Domain structure, stability and interactions in streptokinase

Plasminogen binding by oral streptococci from dental plaque and inflammatory lesions

Plasminogen binding by bacteria is a virulence factor important for the entry and dissemination of bacteria in the body. A wide variety of bacteria bind plasminogen, including both organisms causing disease and components of the normal oral flora. The purpose of this study was to examine the characteristics of plasminogen binding by six clinical isolates of oral streptococci from both dental plaque and inflammatory lesions. All the strains bound plasminogen with approximately the same affinity, and binding was specific and lysine-dependent as evidenced by its inhibition by epsilon-aminocaproic acid. All of the test strains were capable of activating bound plasminogen to plasmin without the addition of a plasminogen activator, and subsequent analysis revealed the presence of streptokinase in all strains. However, the streptococci exhibited fibrinolytic activity only in the presence of plasminogen and this could be inhibited by the addition of epsilon-aminocaproic acid. SDS-PAGE and 2D gel electrophoresis coupled with plasminogen ligand blotting showed that only a subset of the total proteins (2-15) were involved in the binding of plasminogen. Partial identification of the binding proteins revealed that four glycolytic enzymes, enolase, phosphoglycerate kinase, glyceraldehyde-3-phosphate dehydrogenase and phosphoglycerate mutase, were predominant in binding plasminogen. The binding of plasminogen by bacteria from pus did not differ from that of the strains from supragingival plaque. The findings illustrate how apparently innocuous commensal bacteria are capable of utilizing a mechanism that is generally regarded as being of importance to pathogenicity and suggest an additional role of plasminogen binding.

Isolation and Identification of an Endophytic Strain EJS-3 Producing Novel Fibrinolytic Enzymes

An endophytic strain EJS-3, which produces a novel fibrinolytic enzyme, was screened from root tissue of Stemona japonica (Blume) Miq, a chinese traditional medicine. This strain was identified as Paenibacillus polymyxa (DQ120522) by morphological, physiological, and biochemical tests and 16S rRNA gene sequence analysis. Two serine-type fibrinolytic enzymes with a relative molecular weight about 118 and 49 kDa, respectively, which are larger than known fibrinolytic enzymes, were found by the SDS-fibrin zymogram or by fibrin-inhibitor zymography gels. No work on P. polymyxa-producing fibrinolytic enzymes has been reported.

Enhancement of recombinant streptokinase production in Lactococcus lactis by suppression of acid tolerance response

Lactococcus lactis is a potential host for production of recombinant proteins, especially of therapeutic importance. However, in glucose-grown cultures, lowering of pH due to accumulation of lactic acid and the concomitant induction of acid tolerance response (ATR) may affect the recombinant protein produced. In this work, we have analyzed the effect of culture pH and the associated ATR on production of recombinant streptokinase. Streptokinase gene was cloned and expressed as a secretory protein in L. lactis under the control of P170 promoter. It was found to undergo degradation to form inactive products leading to low productivity. The extent of degradation and productivity of streptokinase was greatly influenced by the development of ATR, which was dependent on the pH of the culture and initial phosphate concentration of the medium. It was found that high pH and high initial phosphate concentration leads to suppression of ATR and this results in at least 2.5-fold increase in streptokinase productivity and significant decrease in degradation of streptokinase.

Streptokinase—a clinically useful thrombolytic agent

A failure of hemostasis and consequent formation of blood clots in the circulatory system can produce severe outcomes such as stroke and myocardial infraction. Pathological development of blood clots requires clinical intervention with fibrinolytic agents such as urokinase, tissue plasminogen activator and streptokinase. This review deals with streptokinase as a clinically important and cost-effective plasminogen activator. The aspects discussed include: the mode of action; the structure and structure-function relationships; the structural modifications for improving functionality; recombinant streptokinase; microbial production; and recovery of this protein from crude broths.

Functional roles of streptokinase C-terminal flexible peptide in active site formation and substrate recognition in plasminogen activation

The bacterial protein streptokinase (SK) activates human plasminogen (Pg) into the fibrinolytic protease plasmin (Pm). Roughly 40 residues from the SK C-terminal domain are mobile in the crystal structure of SK complexed with the catalytic domain of Pm, and the functions of this C-tail remain elusive. To better define its roles in Pg activation, we constructed and characterized three C-terminal truncation mutants containing SK residues 1-378, 1-386, and 1-401, respectively. They exhibit gradually reduced amidolytic activity and Pg-activator activity, as well as marginally decreased binding affinity toward Pg, as more of the C-terminus is deleted. As compared with full-length SK, the shortest construct, SK(1-378), exhibits an 80% decrease in amidolytic activity (k(cat)/K(M)), an 80% decrease in Pg-activator activity, and a 30% increase in the dissociation constant toward the Pg catalytic domain. The C-terminal truncation mutations did not attenuate the resistance of the SK-Pm complex to alpha(2)-antiplasmin. Attempts at using a purified C-tail peptide to rescue the activity loss of the truncation mutants failed, suggesting that the integrity of the SK C-terminal peptide is important for the full function of SK.

Thrombolytic therapy

The therapeutic use of thrombolytic agents is the natural result of the increasing understanding of the pathophysiologic mechanisms underlying normal and deranged thrombosis and fibrinolysis. Plasminogen activators capable of increasing the production of plasmin exhibit considerable efficacy in the treatment of a variety of arterial and venous thrombotic disorders. The ideal thrombolytic agent has yet to be developed but the desired clinical result of rapid opening of the thrombosed vessel without reocclusion, without activation of systemic fibrinogenolysis, and without a risk of hemorrhage is well defined. Clinical studies clearly demonstrate that the addition of a variety of adjunctive agents to the available thrombolytics enhances benefit without inordinate risk. The addition of intravascular angioplasty and stenting to thrombolysis increases the potential long-term benefit. Newer thrombolytic agents and new protocols for the use of existing therapies offer the promise of saving many who would otherwise succumb to coronary or cerebral arterial thrombosis or to venous thromboembolism.

Affinity panning of peptide libraries using anti-streptokinase monoclonal antibodies: selection of an inhibitor of plasmin(ogen) active site

To select sequences complementary to their binding sites, two anti-streptokinase (SK) monoclonal antibodies (mAbs), A4.5 and A5.5, were used in biopanning of 15-mer and hexamer phage-displayed peptide libraries, respectively. mAb A4.5 inhibits the catalytic activity of streptokinase-plasminogen activator complex (SKPAC), the binding of plasminogen to SK and the binding of human anti-SK polyclonal Abs to SK. All clones selected from the 15-mer peptide library by mAb A4.5 had identical nucleotide and amino acid sequences, RSVYRCSPFVGCWFG. An 11-mer peptide (peptide A4.5, YRCSPFVGCWF) derived from this sequence inhibited the binding of mAb A4.5 and human anti-SK polyclonal Abs to SK as well as the catalytic activity of both SKPAC and plasmin. The binding of the second mAb (mAb A5.5) to SK is lost upon interaction of SK with plasminogen, suggesting that sequences selected by this mAb are likely associated with the C-terminal cleavage site of SK. Biopanning of a hexamer peptide library with mAb A5.5 selected the sequence RYLQDY that is homologous to residues 324-328, adjacent to one possible C-terminal cleavage site in SK. A 10-mer synthetic peptide (LDFRDLYDPR) corresponding to residues 321-330 in SK specifically inhibited the binding of mAb A5.5 to SK. The selection and characterization of these two peptides enhances our understanding of SK structure, maps an antigenic epitope, and identifies a peptide inhibitor of plasminogen activation.

Unfolding and aggregation during the thermal denaturation of streptokinase

The thermal denaturation of streptokinase from Streptococcus equisimilis (SK) together with that of a set of fragments encompassing each of its three domains has been investigated using differential scanning calorimetry (DSC). Analysis of the effects of pH, sample concentration and heating rates on the DSC thermograms has allowed us to find conditions where thermal unfolding occurs unequivocally under equilibrium. Under these conditions, pH 7.0 and a sample concentration of less than approximately 1.5 mg x mL(-1), or pH 8.0, the heat capacity curves of intact SK can be quantitatively described by three independent two-state transitions, each of which compares well with the two-state transition observed for the corresponding isolated SK domain. The results indicate that each structural domain of SK behaves as a single cooperative unfolding unit under equilibrium conditions. At pH 7.0 and high sample concentration, or at pH 6.0 at any concentration investigated, the thermal unfolding of domain A was accompanied by the time-dependent formation of aggregates of SK. This produces a severe deformation of the DSC curves, which become concentration dependent and kinetically controlled, and thus precludes their proper analysis by standard deconvolution methods. A simple model involving time-dependent, high-order aggregation may account for the observed effects. Limited-proteolysis experiments suggest that in the aggregates the N-terminal segment 1-63 and the whole of SK domain C are at least partially structured, while domain B is highly unstructured. Unfolding of domain A, under conditions where the N-terminal segment 1-63 has a high propensity for beta sheet structure and a partially formed hydrophobic core, gives rise to rapid aggregation. It is likely that this region is able to act as a nucleus for the aggregation of the full-length protein.

Multidomain structure of a recombinant streptokinase. A differential scanning calorimetry study

The temperature dependence of the heat capacity function of a recombinant streptokinase (rSK) has been studied by high-sensitivity differential scanning microcalorimetry and circular dichroism as a function of pH in low- and high-ionic strength buffers. At low ionic strength it is found that this protein, between pH 7 and 10, undergoes four reversible and independent two-state transitions during its unfolding, suggesting the existence of four domains in the native structure of the protein. This result reconciles previous conflicting reports about the number of domains of this protein obtained by differential scanning calorimetry and small-angle X-ray scattering. The number of two-state transitions decreases when the pH of the medium is decreased, without noticeable changes in its circular dichroism spectrum. A plausible localization of the four domains in the streptokinase sequences is proposed and their thermodynamic parameters are given. Increase of ionic strength to 200 mM NaCl affects positively the protein stability and confirms the existence of four reversible two-state transitions. Above 200 mM NaCl the protein stability decreases, resulting in low percentage of reversibility, and even irreversible transitions.

Analysis of the interactions between streptokinase domains and human plasminogen

The contrasting roles of streptokinase (SK) domains in binding human Glu1-plasminogen (Plg) have been studied using a set of proteolytic fragments, each of which encompasses one or more of SK's three structural domains (A, B, C). Direct binding experiments have been performed using gel filtration chromatography and surface plasmon resonance. The latter technique has allowed estimation of association and dissociation rate constants for interactions between Plg and intact SK or SK fragments. Each of the SK fragments that contains domain B (fragments A2-B-C, A2-B, B-C, and B) binds Plg with similar affinity, at a level approximately 100- to 1,000-fold lower than intact SK. Experiments using 10 mM 6-aminohexanoic acid or 50 mM benzamidine demonstrate that either of these two lysine analogues abolishes interaction of domain B with Plg. Isolated domain C does not show detectable binding to Plg. Moreover, the additional presence of domain C within other SK fragments (B-C and A2-B-C) does not alter significantly their affinities for Plg. In addition, Plg-binding by a noncovalent complex of two SK fragments that contains domains A and B is similar to that of domain B. By contrast, species containing domain B and both domains A and C (intact SK and the two-chain complex A1 x A2-B-C) show a significantly higher affinity for Plg, which could not be completely inhibited by saturating amounts of 6-AHA. These results show that SK domain B interacts with Plg in a lysine-dependent manner and that although domains A and C do not appear independently to possess affinity for Plg, they function cooperatively to establish the additional interactions with Plg to form an efficient native-like Plg activator complex.

Crystal structure of the catalytic domain of human plasmin complexed with streptokinase

Streptokinase is a plasminogen activator widely used in treating blood-clotting disorders. Complexes of streptokinase with human plasminogen can hydrolytically activate other plasminogen molecules to plasmin, which then dissolves blood clots. A similar binding activation mechanism also occurs in some key steps of blood coagulation. The crystal structure of streptokinase complexed with the catalytic unit of human plasmin was solved at 2.9 angstroms. The amino-terminal domain of streptokinase in the complex is hypothesized to enhance the substrate recognition. The carboxyl-terminal domain of streptokinase, which binds near the activation loop of plasminogen, is likely responsible for the contact activation of plasminogen in the complex.