Lysine-50 is a likely site for anchoring the plasminogen N-terminal peptide to lysine-binding kringles

The ANXA2/S100A10 Complex—Regulation of the Oncogenic Plasminogen Receptor

The generation of the serine protease plasmin is initiated by the binding of its zymogenic precursor, plasminogen, to cell surface receptors. The proteolytic activity of plasmin, generated at the cell surface, plays a crucial role in several physiological processes, including fibrinolysis, angiogenesis, wound healing, and the invasion of cells through both the basement membrane and extracellular matrix. The seminal observation by Albert Fischer that cancer cells, but not normal cells in culture, produce large amounts of plasmin formed the basis of current-day observations that plasmin generation can be hijacked by cancer cells to allow tumor development, progression, and metastasis. Thus, the cell surface plasminogen-binding receptor proteins are critical to generating plasmin proteolytic activity at the cell surface. This review focuses on one of the twelve well-described plasminogen receptors, S100A10, which, when in complex with its regulatory partner, annexin A2 (ANXA2), forms the ANXA2/S100A10 heterotetrameric complex referred to as AIIt. We present the theme that AIIt is the quintessential cellular plasminogen receptor since it regulates the formation and the destruction of plasmin. We also introduce the term oncogenic plasminogen receptor to define those plasminogen receptors directly activated during cancer progression. We then discuss the research establishing AIIt as an oncogenic plasminogen receptor-regulated during EMT and activated by oncogenes such as SRC, RAS, HIF1α, and PML-RAR and epigenetically by DNA methylation. We further discuss the evidence derived from animal models supporting the role of S100A10 in tumor progression and oncogenesis. Lastly, we describe the potential of S100A10 as a biomarker for cancer diagnosis and prognosis.

Impact of SMTP Targeting Plasminogen and Soluble Epoxide Hydrolase on Thrombolysis, Inflammation, and Ischemic Stroke

Stachybotrys microspora triprenyl phenol (SMTP) is a large family of small molecules derived from the fungus S. microspora. SMTP acts as a zymogen modulator (specifically, plasminogen modulator) that alters plasminogen conformation to enhance its binding to fibrin and subsequent fibrinolysis. Certain SMTP congeners exert anti-inflammatory effects by targeting soluble epoxide hydrolase. SMTP congeners with both plasminogen modulation activity and anti-inflammatory activity ameliorate various aspects of ischemic stroke in rodents and primates. A remarkable feature of SMTP efficacy is the suppression of hemorrhagic transformation, which is exacerbated by conventional thrombolytic treatments. No drug with such properties has been developed yet, and SMTP would be the first to promote thrombolysis but suppress disease-associated bleeding. On the basis of these findings, one SMTP congener is under clinical study and development. This review summarizes the discovery, mechanism of action, pharmacological activities, and development of SMTP.

Mechanism of the action of SMTP-7, a novel small-molecule modulator of plasminogen activation

SMTP-7 is a small molecule that promotes the proteolytic activation of plasminogen by relaxing its conformation. SMTP-7 has excellent therapeutic activities against thrombotic stroke in several rodent models. The objective of this study was to elucidate detailed mechanism of the action of SMTP-7 in vitro. We report here that the action of SMTP-7 requires a cofactor with a long-chain alkyl or alkenyl group, and that the fifth kringle domain (kringle 5) of plasminogen is involved in the SMTP-7 action. In this study, we found that the SMTP-7 action to enhance plasminogen activation depended on the presence of a certain type of surfactant, and we screened biologically relevant molecules for their cofactor activity for the SMTP action. As a result, phospholipids, sphingolipids, and oleic acid were found to be active in assisting the SMTP-7 action. On the contrary, stearic acid and bile acids were inactive. Thus, a certain structural element, not only the surface-activating potential, is required for a compound to act as a cofactor for the SMTP-7 action. The plasminogen molecule consists of a PAN domain, five kringle domains, and a serine protease domain. The cofactor-dependent effects of SMTP-7 was observed with plasminogen species including kringle 5 such as intact plasminogen (Glu-plasminogen), des-PAN plasminogen (Lys-plasminogen), and des-[PAN - (kringles 1-4)] plasminogen (mini-plasminogen). However, SMTP-7 effect was not observed with the smallest plasminogen species des-[PAN - (kringles 1-4) and a half of kringle 5)] plasminogen (micro-plasminogen). Thus, kringle 5 is crucial for the action of SMTP-7.

The interaction of streptococcal enolase with canine plasminogen: the role of surfaces in complex formation

The enolase from Streptococcus pyogenes (Str enolase F137L/E363G) is a homo-octamer shaped like a donut. Plasminogen (Pgn) is a monomeric protein composed of seven discrete separated domains organized into a lock washer. The enolase is known to bind Pgn. In past work we searched for conditions in which the two proteins would bind to one another. The two native proteins in solution would not bind under any of the tried conditions. We found that if the structures were perturbed binding would occur. We stated that only the non-native Str enolase or Pgn would interact such that we could detect binding. We report here the results of a series of dual polarization interferometry (DPI) experiments coupled with atomic force microscopy (AFM), isothermal titration calorimetry (ITC), dynamic light scattering (DLS), and fluorescence. We show that the critical condition for forming stable complexes of the two native proteins involves Str enolase binding to a surface. Surfaces that attract Str enolase are a sufficient condition for binding Pgn. Under certain conditions, Pgn adsorbed to a surface will bind Str enolase.

A new series of the SMTP plasminogen modulators with a phenylamine-based side chain

SMTPs are a family of small-molecule plasminogen modulators that enhance plasminogen activation. SMTP-7, one of the most potent congeners, is effective in treating thrombotic cerebral infarction. The SMTP molecule consists of a tricyclic γ-lactam moiety, a geranylmethyl group, and an N-linked side chain. The presence of both an aromatic group and a negatively ionizable group in the N-linked side chain is crucial for activity. Investigations of the congeners with a phenylglycine-based side chain suggest that a phenolic hydroxy group affects potency. In this study, we isolate and characterize a series of novel SMTP congeners with a phenylamine-based N-linked side chain. Of the 11 congeners isolated, SMTP-19 (with a 4-phenylcarboxylic acid moiety), SMTP-22 (with a 3-hydroxyphenyl-4-carboxylic acid moiety) and SMTP-25 (with a 2-hydroxyphenyl-3-carboxylic acid moiety) are as potent as SMTP-7 in plasminogen-modulating activity. Their isomers with a carboxylic acid group and/or a phenolic hydroxy group at different positions have <40% of the activity of these congeners. Both SMTP-22 and SMTP-25 have >1.7 times more oxygen radical absorbance capacity as compared with SMTP-7.

SMTP (Stachybotrys microspora triprenyl phenol) enhances clot clearance in a pulmonary embolism model in rats

Background

Stachybotrys microspora triprenyl phenols (SMTPs) are a novel family of small molecules that enhance both activation and fibrin-binding of plasminogen. While their effects on fibrinolysis have been characterized in vitro, little is known about their activity in vivo with respect to plasminogen activation and blood clot clearance.

Results

To select a potent SMTP congener for the evaluation of its action in vitro and in vivo, we tested several SMTP congeners with distinct structural properties for their effects on plasminogen activation. As a result, SMTP-7 (orniplabin) was found to have distinguished activity. Several lines of biochemical evidence supported the idea that SMTP-7 acted as a plasminogen modulator. SMTP-7 elevated plasma level of plasmin-α₂-antiplasmin complex, an index of plasmin formation in vivo, 1.5-fold in mice after the intravenous injections at doses of 5 and 10 mg kg^-1. In a rat pulmonary embolism model, SMTP-7 (5 mg kg^-1) enhanced the rate of clot clearance ~3-fold in the absence of exogenous plasminogen activator. Clot clearance was enhanced further by 5 mg kg^-1 of SMTP-7 in combination with single-chain urokinase-type plasminogen activator.

Conclusions

Our results show that SMTP-7 is a superior plasminogen modulator among the SMTP family compounds and suggest that the agent enhances plasmin generation in vivo, leading to clearance of thrombi in a model of pulmonary embolism.

Structure-activity relationships of 11 new congeners of the SMTP plasminogen modulator

The fungal metabolite Stachybotrys microspora triprenyl phenols (SMTPs) are small-molecule plasminogen modulators that enhance plasminogen activation. The SMTP molecule consists of a tricyclic γ-lactam moiety, an isoprene side-chain and an N-linked side-chain. Previous investigations have demonstrated that the N-linked side-chain is crucial for its activity. In this study, we have isolated 11 new SMTP congeners with a variety of N-linked side-chain structures, to investigate structure-activity relationships. Active compounds included congeners with a carboxyl or a sulfonic acid group in the N-linked side-chain, whereas not all the congeners with a carboxyl group were active. Of these congeners, that with methionine or tyrosine as the N-linked side-chain moiety was more active than that with an aliphatic amino acid. Congeners without ionizable group in the N-linked side-chain were essentially inactive.

Small-molecule modulators of zymogen activation in the fibrinolytic and coagulation systems

The coagulation and fibrinolytic systems are central to the hemostatic mechanism, which works promptly on vascular injury and tissue damage. The rapid response is generated by specific molecular interactions between components in these systems. Thus, the regulation mechanism of the systems is programmed in each component, as exemplified by the elegant processes in zymogen activation. This review describes recently identified small molecules that modulate the activation of zymogens in the fibrinolytic and coagulation systems.

Solution structure and functional characterization of human plasminogen kringle 5

The ligand binding properties of the kringle 5 (K5) domain of human plasminogen have been investigated via intrinsic tryptophan fluorescence. The oleic acid (OA) affinity for K5 was quantified, yielding an association constant K(a) approximately 2.08 x 10(4) mM(-1). Simultaneously, it was determined that OA and trans-4-(aminomethyl)cyclohexanecarboxylic acid (AMCHA) (K(a) approximately 50 mM(-1)) compete for binding to K5. The solution structure of K5 in the presence of 11 mM AMCHA was solved via NMR spectroscopy (protein heavy atom RMSD approximately 0.93 +/- 0.12 A). The AMCHA binding site was localized via (1)H/(15)N chemical shift perturbation mapping assisted by in silico docking. We have found that AMCHA binds at the canonical kringle lysine binding site (LBS), structured by the Pro54-Gly60 segment plus the neighboring Phe36, Thr37, Trp62, Leu71, and Tyr72 residues. The segment 30-42, encompassing LBS residues, appears to be endowed with a higher degree of structural flexibility as suggested by the relatively lower value of S(2), the generalized order parameter, consistent with a higher backbone heavy atom RMSD of approximately 1.22 A (vs 0.84 A overall) between the two monomeric units in the crystal unit cell, of potential significance for ligand binding. OA was found to perturb the same area of the protein, namely, the LBS, as well as Tyr74. Combined with previous studies, the observation of OA binding expands the range of ligands that interact with kringle 5 while it widens the scope of potential biological functions for kringle domains.

Stachybotrydial selectively enhances fibrin binding and activation of Glu-plasminogen

Stachybotrydial, a triprenyl phenol metabolite from a fungus, has a plasminogen modulator activity selective to Glu-plasminogen. Stachybotrydial enhanced fibrin binding and activation of Glu-plasminogen (2- to 4-fold enhancement at 60-120 microM) but not of Lys-plasminogen. Approximately 1.2-1.6 moles of [3H]stachybotrydial bound to Glu-plasminogen to exert such effects. The selective modulation of the Glu-plasminogen function by stachybotrydial may be related to alteration of its conformational status.

Plasminogen N-terminal activation peptide modulates the activity of angiostatin-related peptides on endothelial cell proliferation and migration

Angiostatin, a potent inhibitor of angiogenesis, is derived from the fibrinolytic proenzyme, plasminogen, by enzymatic processing. Plasminogen N-terminal activation peptide (PAP) is one of the products concomitantly released aside from angiostatin (kringles 1-4) and mini-plasminogen (kringle 5 plus the catalytic domain) when plasminogen is processed. To determine whether PAP alone or together with the angiostatin-related peptides derived from the processing of plasminogen modulate the proliferation and motility of endothelial cells, we have generated a recombinant PAP and used it to study its effects on endothelial cells in the presence and absence of the angiostatin-related peptides. Our results showed that PAP alone slightly increased the migration but not the proliferation of endothelial cells. However, in the presence of the angiostatin-related peptides, PAP attenuated the inhibitory activity of the angiostatin-related peptides on the proliferation and migration of endothelial cells. The inhibitory effect of PAP on the angiostatin-related peptides could be due to its binding to the kringle domains of the latter peptides.

Isolation and absolute configuration of SMTP-0, a simplest congener of the SMTP family nonlysine-analog plasminogen modulators

SMTP-0, a new simple congener of the SMTP nonlysine-analog plasminogen modulators, was isolated from a culture of Stachybotrys microspora. Based on the physico-chemical data, SMTP-0 was shown to be an enantiomer of the antimicrobial compound stachybotrin B. The absolute configuration of SMTP-0 was determined by the modified Mosher method. The stereochemistry was further confirmed using an epimer of SMTP-0. Unlike most SMTPs with an amino acid side chain linked to the nitrogen atom of the lactam moiety, SMTP-0, which lacks the N-linked side chain, showed no plasminogen modulator activity.

Canine plasminogen: spectral responses to changes in 6-aminohexanoate and temperature

We studied the near UV absorption spectrum of canine plasminogen. There are 19 tryptophans, 19 phenylalanines and 34 tyrosines in the protein. 4th derivative spectra optimized for either tryptophan or tyrosine give a measure of the polarity of the environments of these two aromatic amino acids. Plasminogen at temperatures between 0 degrees C and 37 degrees C exists as a mixture of four conformations: closed-relaxed, open-relaxed, closed-compact, and open-compact. The closed to open transition is driven by addition of ligand to a site on the protein. The relaxed to compact transition is driven by increasing temperature from 0 degrees C to above 15-20 degrees C.When the conformation of plasminogen is mainly closed-relaxed, the 4th derivative spectra suggest that the average tryptophan environment is similar to a solution of 20% methanol at the same temperature. Under the same conditions, 4th derivative spectra suggest that the average tyrosine environment is similar to water. These apparent polarities change as the plasminogen is forced to assume the other conformations. We try to rationalize the information based on the known portions of the plasminogen structure.

Canine Plasminogen: Spectral Responses to Changes in 6-Aminohexanoate and Temperature

We studied the near UV absorption spectrum of canine plasminogen. There are 19 tryptophans, 19 phenylalanines and 34 tyrosines in the protein. 4th derivative spectra optimized for either tryptophan or tyrosine give a measure of the polarity of the environments of these two aromatic amino acids. Plasminogen at temperatures between 0°C and 37°C exists as a mixture of four conformations: closed-relaxed, open-relaxed, closed-compact, and open-compact. The closed to open transition is driven by addition of ligand to a site on the protein. The relaxed to compact transition is driven by increasing temperature from 0°C to above 15-20°C.

When the conformation of plasminogen is mainly closed-relaxed, the 4th derivative spectra suggest that the average tryptophan environment is similar to a solution of 20% methanol at the same temperature. Under the same conditions, 4th derivative spectra suggest that the average tyrosine environment is similar to water. These apparent polarities change as the plasminogen is forced to assume the other conformations. We try to rationalize the information based on the known portions of the plasminogen structure.

Expression and characterization of recombinant human micro-plasminogen

Micro-plasminogen (microPlg) gene amplified from human liver cells by reverse transcription PCR was inserted into expression vector pET-28a (pET-28a/microPlg) and transformed into E. coli strain BL21(DE3). Recombinant human micro-plasminogen (rh-microPlg) was over-expressed as inclusion bodies when induced with IPTG. After renaturation and purification, 16 mg rh-microPlg/l was obtained with a homogeneity of 95% (w/w). Pro-urokinase (proUK)-induced rh-microPlg activation was significantly faster than when Glu-plasminogen was the substrate. The catalytic efficiency of urokinase (UK) activation of rh-microPlg was twice that of Glu-plasminogen. While recombinant human micro-plasmin (rh-microPlm) and Lys-plasmin had a similar amidolytic activity against a small substrate, D-valyl-L-leucyllysine-p-nitroaniline dihydrochloride, Lys-plasmin activated proUK with a catalytic efficiency about fourfold greater than did rh-microPlm. These results suggested that the kringle 1-5 domain of plasminogen and plasmin may modify both UK activation of plasminogen and plasmin activation of proUK, respectively.

The role of β2-glycoprotein I (β2GPI) in the activation of plasminogen

Beta2-glycoprotein I (beta2GPI) is a glycoprotein of unknown physiological function. It is the main target antigen for antiphospholipid antibodies in patients with antiphospholipid syndrome (APS). beta2GPI binds with high affinity to the atherogenic lipoprotein Lp(a) which shares structural homology with plasminogen, a key molecule in the fibrinolytic system. Impaired fibrinolysis has been described in APS. The present work reports the interaction between beta2GPI and Glu-Plasminogen which may explain the recently described proteolytic effect of plasmin on beta2GPI. In the process of Glu-Plasminogen activation, we found an increase in plasmin generation both at fibrin and cellular surface level as a function of the concentration of beta2GPI added, suggesting an important role as a cofactor in the trimolecular complex beta2GPI-Plasminogen-tPA. This phenomenon represents a novel regulatory step both in the positive feedback mechanism for extrinsic fibrinolysis and in antithrombotic regulation. IgG anti-beta2GPI antibodies recognized the beta2GPI at the endothelial surface inducing its activation with an increase of ICAM-I and a decrease in the expression of thrombomodulin favoring a pro-thrombotic state in the vascular endothelium. The interference in the plasmin conversion by anti-beta2GPI antibodies could generate thrombosis as observed in APS.

Nonlysine-analog plasminogen modulators promote autoproteolytic generation of plasmin(ogen) fragments with angiostatin-like activity

We recently discovered several nonlysine-analog conformational modulators for plasminogen. These include SMTP-6, thioplabin B and complestatin that are low molecular mass compounds of microbial origin. Unlike lysine-analog modulators, which increase plasminogen activation but inhibit its binding to fibrin, the nonlysine-analog modulators enhance both activation and fibrin binding of plasminogen. Here we show that some nonlysine-analog modulators promote autoproteolytic generation of plasmin(ogen) derivatives with its catalytic domain undergoing extensive fragmentation (PMDs), which have angiostatin-like anti-endothelial activity. The enhancement of urokinase-catalyzed plasminogen activation by SMTP-6 was followed by rapid inactivation of plasmin due to its degradation mainly in the catalytic domain, yielding PMD with a molecular mass ranging from 68 to 77 kDa. PMD generation was observed when plasmin alone was treated with SMTP-6 and was inhibited by the plasmin inhibitor aprotinin, indicating an autoproteolytic mechanism in PMD generation. Thioplabin B and complestatin, two other nonlysine-analog modulators, were also active in producing similar PMDs, whereas the lysine analog 6-aminohexanoic acid was inactive while it enhanced plasminogen activation. Peptide sequencing and mass spectrometric analyses suggested that plasmin fragmentation was due to cleavage at Lys615-Val616, Lys651-Leu652, Lys661-Val662, Lys698-Glu699, Lys708-Val709 and several other sites mostly in the catalytic domain. PMD was inhibitory to proliferation, migration and tube formation of endothelial cells at concentrations of 0.3-10 microg.mL(-1). These results suggest a possible application of nonlysine-analog modulators in the treatment of cancer through the enhancement of endogenous plasmin(ogen) fragment formation.

Enhancement of plasminogen activation by surfactin C: augmentation of fibrinolysis in vitro and in vivo

The reciprocal activation of plasminogen and prourokinase (pro-u-PA) is an important mechanism in the initiation and propagation of local fibrinolytic activity. We have found that a bacterial lipopeptide compound, surfactin C (3-20 microM), enhances the activation of pro-u-PA in the presence of plasminogen. This effect accompanied increased conversions of both pro-u-PA and plasminogen to their two-chain forms. Surfactin C also elevated the rate of plasminogen activation by two-chain urokinase (tcu-PA) while not affecting plasmin-catalyzed pro-u-PA activation and amidolytic activities of tcu-PA and plasmin. The intrinsic fluorescence of plasminogen was increased, and molecular elution time of plasminogen in size-exclusion chromatography was shortened in the presence of surfactin C. These results suggested that surfactin C induced a relaxation of plasminogen conformation, thus leading to enhancement of u-PA-catalyzed plasminogen activation, which in turn caused feedback pro-u-PA activation. Surfactin C was active in enhancing [125I]fibrin degradation both by pro-u-PA/plasminogen and tcu-PA/plasminogen systems. In a rat pulmonary embolism model, surfactin C (1 mg/kg, i.v.) elevated 125I plasma clot lysis when injected in combination with pro-u-PA. The present results provide first evidence that pharmacological relaxation of plasminogen conformation leads to enhanced fibrinolysis in vivo.

Gelatin-binding region of human matrix metalloproteinase-2: solution structure, dynamics, and function of the COL-23 two-domain construct

Human matrix metalloproteinase-2 (MMP-2) contains an array of three fibronectin type II (FII) modules postulated to interact with gelatin (denatured collagen). Here, we verify that the NMR solution structure of the third FII repeat (COL-3) is similar to that of the second FII repeat (COL-2); characterize its ligand-binding properties; and derive dynamics properties and relative orientation in solution for the two domains of the COL-23 fragment, a construct comprising COL-2 and COL-3 in tandem, with each domain possessing a putative collagen-binding site. Interaction of the synthetic gelatin-like octadecapeptide (Pro-Pro-Gly)(6) (PPG6) with COL-3 is weaker than with COL-2. We found that a synthetic peptide comprising segment 33-42 (peptide 33-42) from the MMP-2 prodomain interacts with COL-3 and, albeit with lower affinity, with COL-2 in a way that mimics PPG6 binding. COL-3 strongly prefers peptide 33-42 over PPG6, which suggests that intramolecular interactions with the prodomain could modulate binding of pro-MMP-2 to its gelatin substrate. In COL-23, the two modules retain their structural individuality and tumble independently. Overall, the NMR data indicate that the relative orientation of the modules in COL-23 is not fixed in solution, that the modules do not interact with one another, and that COL-23 is rather flexible. The binding sites face opposite each other, and their responses to, and normalized affinities for, the longer ligand PPG12 are virtually identical to those of the individual domains for PPG6, thus precluding co- operativity, although they may interact simultaneously with multiple sites of the extracellular matrix.

Understanding the fluorescence changes of human plasminogen when it binds the ligand, 6-aminohexanoate: a synthesis

This work attempts to explain several aspects of the response of plasminogen to 6-aminohexanoate (6-AH). These responses include the overall fluorescent changes that occur when plasminogen binds the ligand, the changes shown by the individual domains when they bind the ligand, and the changes in structure shown by the holoprotein when it binds 6-AH. The results have implications for understanding the physicochemical behavior of all kringle based proteins.

Homologous plasminogen N-terminal and plasminogen-related gene A and B peptides. Characterization of cDNAs and recombinant fusion proteins

The cDNA corresponding to exons 2-4 of the processed human plasminogen (Pgn) gene, encoding the N-terminal peptide domain (NTP), has been cloned, expressed in Escherichia coli as a recombinant protein (r-NTP) containing a hexahistidine tag, and refolded to the native structure that contains two internal cystine bridges. RNA expression of the two Pgn-related genes, PRG A and PRG B, that potentially encode 9-kDa polypeptides having extensive similarity to the NTP has been investigated. Using RNA-based PCR with liver RNA as template, we demonstrate that PRG A encodes a detectable mRNA species. PRG A and PRG B have been found to be transcribed in the liver and yield virtually identical mRNAs. Neither of the PRGs are expressed in a variety of other normal tissues, as determined by Northern blot analysis. Factor-Xa digestion of the tagged r-NTP yields cleavage products which indicates that the expressed r-NTP domain of Pgn is endowed with a flexible conformation. Recombinant PRG B protein (r-PRG B) fused to a hexahistidine tag was purified and analyzed for structural integrity. Preliminary 1H-NMR spectroscopic data for r-NTP and r-PRG B indicate relatively fast amide 1H-2H exchange in 2H2O and close conformational characteristics for the two homologous polypeptides. Far ultraviolet-CD spectra for r-NTP and r-PRG B at pH 7.0 indicate similar defined secondary structure content for both domains, with 13-17% alpha-helix and 24-27% antiparallel beta-sheet. The fact that two transcriptionally active genes encode almost identical polypeptides supports the hypothesis that the Pgn NTP, together with the putative polypeptides encoded by the PRGs, may serve an important function, such as controlling the conformation of Pgn and thus its susceptibility to tissue activators.

Structural/functional properties of the Glu1-HSer57 N-terminal fragment of human plasminogen: conformational characterization and interaction with kringle domains

The Glu1-Val79 N-terminal peptide (NTP) domain of human plasminogen (Pgn) is followed by a tandem array of five kringle (K) structures of approximately 9 kDa each. K1, K2, K4, and K5 contain each a lysine-binding site (LBS). Pgn was cleaved with CNBr and the Glul-HSer57 N-terminal fragment (CB-NTP) isolated. In addition, the Ile27-Ile56 peptide (L-NTP) that spans the doubly S-S bridged loop segment of NTP was synthesized. Pgn kringles were generated either by proteolytic fragmentation of Pgn (K4, K5) or via recombinant gene expression (rK1, rK2, and rK3). Interactions of CB-NTP with each of the Pgn kringles were monitored by 1H-NMR at 500 MHz and values for the equilibrium association constants (Ka) determined: rK1, Ka approximately 4.6 mM(-1); rK2, Ka approximately 3.3 mM(-1); K4, Ka approximately 6.2 mM-'; K5, K, 2.3 mM(-1). Thus, the lysine-binding kringles interact with CB-NTP more strongly than with Nalpha-acetyl-L-lysine methyl ester (Ka < 0.6 mM(-l), which reveals specificity for the NTP. In contrast, CB-NTP does not measurably interact with rK3. which is devoid of a LBS. CB-NTP and L-NTP 1H-NMR spectra were assigned and interproton distances estimated from 1H-1H Overhauser (NOESY) experiments. Structures of L-NTP and the Glul-Ile27 segment of CB-NTP were computed via restrained dynamic simulated annealing/energy minimization (SA/EM) protocols. Conformational models of CB-NTP were generated by joining the two (sub)structures followed by a round of constrained SA/EM. Helical turns are indicated for segments 6-9, 12-16, 28-30, and 45-48. Within the Cys34-Cys42 loop of L-NTP, the structure of the Glu-Glu-Asp-Glu-Glu39 segment appears to be relatively less defined, as is the case for the stretch containing Lys5O within the Cys42-Cys54 segment, consistent with the latter possibly interacting with kringle domains in intact Glul-Pgn. Overall, the CB-NTP and L-NTP fragments are of low regular secondary structure content-as indicated by UV-CD spectra- and exhibit fast amide 1H-2H exchange in 2H2O, suggestive of high flexibility.