Proton magnetic resonance study of kringle 1 from human plasminogen. Insights into the domain structure

Recombinant gene expression and 1H NMR characteristics of the kringle (2 + 3) supermodule: spectroscopic/functional individuality of plasminogen kringle domains

The plasminogen kringle 2 (K2HPg) and kringle 3 (K3HPg) modules occur in tandem within the polypeptide segment that affords the heavy chain of plasmin. The K2HPg and K3HPg are unique among the plasminogen kringle domains in that they also are linked to each other via the Cys169-Cys297 (Cys4 of K2HPg to Cys43 of K3HPg, kringle numbering convention) disulfide bridge, thus generating a K2HPg-K3HPg "supermodule". The kringle (2 + 3) sequence of human plasminogen (r-EE[K2HPgK3HPg]DS) was expressed in Escherichia coli, using an expression vector containing the phage T5 promoter/operator N250PSN250P29 and the codons for an N-terminal hexahistidine tag to ensure the isolation of the recombinant protein by affinity chromatography on Ni(2+)-nitrilotriacetic acid/agarose under denaturing and reducing conditions. Kringle (2 + 3) was refolded in the presence of glutathione redox buffer. By taking advantage of the lysine affinity of kringle 2, the protein was purified by affinity chromatography on lysine-Bio-Gel. Recombinant kringle (2 + 3) was identified by amino acid composition, N-terminal sequence and mass determination. The 1H NMR spectrum shows that the intact r-K2HPgK3HPg is properly folded. By reference to spectra of the individual kringles, r-K2HPg and r-K3HPg, resonances of the K2HPg and K3HPg components in the spectrum of the intact r-K2HPgK3HPg can be readily distinguished. The strictly conserved Leu46 residue (kringle residue number convention) yields delta-methyl signals that are characteristic for K2HPg and K3HPg, exhibiting chemical shifts of -0.87 and -0.94 ppm, respectively, which are distinct from those of K1HPg, K4HPg, and K5HPg, (-1.04 to -1.05 ppm). Thus, the high-field Leu46 signals from K2HPg and K3HPg are well resolved from those of other kringles and can be identified unambiguously in spectra of the K1HPgK2HPgK3HPg elastolytic fragment of plasminogen as well as in spectra of Glu-plasminogen. Overall, r-K2HPgK3HPg exhibits broader resonance line widths than does the K1HPg component, consistent with a lesser mobility of the K2HPgK3HPg segment within the K1HPgK2HPgK3HPg fragment, a reflection of the extra structural constraint imposed by the disulfide bridge linking K2HPg to K3HPg. The ligand 6-aminohexanoic acid (6-AHA), which is known to interact with r-K2HPg but not with r-K3HPg, selectively perturbs K2 aromatic signals in the intact r-K2HPgK3HPg spectrum while leaving K3 resonances largely unaffected. Association constant (K(a)) values for 6-AHA determined from 1H NMR ligand titration experiments yield K(a) approximately 2.2 +/- 0.3 mM(-1) for the intact r-K2HPgK3HPg, comparable to K(a) approximately 2.3 +/- 0.2 mM(-1) determined for the isolated r-K2HPg, which demonstrates that the interactions of 6-AHA with the K2HPg ligand-binding site are not significantly affected by the neighboring K3HPg domain within the intact r-K2HPgK3HPg supermodule.

Solution structure of the epsilon-aminohexanoic acid complex of human plasminogen kringle 1

The solution structure of the human plasminogen kringle 1 domain complexed to the antifibrinolytic drug 6-aminohexanoic acid (epsilon Ahx) was obtained on the basis of 1H-NMR spectroscopic data and dynamical simulated annealing calculations. Two sets of structures were derived starting from (a) random coil conformations and (b) the (mutated) crystallographic structure of the homologous prothrombin kringle 1. The two sets display essentially the same backbone folding (pairwise root-mean-square deviation, 0.15 nm) indicating that, regardless of the initial structure, the data is sufficient to locate a conformation corresponding to an essentially unique energy minimum. The conformations of residues connected to prolines were localized to energetically preferred regions of the Ramachandran map. The Pro30 peptide bond is proposed to be cis. The ligand-binding site of the kringle 1 is a shallow cavity composed of Pro33, Phe36, Trp62, Tyr64, Tyr72 and Tyr74. Doubly charged anionic and cationic centers configured by the side chains of Asp55 and Asp57, and Arg34 and Arg71, respectively, contribute to anchoring the zwitterionic epsilon Ahx molecule at the binding site. The ligand exhibits closer contacts with the kringle anionic centers (approximately 0.35 nm average O...H distance between the Asp55/Asp57 carboxylate and ligand amino groups) than with the cationic ones (approximately 0.52 nm closest O...H distances between the ligand carboxylate and the Arg34/Arg71 guanidino groups). The epsilon Ahx hydrocarbon chain rests flanked by Pro33, Tyr64, Tyr72 and Tyr74 on one side and Phe36 on the other. Dipolar (Overhauser) connectivities indicate that the ligand aliphatic moiety establishes close contacts with the Phe36 and Trp62 aromatic rings. The computed structure suggests that the epsilon Ahx molecule adopts a kinked conformation when complexed to kringle 1, effectively shortening its dipole length to approximately 0.65 nm.

1H-NMR assignments and secondary structure of human plasminogen kringle 1

The 1H-NMR spectrum of the kringle 1 domain of human plasminogen complexed with 6-aminohexanoic acid, an antifibrinolytic drug, has been assigned. Elements of secondary structure have been identified on the basis of sequential, medium and long-range dipolar interactions, back-bone amide spin-spin couplings (3JHN-H alpha) and 1H-2H exchange rates. The kringle contains scarcely any repetitive secondary structure: eight reverse turns and two short beta-sheets. These comprise 40% and 12% of the domain, respectively. No alpha-helix was found. An aromatic cluster formed by His31, Phe36, Trp62, Phe64, Tyr72 and Tyr74 is indicated by several inter-residue Overhauser connectivities. Contacts between the methyl groups of Leu46 and the side chains of Phe36, Trp62 and Trp25 are observed. A second hydrophobic cluster formed by Tyr9, Ile77 and Leu78 is also indicated. A comparison of secondary structure elements among plasminogen kringles 1 and 4 and tissue-type plasminogen activator kringle 2 suggests that there is variability in the position and number of reverse turns on going from one kringle to another; however, the beta-sheets are conserved among the homologs.

Expression, purification and characterization of the recombinant kringle 2 and kringle 3 domains of human plasminogen and analysis of their binding affinity for omega-aminocarboxylic acids

The kringle 2 (E161T/C162S/EEE[K2HPg/C169S]TT) and the kringle 3 (TYQ[K3HPg]DS) domains of human plasminogen (HPg) were expressed in Escherichia coli in an expression vector with the phage T5 promotor/operator element N250PSN250P29 and the cDNA sequence for a hexahistidine tail to facilitate the isolation of the recombinant protein. A coagulation factor Xa (FXa)-sensitive cleavage site was introduced to remove the N-terminal histidine tag. In r-K2, mutations E161T and C162S were introduced to enhance the FXa cleavage yield and C169S to replace the cysteine residue, participating in the inter-kringle disulfide bridge between kringles 2 and 3. Recombinant proteins were isolated by affinity chromatography on Ni(2+)-nitrilotriacetic acid/agarose and refolded under denaturing and reducing conditions followed by a non-denaturing and oxidising environment. The free thiol group in position 297 in r-K3 was selectively alkylated with iodoacetamide. The hexahistidine tail was successfully removed with FXa. The N-terminal sequence, the amino acid composition and the molecular mass analyses are in agreement with the expected data. The correct arrangement of the disulfide bonds was verified by sequence analysis of the corresponding thermolytic and subtilisin fragments. r-K2 exhibits weak binding to lysine-Bio-Gel. The weak binding affinity of r-K2 for omega-aminocarboxylic acids is confirmed by intrinsic fluorescence titration with 6-aminohexanoic acid (NH2C5COOH) indicating a Kd of approximately 401 microM. In contrast, r-K3 seems to be devoid of a binding affinity for omega-aminocarboxylic acids. Considering earlier determined Kd values of kringle 1, kringle 4 and kringle 5, the binding affinity of HPg kringle domains for NH2C5COOH is proposed to decrease in the following order, kringle 1 > kringle 4 > kringle 5 > kringle 2 > kringle 3.

Fluorescence spectroscopic analysis of ligand binding to kringle 1 + 2 + 3 and kringle 1 fragments from human plasminogen

The ligand binding of kringle 1 + 2 + 3 and kringle 1 from human plasminogen has been investigated by fluorescence spectroscopy. Analysis of fluorescence titration of kringle 1 + 2 + 3 with 6-aminohexanoic acid shows that this fragment, besides the high-affinity lysine-binding site with Kd = 2.9 microM, contains two additional lysine-binding sites which differ in binding strength (Kd = 28 microM and Kd = 220 microM). This strongly suggests the existence of a lysine-binding site in each of the first three kringles. 6-Aminohexanoic acid, pentylamine, pentanoic acid and arginine were used for investigation of the ligand specificity of isolated kringle 1 prepared by pepsin hydrolysis of kringle 1 + 2 + 3. It has been established that kringle 1 has high affinity to 6-aminohexanoicacid, pentylamine and arginine (Kd values are 3.2 microM, 4.8 microM and 4.3 microM, respectively). At the same time pentanoic acid did not bind with kringle 1. These facts indicate, firstly, a broad ligand specificity of kringle 1 and, secondly, the paramount importance of the positively charged group of the ligand for its interaction with lysine-binding site of this kringle.

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)

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.

The effect of polymerised fibrin on the catalytic activities of one-chain tissue-type plasminogen activator as revealed by an analogue resistant to plasmin cleavage

A one-chain recombinant tissue-type plasminogen activator (EC 2.4.31.-) (tPA) analogue was constructed in which Arg-275 of the activation site was changed to Gly by site-directed mutagenesis. This analogue, tPA-Gly275, was very resistant to plasmin (EC 2.4.21.5) cleavage. It has been used to gain information about the activity of the uncleaved one-chain tPA form, also when plasmin is generated as a result of a plasminogen activation reaction. The amidolytic activity of tPA-Gly275 with less than Glu-Gly-Arg-pNA was investigated and compared to that of one-chain and two-chain wild-type recombinant tPA. A small but significant intrinsic amidolytic activity was observed with the analogue as well as the wild-type one-chain tPA form. However, it was much lower than that of two-chain tPA. Polymerised fibrin enhanced the amidolytic activity of both one-chain tPA forms but not of two-chain tPA. Measurements of the plasminogen activation kinetics in the absence of fibrin revealed that tPA-Gly275 possessed a significant intrinsic activity. However, it was 30-fold lower than that of two-chain tPA. Addition of polymerised fibrin profoundly enhanced the plasminogen activation rate of both tPA-Gly275 and wild-type one- and two-chain tPA to approximately the same maximal level. The results were interpreted to mean that fibrin binding can induce an activated state of the intact tPA one-chain form.

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.