purification of recombinant kringle 1 of human plasminogen and analysis of its interaction with omega-amino acids

An Escherichia coli expression vector, containing the alkaline phosphatase promoter and the stII heat-stable enterotoxin signal sequence, along with the cDNA of the kringle 1 (K1) region of human plasminogen (HPg), has been employed to express into the periplasmic space amino acid residues 82-163 (E163----D) of HPg. This region of the molecule contains the entire K1 domain (residues C84-C162) of HPg, as well as two non-kringle amino-terminal amino acids (S82-E83) that are present in their normal locations in HPg and a carboxyl-terminal amino acid, D163, that results from mutation of the E163, normally present at this location in the HPg amino acid sequence. After purification of r-K1 by chromatographic techniques, we have investigated its omega-amino acid binding properties by titration calorimetry, intrinsic fluorescence, and differential scanning microcalorimetry (DSC). The antifibrinolytic agent, epsilon-aminocaproic acid (EACA), possesses a single binding site for r-K1. The thermodynamic properties of this interaction, studied by calorimetric titrations of the heats of binding with this ligand, reveal a Kd of 12 +/- 2 microM at 25 degrees C and pH 7.4, a corresponding delta G of -6.7 +/- 0.1 kcal/mol, a delta H of -3.6 +/- 0.1 kcal/mol, and a delta S of 10.5 +/- 0.8 eu. The intrinsic fluorescence of r-K1 decreases by approximately 44% when its binding site is saturated with EACA, and titrations of this perturbation with EACA lead to calculation of a Kd of approximately 13 microM, a value in good agreement with that obtained from titration calorimetric analysis. EACA represents the strongest binding ligand of a variety of simple aliphatic omega-amino acids examined. A cyclic analogue of EACA, trans-4-(aminomethyl)cyclohexanecarboxylic acid, interacts with r-K1 with an approximate 12-fold tighter Kd (1.0 +/- 0.2 microM). Investigations by DSC, at pH 7.4, demonstrate that a significant stabilization of the r-K1 structure occurs when EACA binds to this domain. The temperature of maximum heat capacity change (Tm) in the thermal denaturation of r-K1 increases from approximately 340.8 to 359.1 K as a consequence of EACA binding. These studies demonstrate that a fully functional EACA-binding kringle from HPg can be expressed and secreted in E. coli, purified by techniques that do not require refolding, and investigated as an independent structural unit.

Plasminogen activator activities of equimolar complexes of streptokinase with variant recombinant plasminogens

The steady-state kinetic characteristics of the amidolytic and plasminogen activator activities of equimolar streptokinase (SK)-human plasminogen (HPg) and SK-human plasmin (HPm) complexes have been determined, exploiting the generation and use of cleavage site resistant mutants of HPg to stabilize plasminogen within the complex. Whereas amidolytic kinetic constants for equimolar complexes of SK with the following proteins, viz., plasma HPm, insect (i) cell-expressed wild-type (wt) recombinant (r) HPm, R561E-irHPg, and Chinese hamster ovary cell (c)-expressed R561S-crHPg, are similar, it has been found that the various SK-HPg complexes are far better enzymes than SK-HPm complexes for activation of bovine plasminogen, a species of plasminogen that is resistant to activation by SK, alone. In addition, it is emphasized that as a result of mutating the cleavage site in plasminogen, it is possible to express this protein in mammalian cells, and thus provide it for use in complex with SK as a more efficient plasminogen activator than plasma plasminogen, which is rapidly converted to HPm within the SK complex. This finding has important implications in the assessment of thrombolytic therapeutic reagent employing SK-plasminogen and SK-plasmin complexes.

Characterization of an extremely large, ligand-induced conformational change in plasminogen

Native human plasminogen has a radius of gyration of 39 angstroms. Upon occupation of a weak lysine binding site, the radius of gyration increases to 56 angstroms, an extremely large ligand-induced conformational change. There are no intermediate conformational states between the closed and open form. The conformational chang is not accompanied by a change in secondary structure, hence the closed conformation is formed by interaction between domains that is abolished upon conversion to the open form. This reversible change in conformation, in which the shape of the protein changes from that best described by a prolate ellipsoid to a flexible structure best described by a Debye random coil, is physiologically relevant because a weak lysine binding site regulates the activation of plasminogen.

Folding of Eukaryotic Proteins Produced in Escherichia Coli

Although intracellular expression in E. coli may result in accumulation of the eukaryotic protein in inclusion bodies, the protein may often be recovered by first solubilizing with denaturant followed by refolding. Some general guidelines for developing a refolding procedure are apparent but the specific protocol must be empirically determined for each protein. Convenient and rapid assays for detecting native protein are critical for developing a refolding procedure. Maintaining solubility during refolding is a common feature of recovery processes. Proper folding should be assessed by a number of methods including activity, spectroscopic and stability measurements. For some proteins, properly folded protein may be obtained by secretion from E. coli; however, secretion does not ensure correct folding and protection from proteolytic degradation.

Expression of human plasminogen cDNA in a baculovirus vector-infected insect cell system

A cDNA that encodes the human plasminogen (HPg) amino acid sequence has been inserted adjacent to the polyhedrin promoter in the genome of the baculovirus, Autographa californica nuclear polyhedrosis virus, which was then used to infect cultured cells of the farm armyworm, Spodoptera frugiperda. Under the conditions of cell growth employed, recombinant (rec)-HPg was secreted into the medium after 24 h postinfection (p.i.), at which point virtually no rec-HPg antigen remained inside the cells. At 48 h p.i., a maximal level of intact rec-HPg was present in the medium, which underwent substantial proteolytic digestion after that time. The rec-HPg produced by this expression system possessed a molecular weight equivalent to that of plasma [Glu1]-plasminogen. In addition, the rec-HPg adsorbed to Sepharose-lysine, and was eluted with epsilon-aminocaproic acid (EACA). The recombinant protein also interacted with polyclonal antibodies generated to plasma HPg, as well as with a monoclonal antibody directed against a distinct region (kringle 1-3) of the plasma HPg molecule. Finally, the insect-expressed rec-HPg was activatable to plasmin (HPm) by urokinase. The results demonstrate that this expression system produces a full-length functional single-chain rec-HPg, which can be isolated intact from the culture medium, with some consideration for the temporal events that occur in secretion and longer-term degradation of the protein. The fact that this rec-HPg was converted to HPm with a plasminogen activator, and that it interacted with anti-plasma HPg polyclonal and monoclonal antibodies, as well as with the ligand, EACA, indicates that the molecule retains many of its important functional properties and is folded in an integral manner.

A potential basis for the thrombotic risks associated with lipoprotein(a)

Lipoprotein(a) (Lp(a)) has been strongly linked with atherosclerosis and is an independent risk factor for myocardial infarction. Distinguishing Lp(a) from other low-density lipoprotein particles is its content of a unique apoprotein, apo(a). The recently described sequence of apo(a) indicates a remarkable homology with plasminogen, the zymogen of the primary thrombolytic enzyme, plasmin. Lp(a) may contain 37 or more disulphide-looped kringle structures, which are 75-85% identical to the fourth kringle of plasminogen. Plasminogen receptors are widely distributed on blood cells and are present at extremely high density on endothelial cells. These receptors promote thrombolysis by accelerating plasminogen activation and protecting plasmin from inhibition. If, by molecular mimicry, Lp(a) competes with plasminogen for receptors, then thrombolysis would be inhibited and thrombosis promoted. Here we provide support for such a mechanism being responsible for the thrombotic risks associated with elevated Lp(a) by demonstrating that Lp(a) inhibits plasminogen binding to cells.

Purification and characterization of tissue plasminogen activator kringle-2 domain expressed in Escherichia coli

We have expressed the 174-263 fragment (kringle-2 domain) of human tissue-type plasminogen activator (t-PA) in Escherichia coli by secretion into the periplasmic space using the alkaline phosphatase promoter and stII enterotoxin signal sequence. A large portion of the secreted protein is associated with an insoluble cellular fraction. This material can be solubilized by extraction with denaturant and reducing agent and then recovered in active form by refolding in the presence of reduced and oxidized glutathione. Kringle-2 is then easily purified by affinity chromatography on lysine-Sepharose followed by cation-exchange chromatography. The isolated protein has an amino acid composition and N-terminal sequence as expected for the 174-263 fragment of t-PA, indicating that the signal peptide has been properly removed. Circular dichroic spectra suggest that the protein is folded similar to the kringle-4 domain of plasminogen [Castellino et al. (1986) Arch. Biochem. Biophys. 247, 312-320]. Equilibrium dialysis experiments indicate a single binding site on kringle-2 for L-lysine having a KD of 100 microM. Using a method based on elution of kringle from lysine-Separose with omega-aminocarboxylic acids [Winn et al. (1980) Eur. J. Biochem. 104, 579-586], we have shown the lysine binding site of t-PA kringle-2 to have a preference for a ligand with 8.8-A separation between amine and carboxylate functions. Charge interactions with the epsilon-amino group of L-lysine are important in binding since the affinities for N epsilon-acetyl-L-lysine, L-arginine, and gamma-guanidinobutyric acid are decreased greater than 2000-fold, 200-fold, and 12-fold, respectively, relative to the affinity for L-lysine.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of ligand binding to kringles 4 and 5 fragments from human plasminogen

The interaction of the isolated kringles 4 and 5 from human plasminogen with 6-aminohexanoic acid, pentylamine, pentanoic acid and arginine has been quantitatively characterized by scanning calorimetry and fluorescent spectroscopy. It has been found that the ligands with the positively charged group have a good binding ability while pentanoic acid in comparison with 6-aminohexanoic acid being devoid of amino group does not interact with the kringles under study. The positively charged group of the ligand is suggested to play a crucial role in ligand binding with the lysine-binding site.

A differential scanning calorimetric investigation of the domains of recombinant tissue plasminogen activator

The differential scanning calorimetric (DSC) properties of a series of recombinant tissue plasminogen activators (rt-PA) have been examined. The endotherm obtained for native rt-PA can be deconvoluted into a pair of two-state transitions, which indicates that two separate observable regions of the molecule undergo independent melting. A distinguishing feature of native rt-PA is the dependence of the temperature of the maximum heat capacity of the endothermic transitions (Tm) on the thermal scan rate of the samples, suggesting that a kinetic process, involving conversion of a reversibly denatured to an irreversibly denatured form of the protein, with an energy of activation of approximately 81.5 kcal/mol, characterizes its thermal denaturation. A comparison of the conformational properties of rt-PA preparations which have been produced in different expression systems, as revealed by DSC analysis of their thermal denaturation characteristics, has demonstrated that subtle differences do occur. Similar studies with deletion mutants of rt-PA suggest that the growth factor domain (EGF) plays a role in its overall thermal stability, when the protein also contains the kringle 1 region, and removal of the EGF domain leads to conformational alterations in other areas of the molecule.

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

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

Differential autolysis of human plasmin at various pH levels

Denaturation of human plasmin in solutions of various pH levels was studied. The denaturation and loss of catalytic activity of plasmin in solutions of between pH 3.5 and 10.5 is a second-order kinetics. In alkaline solutions of pH levels greater than 11.5, plasmin undergoes a first-order denaturation. The second-order denaturation of plasmin is mainly due to the autolytic reactions between plasmin molecules. Two autolytic processes of human plasmin in aqueous solution were observed. In a slightly acidic solution (pH 6.5) the light (B) chain was found to be cleaved faster than the heavy (A) chain of plasmin. On the other hand, the heavy (A) chain was cleaved in an alkaline solution of pH near 11.0. A cleaved heavy (A) chain of molecular weight 58,000 was observed. Both the heavy (A) chain and the light (B) chain were found to be cleaved at pH levels between 6.5 and 11.0. The loss of the esterase activity of plasmin samples in the autolytic process is in parallel with the decline of intact light (B) chain. The autolytic cleavage of the heavy (A) chain led to the formation of a new type of catalytically active plasmin.

Analysis of the aromatic 1H-NMR spectrum of the kringle 5 domain from human plasminogen. Evidence for a conserved kringle fold

A kringle 5 domain fragment from human plasminogen has been investigated by 1H-NMR spectroscopy at 300 MHz and 620 MHz. The study focuses on the kringle 5 aromatic spectrum as aromatic side chains appear to mediate the binding of benzamidine. Spin-echo experiments and acid/base-titration studies in conjunction with two-dimensional double-quantum and chemical-shift-correlated spectroscopies were used to identify individual spin systems. Sequence-specific assignments of aromatic resonances are derived from direct comparison of the kringle 5 spectrum with spectra of the homologous kringle 1 and kringle 4 domains of plasminogen. As previously observed for kringles 1 and 4, the pattern we detect for Tyr9 in kringle 5 reflects a slow conformational exchange between two states in equilibrium, one in which the Tyr9 ring is freely mobile and one in which its flip dynamics are constrained. Proton Overhauser experiments in 1H2O and in 2H2O have been used to probe aromatic ring interactions and to identify residues which are part of the hydrophobic core centered at the Leu46 side chain. Overall, the data indicate a strong structural homology among the three plasminogen kringles.

Secondary and tertiary structure of apolipoproteins

The advent of these and other high-powered techniques for the detailed study of apoLP organization will allow us to obtain a high resolution picture of apoLP conformation both in solution and on native lipoprotein particles.

Proton magnetic resonance study of lysine-binding to the kringle 4 domain of human plasminogen. The structure of the binding site

The binding of L-Lys, D-Lys and epsilon-aminocaproic acid (epsilon ACA) to the kringle 4 domain of human plasminogen has been investigated via one and two-dimensional 1H-nuclear magnetic resonance spectroscopy at 300 and 600 MHz. Ligand-kringle association constants (Ka) were determined assuming single site binding. At 295 K, pH 7.2, D-Lys binds to kringle 4 much more weakly (Ka = 1.2 mM-1) than does L-Lys (Ka = 24.4 mM-1). L-Lys binding to kringle 4 causes the appearance of ring current-shifted high-field resonances within the -1 approximately less than delta approximately less than 0 parts per million range. The ligand origin of these signals has been confirmed by examining the spectra of kringle 4 titrated with deuterated L-Lys. A systematic analysis of ligand-induced shifts on the aromatic resonances of kringle 4 has been carried out on the basis of 300 MHz two-dimensional chemical shift correlated (COSY) and double quantum correlated spectroscopies. Significant differences in the effect of L-Lys and D-Lys binding to kringle 4 have been observed in the aromatic COSY spectrum. In particular, the His31 H4 and Trp72 H2 singlets and the Phe64 multiplets appear to be the most sensitive to the particular enantiomers, indicating that these residues are in proximity to the ligand C alpha center. In contrast, the rest of the indole spectrum of Trp72 and the aromatic resonances of Trp62 and Tyr74, which are affected by ligand presence, are insensitive to the optical nature of the ligand isomer. These results, together with two-dimensional proton Overhauser studies and ligand-kringle saturation transfer experiments reported previously, enabled us to generate a model of the kringle 4 ligand-binding site from the crystallographic co-ordinates of the prothrombin kringle 1. The latter, although lacking recognizable lysine-binding capability, is otherwise structurally homologous to the plasminogen kringles.

Isolation, purification and 1H-NMR characterization of a kringle 5 domain fragment from human plasminogen

A scheme is proposed for generating the intact Val-448-Phe-545 polypeptide of human plasminogen which contains the fifth kringle domain of the plasmin heavy chain. The procedure is based on a pepsin fragmentation of miniplasminogen and involves the purification of the kringle 5-containing fragment by gel filtration and ion-exchange chromatography. The final product is characterized by amino acid analysis, N- and C-terminal analyses, and high-resolution 1H-NMR spectroscopy at both 300 MHz and 611 MHz. We detect a (40:60%) Asp/Asn heterogeneity at site 452 of the Glu-plasminogen molecule. In the conventional kringle numbering system, the kringle 5 domain extends from Cys-1 to Cys-80, which corresponds to Cys-461 to Cys-540 in plasminogen. A preliminary 1H-NMR characterization of kringle 5 focuses on the global conformational features of the polypeptide. Assignments are given for a number of resonances, including the Tyr-72, the His imidazoles' and the Trp indoles' spin systems. Comparison with human plasminogen kringles 1 and 4 shows that the kringle 5 conformation is highly structured and very similar to that of the homologous domains. This conservancy is particularly striking in the environment surrounding Leu-46 and in the overall features of the aromatic spectrum. There are some differences, particularly in the buried His-33 imidazole group, whose H2 resonance is shifted to 9.67 ppm. A preliminary study of benzamidine-binding shows that the ligand interacts weakly (Ka approximately equal to 1.7 mM -1) mainly through the amidino functional group. Trp-62 and Tyr-72 are significantly perturbed by benzamidine, suggesting that these residues are part of the ligand-binding site.