Converting tissue type plasminogen activator into a zymogen. Important role of Lys156

A low bleeding risk thrombolytic agent: citPA5

AIMS

Alteplase is a cornerstone thrombolytic agent in clinical practice, but presents a potential bleeding risk. Stroke patients need pre-screening to exclude hemorrhagic stroke before using Alteplase. In this study, we develop a new thrombolytic agent citPA5, characterized by an enhanced safety profile and minimal bleeding tendency.

METHODS AND RESULTS

A clot lysis agent, named citPA5, is developed based on rtPA with point mutations to completely suppress its proteolytic activity in the absence of fibrin. In the presence of fibrin, citPA5 exhibited significantly higher fibrinolytic activity (a 15.8-fold increase of kcat/Km). Furthermore, citPA5 showed resistance to endogenous fibrinolysis inhibitor, PAI-1, resulting in enhanced potency. In a series of safety evaluation experiments, including thrombelastography (TEG) assay, mice tail bleeding assay, and a murine intracerebral hemorrhage (ICH) model, citPA5 did not cause systemic bleeding or worsen intracerebral hemorrhage compared to Alteplase. This highlights the low risk of bleeding associated with citPA5. Finally, we found that citPA5 effectively improved cerebral blood flow and reduced infarct volume in a carotid embolism-induced stroke (CES) model.

CONCLUSIONS

This clot lysis agent, citPA5, not only exhibits a low risk of bleeding but also demonstrates highly effective thrombolysis capabilities. As a result, citPA5 shows great potential for administration prior to the classification of stroke types, making it possible for use in ambulances at the onset of stroke when symptoms are identified. The findings presented in this study also suggest that this strategy could be applied to develop a new generation of fibrinolytic drugs that offer greater safety and specificity in targeting fibrin.

Structural Biology and Protein Engineering of Thrombolytics

Myocardial infarction and ischemic stroke are the most frequent causes of death or disability worldwide. Due to their ability to dissolve blood clots, the thrombolytics are frequently used for their treatment. Improving the effectiveness of thrombolytics for clinical uses is of great interest. The knowledge of the multiple roles of the endogenous thrombolytics and the fibrinolytic system grows continuously. The effects of thrombolytics on the alteration of the nervous system and the regulation of the cell migration offer promising novel uses for treating neurodegenerative disorders or targeting cancer metastasis. However, secondary activities of thrombolytics may lead to life-threatening side-effects such as intracranial bleeding and neurotoxicity. Here we provide a structural biology perspective on various thrombolytic enzymes and their key properties: (i) effectiveness of clot lysis, (ii) affinity and specificity towards fibrin, (iii) biological half-life, (iv) mechanisms of activation/inhibition, and (v) risks of side effects. This information needs to be carefully considered while establishing protein engineering strategies aiming at the development of novel thrombolytics. Current trends and perspectives are discussed, including the screening for novel enzymes and small molecules, the enhancement of fibrin specificity by protein engineering, the suppression of interactions with native receptors, liposomal encapsulation and targeted release, the application of adjuvants, and the development of improved production systems.

The role of endogenous tissue-type plasminogen activator in neuronal survival after ischemic stroke: friend or foe?

Endogenous protease tissue-type plasminogen activator (tPA) has highly efficient fibrinolytic activity and its recombinant variants alteplase and tenecteplase are established as highly effective thrombolytic drugs for ischemic stroke. Endogenous tPA is constituted of five functional domains through which it interacts with a variety of substrates, binding proteins and receptors, thus having enzymatic and cytokine-like effects to act on all cell types of the brain. In the past 2 decades, numerous studies have explored the clinical relevance of endogenous tPA in neurological diseases, especially in ischemic stroke. tPA is released from many cells within the brain parenchyma exposed to ischemia conditions in vitro and in vivo, which is believed to control neuronal fate. Some studies proved that tPA could induce blood-brain barrier disruption, neural excitotoxicity and inflammation, while others indicated that tPA also has anti-excitotoxic, neurotrophic and anti-apoptotic effects on neurons. Therefore, more work is needed to elucidate how tPA mediates such opposing functions that may amplify tPA from a therapeutic means into a key therapeutic target in endogenous neuroprotection after stroke. In this review, we summarize the biological characteristics and pleiotropic functions of tPA in the brain. Then we focus on possible hypotheses about why and how endogenous tPA mediates ischemic neuronal death and survival. Finally, we analyze how endogenous tPA affects neuron fate in ischemic stroke in a comprehensive view.

Neuroserpin Differentiates Between Forms of Tissue Type Plasminogen Activator via pH Dependent Deacylation

Tissue-type plasminogen activator (t-PA), initially characterized for its critical role in fibrinolysis, also has key functions in both physiologic and pathologic processes in the CNS. Neuroserpin (NSP) is a t-PA specific serine protease inhibitor (serpin) found almost exclusively in the CNS that regulates t-PA's proteolytic activity and protects against t-PA mediated seizure propagation and blood-brain barrier disruption. This report demonstrates that NSP inhibition of t-PA varies profoundly as a function of pH within the biologically relevant pH range for the CNS, and reflects the stability, rather than the formation of NSP: t-PA acyl-enzyme complexes. Moreover, NSP differentiates between the zymogen-like single chain form (single chain t-PA, sct-PA) and the mature protease form (two chain t-PA, tct-PA) of t-PA, demonstrating different pH profiles for protease inhibition, different pH ranges over which catalytic deacylation occurs, and different pH dependent profiles of deacylation rates for each form of t-PA. NSP's pH dependent inhibition of t-PA is not accounted for by differential acylation, and is specific for the NSP-t-PA serpin-protease pair. These results demonstrate a novel mechanism for the differential regulation of the two forms of t-PA in the CNS, and suggest a potential specific regulatory role for CNS pH in controlling t-PA proteolytic activity.

Inhibition of PAI-1 antiproteolytic activity against tPA by RNA aptamers

Plasminogen activator inhibitor-1 (PAI-1; SERPINE1) inhibits the plasminogen activators: tissue-type plasminogen activator (tPA) and urokinase-type plasminogen activator (uPA). Elevated levels of PAI-1 have been correlated with an increased risk for cardiovascular disease. Pharmacologically suppressing PAI-1 might prevent, or successfully treat PAI-1 related vascular diseases. This can potentially be accomplished by using small RNA molecules (aptamers). This study's goal is to develop RNA aptamers to a region of PAI-1 that will prevent the ability of PAI-1 to interact with the plasminogen activators. The aptamers were generated through a systematic evolution of ligands via exponential enrichment approach that ensures the creation of RNA molecules that bind to our target protein, PAI-1. In vitro assays were used to determine the effect of these aptamers on PAI-1's inhibitory activity. Three aptamers that bind to PAI-1 with affinities in the nanomolar range were isolated. The aptamer clones R10-4 and R10-2 inhibited PAI-1's antiproteolytic activity against tPA and disrupted PAI-1's ability to form a stable covalent complex with tPA. Increasing aptamer concentrations correlated positively with an increase in cleaved PAI-1. To the best of our knowledge, this is the first report of RNA molecules that inhibit the antiproteolytic activity of PAI-1.

Effects of low frequency ultrasound on some properties of fibrinogen and its plasminolysis

Background

Pharmacological thrombolysis with streptokinase, urokinase or tissue activator of plasminogen (t-PA), and mechanical interventions are frequently used in the treatment of both arterial and venous thrombotic diseases. It has been previously reported that application of ultrasound as an adjunct to thrombolytic therapy offers unique potential to improve effectiveness. However, little is known about effects of the ultrasound on proteins of blood coagulation and fibrinolysis. Here, we investigated the effects of the ultrasound on fibrinogen on processes of coagulation and fibrinogenolysis in an in vitro system.

Results

Our study demonstrated that low frequency high intensity pulse ultrasound (25.1 kHz, 48.4 W/cm2, duty 50%) induced denaturation of plasminogen and t-PA and fibrinogen aggregates formation in vitro. The aggregates were characterized by the loss of clotting ability and a greater rate of plasminolysis than native fibrinogen. We investigated the effect of the ultrasound on individual proteins. In case of plasminogen and t-PA, ultrasound led to a decrease of the fibrinogenolysis rate, while it increased the fibrinogenolysis rate in case of fibrinogen. It has been shown that upon ultrasound treatment of mixture fibrinogen or fibrin with plasminogen, t-PA, or both, the rate of proteolytic digestion of fibrin(ogen) increases too. It has been shown that summary effect on the fibrin(ogen) proteolytic degradation under the conditions for combined ultrasound treatment is determined exclusively by effect on fibrin(ogen).

Conclusions

The data presented here suggest that among proteins of fibrinolytic systems, the fibrinogen is one of the most sensitive proteins to the action of ultrasound. It has been shown in vitro that ultrasound induced fibrinogen aggregates formation, characterized by the loss of clotting ability and a greater rate of plasminolysis than native fibrinogen in different model systems and under different mode of ultrasound treatment. Under ultrasound treatment of plasminogen and/or t-PA in the presence of fibrin(ogen) the stabilizing effect fibrin(ogen) on given proteins was shown. On the other hand, an increase in the rate of fibrin(ogen) lysis was observed due to both the change in the substrate structure and promoting of the protein-protein complexes formation.

Underlying mechanism of ASIC1a involved in acidosis-induced cytotoxicity in rat C6 glioma cells

AIM

To investigate the underlying mechanism of acid-sensing ion channel (ASIC) 1a involved in the acidosis-induced cytotoxicity of rat C6 glioma cells.

METHODS

The stable ASIC1a-silenced C6 cells built with the RNA interference technology were confirmed by RT-PCR and Western blot analysis. Intracellular calcium ([Ca2+]i) in both the wild-type rat C6 glioma cells and the ASIC1a-silenced C6 cells were analyzed before and after acid application/exposure with the calcium imaging experiment.

RESULTS

The rapid extracellular pH drop induced the increase of [Ca2+]i in the wild-type C6 cells, but not in the ASIC1a-silenced C6 cells. During the prolonged acid exposure, [Ca2+]i was lower in the ASIC1a-silenced C6 cells than that in the control cells.

CONCLUSION

The resultant toxicity of [Ca2+]i might contribute to the acidosis-induced cytotoxicity.

Prediction of in vitro response to interferon-alpha in renal cell carcinoma cell lines

We analyzed the correlation between interferon-alpha (IFNalpha) response and gene expression profiles to predict IFNalpha sensitivity and identified key molecules regulating the IFNalpha response in renal cell carcinoma (RCC) cell lines. To classify eight RCC cell lines of the SKRC series into three subgroups according to IFNalpha sensitivity, that is, sensitive, resistant and intermediate group, responses to IFNalpha (300-3000 IU/mL) were quantified by WST-1 assay. Microarray, followed by supervised hierarchical clustering analysis, was applied to selected genes according to IFNalpha sensitivity. In order to find alteration of expression profiles induced by IFNalpha, sequential microarray analyses were performed at 3, 6, and 12 h after IFNalpha treatment of RCC cell lines and mRNA expression level was confirmed using quantitative real time polymerase chain reaction. According to the sequential microarray analysis between IFNalpha-sensitive and -resistant line, seven genes were selected as candidates for IFNalpha-sensitivity-related genes in RCC cell lines. Among these seven genes, we further developed a model to predict tumor inhibition with four genes, that is, adipose differentiation-related protein, microphthalmia associated transcription factor, mitochondrial tumor suppressor 1, and troponin T1 using multiple linear regression analysis (coefficient=0.948, P=0.0291) and validated the model using other RCC cell lines including six primary cultured RCC cells. The expression levels of the combined selected genes may provide predictive information on the IFNalpha response in RCC. Furthermore, the IFNalpha response to RCC might be modulated by regulation of the expression level of these molecules.

Separate elements are required for ligand-dependent and -independent internalization of metastatic potentiator CXCR4

The C-terminal cytoplasmic domain of the metastatic potentiator CXCR4 regulates its function and spatiotemporal expression. However, little is known about the mechanism underlying constitutive internalization of CXCR4 compared to internalization mediated by its ligand, stromal cell-derived factor-1 alpha (SDF-1alpha)/CXCL12. We established a system to analyze the role of the CXCR4 cytoplasmic tail in steady-state internalization using the NP2 cell line, which lacks endogenous CXCR4 and SDF-1alpha. Deleting more than six amino acids from the C-terminus dramatically reduced constitutive internalization of CXCR4. Alanine substitution mutations revealed that three of those amino acids Ser(344) Glu(345) Ser(346) are essential for efficient steady-state internalization of CXCR4. Mutating Glu(345) to Asp did not disrupt internalization, suggesting that the steady-state internalization motif is S(E/D)S. When responses to SDF-1alpha were tested, cells expressing CXCR4 mutants lacking the C-terminal 10, 14, 22, 31 or 44 amino acids did not show downregulation of cell surface CXCR4 or the cell migration induced by SDF-1alpha. Interestingly, however, we identified two mutants, one with E344A mutation and the other lacking the C-terminal 17 amino acids, that were defective in constitutive internalization but competent in ligand-promoted internalization and cell migration. These data demonstrate that ligand-dependent and -independent internalization is genetically separable and that, between amino acids 336 and 342, there is a negative regulatory element for ligand-promoted internalization. Potential involvement of this novel motif in cancer metastasis and other CXCR4-associated disorders such as warts, hypogammaglobulinemia, infections and myelokathexis (WHIM) syndrome is discussed.

Mutagenesis at Pro309 of single-chain urokinase-type plasminogen activator alters its catalytic properties

The charge of Lys300(c143) located within a flexible loop(297-313) of sc-uPA has been identified as an important determinant for its high intrinsic activity. Mutations affecting the flexibility of the loop also modulate the intrinsic activity. Glu-plasminogen activation by sc-uPA is strongly promoted by fibrin fragment E but not fibrin fragment D-dimer, whereas plasminogen activation by t-PA is strongly promoted by fragment D-dimer but not fragment E. To further investigate the effect of conformation changes in the flexible loop on catalytic properties of sc-uPA, cassette mutations at Pro309(c152) were made and characterized. It was found that the activation of Pro309(c152) mutants by Lys-plasmin was only moderately affected. In contrast, the intrinsic and two-chain activities of Pro309(c152) mutants against S2444 were both significantly decreased. The two-chain activities of these mutants against Glu-plasminogen were also reduced in a range of 1.1- to 127-fold. The mutations of Pro309(c152) to Trp/Phe and Arg/Asp more significantly affected both intrinsic and two-chain activities, while only a moderate decrease in activity was found with mutations to Ala/Ser/Thr. In contrast to wild-type sc-uPA, plasminogen activation by Pro309(c152) mutants was found to be promoted by both fibrin fragment E and D-dimer. In the presence of 2.0 microM D-dimer, plasminogen activation by mutant Pro309(c152) --> His was promoted by 22-fold, while only 2.0-fold promotion was found with mutant Pro309(c152) --> Gly. In conclusion, these findings demonstrated that conformation changes in the flexible loop of sc-uPA not only affect its intrinsic and two-chain activity, but also extend its promotion of plasminogen activation by fragment E to D-dimer.

Characterization of Lys-698 to met substitution in human plasminogen catalytic domain

Streptokinase (SK) is a human plasminogen (Pg) activator secreted by streptococci. The activation mechanism of SK differs from that of physiological Pg activators in that SK is not a protease and cannot proteolytically activate Pg. Instead, it forms a tight complex with Pg that proteolytically activates other Pg molecules. The residue Lys-698 of human Pg was hypothesized to participate in triggering activation in the SK-Pg complex. Here, we report a study of the Lys-698 to Met substitution in the catalytic domain of Pg (microPg) containing the proteolytic activation-resistant background (R561A). While it remains competent in forming a complex with SK, maintaining a comparable equilibration dissociation constant (K(D)), the recombinant protein shows a nearly 60-fold reduction in amidolytic activity relative to its R561A background when mixed with native SK. A 2.3 A crystal structure of this mutant microPg confirmed the correct folding of this recombinant protein. Combined with other biochemical data, these results support the premise that Lys-698 of human Pg plays a functional role in the so-called N-terminal insertion activation mechanism by SK.

Functional characterization of a H+/nucleoside co-transporter (CaCNT) from Candida albicans, a fungal member of the concentrative nucleoside transporter (CNT) family of membrane proteins

Human and other mammalian concentrative (Na(+)-linked) nucleoside transport proteins belong to a membrane protein family (CNT, TC 2.A.41) that also includes Escherichia coli H(+)-dependent nucleoside transport protein NupC. Here, we report the cDNA cloning and functional characterization of a CNT family member from the pathogenic yeast Candida albicans. This 608 amino acid residue H(+)/nucleoside symporter, designated CaCNT, contains 13 predicted transmembrane domains (TMs), but lacks the exofacial, glycosylated carboxyl-terminus of its mammalian counterparts. When produced in Xenopus oocytes, CaCNT exhibited transport activity for adenosine, uridine, inosine and guanosine but not cytidine, thymidine or the nucleobase hypoxanthine. Apparent K(m) values were in the range 16-64 micro M, with V(max) : K(m) ratios of 0.58-1.31. CaCNT also accepted purine and uridine analogue nucleoside drugs as permeants, including cordycepin (3'-deoxyadenosine), a nucleoside analogue with anti-fungal activity. Electrophysiological measurements under voltage clamp conditions gave a H(+) to [(14)C]uridine coupling ratio of 1 : 1. CaCNT, obtained from logarithmically growing cells, is the first described cation-coupled nucleoside transporter in yeast, and the first member of the CNT family of proteins to be characterized from a unicellular eukaryotic organism.

NEDD8 protein is involved in ubiquitinated inclusion bodies

Proteolysis by the ubiquitin-proteasome system is considered to play a pathological role in several degenerative diseases that involve ubiquitinated inclusion bodies. In recent years, several ubiquitin-like proteins have been isolated, but it is uncertain whether their roles are associated with protein degradation through the ubiquitin-proteasome system. NEDD8 (neural precursor cell-expressed and developmentally down-regulated gene), which consists of 81 amino acid residues, possesses the highest sequence similarity to ubiquitin. Recent studies have indicated that NEDD8 is covalently ligated to cullin family proteins, which are components of certain ubiquitin E3 ligases, by a pathway analogous to that of ubiquitin. Thus, by focusing on the structural and functional association between NEDD8 and ubiquitin, it would be of interest to know whether the NEDD8 system is involved in pathological disorders of the ubiquitin-proteasome system. This study has examined the immunohistochemical distribution of NEDD8 protein by using a highly purified antibody in normal tissues and in tissues known to contain ubiquitinated inclusions. NEDD8 protein expression was widely observed in most types of tissues. Furthermore, accumulation of the NEDD8 protein was commonly observed in ubiquitinated inclusion bodies, including Lewy bodies in Parkinson's disease, Mallory bodies in alcoholic liver disease, and Rosenthal fibres in astrocytoma. Two of ten cases of neurofibrillary tangles and senile plaques from patients with Alzheimer's disease showed intense staining for NEDD8 as well as for ubiquitin. These findings suggest the possibility that the NEDD8 system is involved in the metabolism of these inclusion bodies via the ubiquitin-proteasome system.

Acyl-enzyme complexes between tissue-type plasminogen activator and neuroserpin are short-lived in vitro

The serine protease tissue-type plasminogen activator (t-PA) initiates the fibrinolytic protease cascade and plays a significant role in motor learning, memory, and neuronal cell death induced by excitotoxin and ischemia. In the fibrinolytic system, the serpin PAI-1 negatively regulates the enzymatic activity of both single-chain and two-chain t-PA (sct-PA and tct-PA). In the central nervous system, neuroserpin (NSP) is a serpin thought to regulate t-PA enzymatic activity. We report that although both sct-PA and tct-PA rapidly form acyl-enzyme complexes with NSP in vitro, the interactions are short-lived, rapidly progressing to complete cleavage of NSP and regeneration of fully active enzyme. All NSP molecules appear to transit through the detectable acyl-enzyme intermediate and progress to completion of cleavage; no subpopulation that functions as a pure substrate was detected. Likewise, all molecules were reactive, with no evidence of a latent subpopulation. The interactions between NSP and t-PA were distinct from those between plasmin and NSP, wherein the same peptide bond was cleaved but there was no evidence of a detectable plasmin-NSP acyl-enzyme complex. The interactions between t-PA and NSP contrast with the formation of long-lived, physiologically irreversible acyl-enzyme complexes between t-PA and PAI-1, suggesting that the physiologic effect of t-PA-NSP interactions may be more complex than previously thought.

Residue Met(156) contributes to the labile enzyme conformation of coagulation factor VIIa

Serine protease activation is typically controlled by proteolytic cleavage of the scissile bond, resulting in spontaneous formation of the activating Ile(16)-Asp(194) salt bridge. The initiating coagulation protease factor VIIa (VIIa) differs by remaining in a zymogen-like conformation that confers the control of catalytic activity to the obligatory cofactor and receptor tissue factor (TF). This study demonstrates that the unusual hydrophobic Met(156) residue contributes to the propensity of the VIIa protease domain to remain in a zymogen-like conformation. Mutation of Met(156) to Gln, which is found in the same position of the highly homologous factor IX, had no influence on the amidolytic and proteolytic activity of TF-bound VIIa. Furthermore, the mutation did not appreciably stabilize the labile Ile(16)-Asp(194) salt bridge in the absence of cofactor. VIIa(Gln156) had increased affinity for TF, consistent with a long range conformational effect that stabilized the cofactor binding site in the VIIa protease domain. Notably, in the absence of cofactor, amidolytic and proteolytic function of VIIa(Gln156) were enhanced 3- and 9-fold, respectively, compared with wild-type VIIa. The mutation thus selectively influenced the catalytic activity of free VIIa, identifying the Met(156) residue position as a determinant for the zymogen-like properties of free VIIa.

Zymogen activation in the streptokinase-plasminogen complex. Ile1 is required for the formation of a functional active site

Plasminogen (Plgn) is usually activated by proteolysis of the Arg561-Val562 bond. The amino group of Val562 forms a salt-bridge with Asp740, which triggers a conformational change producing the active protease plasmin (Pm). In contrast, streptokinase (SK) binds to Plgn to produce an initial inactive complex (SK.Plgn) which subsequently rearranges to an active complex (SK.Plgn*) although the Arg561-Val562 bond remains intact. Therefore another residue must substitute for the amino group of Val562 and provide a counterion for Asp740 in this active complex. Two candidates for this counterion have been suggested: Ile1 of streptokinase and Lys698 of Plgn. We have investigated the reaction of SK mutants and variants of the protease domain of microplasminogen (muPlgn) in order to determine if either of these residues is the counterion. The mutation of Ile1 of SK decreases the activity of SK.Plgn* by 100-fold (Ile1Val) to >/= 104-fold (Ile1-->Ala, Gly, Trp or Lys). None of these mutations perturb the binding affinity of SK, which suggests that Ile1 is not required for formation of SK.Plgn but is necessary for SK.Plgn*. The substitution of Lys698 of muPlgn decreases the activity of SK.Plgn* by only 10-60-fold. In contrast with the Ile1 substitutions, the Lys698 mutations also decreased the dissociation constant of the SK complex by 15-50-fold. These observations suggest that Lys698 is involved in formation of the initial SK.Plgn complex. These results support the hypothesis that Ile1 provides the counterion for Asp740.

Molecular mechanisms of plasminogen activation: bacterial cofactors provide clues

Plasminogen activation is a key event in the fibrinolytic system that results in the dissolution of blood clots, and also promotes cell migration and tissue remodelling. The recent structure determinations of microplasmin in complex with the bacterial plasminogen activators staphylokinase and streptokinase have provided novel insights into the molecular mechanisms of plasminogen activation and cofactor function. These bacterial proteins are cofactor molecules that contribute to exosite formation and enhance the substrate presentation to the enzyme. At the same time, they modulate the specificity of plasmin towards substrates and inhibitors, making a 'specificity switch' possible.

Two distinct urokinase-serpin interactions regulate the initiation of cell surface-associated plasminogen activation

Single-chain urokinase-type plasminogen activator (scu-PA) possesses enzymatic activity that increases by 2-3 orders of magnitude upon binding to its cellular cofactor, the u-PA receptor (u-PAR), hence activating an enzymatic cascade initially composed of zymogens. The present study demonstrates that plasminogen activator inhibitor type 3 (PAI-3) reversibly inhibits scu-PA in solution, maintaining the system "off." Because the scu-PA/PAI-3 interaction is reversible, cellular expression of u-PAR allows partitioning of scu-PA from PAI-3 to u-PAR with resultant expression of full enzymatic activity. PAI-3 that was originally complexed to scu-PA remains in solution, retaining its functional activity. Importantly, the scu-PA on cell surface u-PAR is protected from PAI-3 inhibition, remaining an effective activator in a PAI-rich environment. Plasmin formed as a result of scu-PA activity then cleaves scu-PA to the mature protease, two-chain u-PA (tcu-PA), which is efficiently and irreversibly inhibited by PAI-3 via the standard serpin mechanism, even on u-PAR. This data generates a new hypothesis which, in contrast to the previous paradigm, holds that receptor bound scu-PA is the initiating enzyme and that tcu-PA is generated not to augment enzymatic activity but rather to allow for inhibition and therefore appropriate regulation of the process.

Distinct contributions of residue 192 to the specificity of coagulation and fibrinolytic serine proteases

Archetypal members of the chymotrypsin family of serine proteases, such as trypsin, chymotrypsin, and elastase, exhibit relatively broad substrate specificity. However, the successful development of efficient proteolytic cascades, such as the blood coagulation and fibrinolytic systems, required the evolution of proteases that displayed restricted specificity. Tissue-type plasminogen activator (t-PA), for example, possesses exquisitely stringent substrate specificity, and the molecular basis of this important biochemical property of t-PA remains obscure. Previous investigations of related serine proteases, which participate in the blood coagulation cascade, have focused attention on the residue that occupies position 192 (chymotrypsin numbering system), which plays a pivotal role in determining both the inhibitor and substrate specificity of these enzymes. Consequently, we created and characterized the kinetic properties of new variants of t-PA that contained point mutations at position 192. These studies demonstrated that, unlike in coagulation serine proteases, Gln-192 does not contribute significantly to the substrate or inhibitor specificity of t-PA in physiologically relevant reactions. Replacement of Gln-192 with a glutamic acid residue did, however, decrease the catalytic efficiency of mature, two-chain t-PA toward plasminogen in the absence of a fibrin co-factor.

Structural and energetic determinants of the S1-site specificity in serine proteases

In recent years the number of determined three-dimensional structures of serine proteases that are accompanied by detailed mutational studies has grown rapidly. In particular, spatial structures have been described for enzymes involved in processes of critical medical significance, often related to severe pathophysiological diseases. There has also been significant progress in the understanding of the structural grounds for the substrate specificity of serine proteases. This review is concerned mainly with primary structural determinants of the S1 specificity, the crucial component of substrate selectivity, often in relation to more distant specificity elements, which cooperatively influence the S1 site.

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.

NH2-terminal structural motifs in staphylokinase required for plasminogen activation

Staphylokinase (Sak) forms an inactive 1:1 stoichiometric complex with plasminogen which requires both conversion of plasminogen to plasmin and hydrolysis of the Lys10-Lys11 peptide bond of Sak to become a potent plasminogen activator (Schlott, B., Guhrs, K.-H., Hartmann, M., Rocker, A., and Collen, D. (1997) J. Biol. Chem. 272, 6067-6072). Exposure of a positively charged NH2-terminal amino acid after hydrolysis of Sak is a major determinant of the plasminogen-activating potential, but in itself is neither necessary nor sufficient. Here, the structural motifs of the NH2-terminal region Lys11-Gly-Asp-Asp-Ala-Ser16-Tyr-Phe-Glu of processed Sak, required for plasminogen activating potential, were studied by deletion and substitution mutagenesis. Expression in Escherichia coli of variants with deletion of 11, 14, 15, or 16 NH2-terminal amino acids yielded correctly processed but inactive molecules. Expression of their homologues with the NH2-terminal amino acid substituted with Lys-generated derivatives from which the NH2-terminal initiation Met was no longer removed, yielding inactive (</= 10%) Sak42DDeltaN11(M),G12K, active (>50%) Sak42DDeltaN14(M), A15K and Sak42DDeltaN15(M),S16K, and inactive Sak42DDeltaN16(M),Y17K. Lys variants without NH2-terminal Met, generated from fusion proteins in which a His6 tag and a factor Xa recognition sequence were linked to the NH2 terminus of the Sak variants, were indistinguishable from their NH2-terminal Met-containing counterparts. All variants studied had intact affinities for plasminogen as measured by biospecific interaction analysis. The activity of Sak42DDeltaN11(M),G12K could be restored by additional substitution of both Asp13 and Asp14 with Asn, yielding active Sak42DDeltaN11(M),G12K, D13N, D14N, whereas substitution in Sak42DDeltaN16(M),Y17K of Phe18 and Glu19 with Asn yielded inactive Sak42DDeltaN16(M),Y17K,F18N,E19N. These data, in combination with the recent finding that the 20 NH2-terminal amino acids of Sak lack secondary structure, suggest that the NH2-terminal region of Sak is not required for binding to plasmin/plasminogen, but that a positively charged amino acid in the ultimate or penultimate NH2-terminal position corresponding to amino acids 11-16 of this flexible region participates in the reconfiguration of the active site of the plasmin molecule to endow it with plasminogen-activating potential.

An active zymogen: unravelling the mystery of tissue-type plasminogen activator

In contrast to almost all other proteinases, human tissue-type plasminogen activator (tPA) is also proteolytically active in its zymogen or single-chain form. The closely related plasminogen activator isolated from vampire bat saliva (vPA) acts exclusively in the single-chain form, lacking the requisite cleavage site for proteolytic activation. Recent structural studies on the proteolytic domains of vPA and human tPA in two- and single-chain forms reveal the mechanism of this anomalous activity. The PA-catalyzed proteolytic conversion of plasminogen to plasmin, responsible for the initiation of fibrinolysis, is fibrin-dependent; comparative structural analysis of the plasminogen activators provides clues as to the role of fibrin as cofactor.

Tissue-type plasminogen activator: variants and crystal/solution structures demarcate structural determinants of function

NMR and crystal structure of many components of tissue-type plasminogen activator (t-PA) are now available: the finger-EGF pair and the kringle-2 domain structures have been solved, as have the proteolytic domains of vampire bat PA and human t-PA in two- and single-chain forms. These structures confirm the trypsin-like arrangement of the proteolytic domain of t-PA and show how surface loops near the catalytic centre contribute to the narrow specificity of t-PA. Together with mutational experiments, they identify the Lys156 sidechain as a cause of the amidolytic activity of single-chain t-PA, as it can provide a substitute salt bridge partner for Asp194 in the absence of the Ile16 N terminus of the two-chain form. These new findings provide new ideas for the design of PA variants with improved therapeutic properties.

Identification of a flexible loop region (297-313) of urokinase-type plasminogen activator, which helps determine its catalytic activity

Pro-urokinase has a much higher intrinsic catalytic activity than other zymogens of the serine protease family. Lys300(c143) in an apparent "flexible loop" region (297-313) was previously shown to be an important determinant of this intrinsic catalytic activity. This was related to the loop allowing the positive charge of Lys300(c143) to transiently interact with Asp355(c194), thereby inducing an active conformation of the protease domain (Liu, J. N., Tang, W., Sun, Z., Kung, W., Pannell, R., Sarmientos, P., and Gurewich, V. (1996) Biochemistry 35, 14070-14076). To further test this hypothesis, the charge at position 300(c143) and the flexibility of the loop were altered using site-directed mutagenesis designed according to a computer model to affect the interaction between Lys300(c143) and Asp355(c194). When the charge at Lys300(c143) but not Lys313(c156) was reduced, a significant reduction in the intrinsic catalytic activity occurred. Similarly, when the flexibility (wobbliness) of the loop was enhanced reducing the size of side chain, the intrinsic catalytic activity was also reduced. By contrast, when the loop was made less flexible, the intrinsic catalytic activity was increased. These findings were consistent with the hypothesis. The effects of these mutations on two-chain activity were less and often discordant with the intrinsic catalytic activity, indicating that they can be modulated independently. This structure-function disparity can be exploited to create a more zymogenic pro-urokinase (lower intrinsic catalytic activity) with a high catalytic activity, as exemplified by two of the mutants. The changes in intrinsic catalytic activity and two-chain activity induced by the mutations were due to changes in kcat rather than Km. Some significant structure-function differences between pro-urokinase and its highly homologous counterpart, tissue plasminogen activator, were also found.

Variants of tissue-type plasminogen activator that display extraordinary resistance to inhibition by the serpin plasminogen activator inhibitor type 1

Fibrinolysis is regulated in part by the interaction of tissue-type plasminogen activator (t-PA) and plasminogen activator inhibitor type 1 (PAI-1). Previous investigations suggest that three specific arginine residues, Arg-298, Arg-299, and Arg-304 of t-PA, play a critical role in this important regulatory interaction. Our earlier studies have demonstrated that conversion of any of these three residues to a glutamic acid residue reduced the rate of inhibition of t-PA by PAI-1 by factors varying from 58-64. In addition, we have reported that the second order rate constant for inhibition by PAI-1 of the variant t-PA/K296E,R298E,R299E is reduced by a factor of approximately 2800 compared with that of wild type t-PA. In this study, we have significantly extended our earlier observations by identifying t-PA variants that are substantially more resistant to inhibition by PAI-1 than any previously reported variants of t-PA or urokinase-type plasminogen activator. Single-chain t-PA/R275E,R298E, R299E,R304E, for example, is inhibited by PAI-1 approximately 120, 000 times less rapidly than single-chain, wild type t-PA. We also report the first direct comparison of the effects of charge reversal mutations of Arg-298, Arg-299, and/or Arg-304 on the properties of the single- and two-chain forms of t-PA. While these mutations confer extraordinary resistance to inhibition by PAI-1 to both forms of the enzyme, our observations reveal that the single-chain enzyme is affected to a greater extent than the two-chain enzyme. Two-chain, wild type t-PA is inhibited by PAI-1 approximately 1.4 times more rapidly than single-chain t-PA. The corresponding ratio increases to 7.6 or 6.7, respectively, for variants of t-PA containing the R298E, R299E or R298E,R299E,R304E mutations.