Alteration of the specificity of ecotin, an E. coli serine proteinase inhibitor, by site directed mutagenesis

Synergy of protease-binding sites within the ecotin homodimer is crucial for inhibition of MASP enzymes and for blocking lectin pathway activation

Ecotin is a homodimeric serine protease inhibitor produced by many commensal and pathogenic microbes. It functions as a virulence factor, enabling survival of various pathogens in the blood. The ecotin dimer binds two protease molecules, and each ecotin protomer has two protease-binding sites: site1 occupies the substrate-binding groove, whereas site2 engages a distinct secondary region. Owing to the twofold rotational symmetry within the ecotin dimer, sites 1 and 2 of a protomer bind to different protease molecules within the tetrameric complex. Escherichia coli ecotin inhibits trypsin-like, chymotrypsin-like, and elastase-like enzymes, including pancreatic proteases, leukocyte elastase, key enzymes of blood coagulation, the contact and complement systems, and other antimicrobial cascades. Here, we show that mannan-binding lectin-associated serine protease-1 (MASP-1) and MASP-2, essential activators of the complement lectin pathway, and MASP-3, an essential alternative pathway activator, are all inhibited by ecotin. We decipher in detail how the preorganization of site1 and site2 within the ecotin dimer contributes to the inhibition of each MASP enzyme. In addition, using mutated and monomeric ecotin variants, we show that site1, site2, and dimerization contribute to inhibition in a surprisingly target-dependent manner. We present the first ecotin:MASP-1 and ecotin:MASP-2 crystal structures, which provide additional insights and permit structural interpretation of the observed functional results. Importantly, we reveal that monomerization completely disables the MASP-2-inhibitory, MASP-3-inhibitory, and lectin pathway-inhibitory capacity of ecotin. These findings provide new opportunities to combat dangerous multidrug-resistant pathogens through development of compounds capable of blocking ecotin dimer formation.

Characterization of ecotin homologs from Campylobacter rectus and Campylobacter showae

Ecotin, first described in Escherichia coli, is a potent inhibitor of a broad range of serine proteases including those typically released by the innate immune system such as neutrophil elastase (NE). Here we describe the identification of ecotin orthologs in various Campylobacter species, including Campylobacter rectus and Campylobacter showae residing in the oral cavity and implicated in the development and progression of periodontal disease in humans. To investigate the function of these ecotins in vitro, the orthologs from C. rectus and C. showae were recombinantly expressed and purified from E. coli. Using CmeA degradation/protection assays, fluorescence resonance energy transfer and NE activity assays, we found that ecotins from C. rectus and C. showae inhibit NE, factor Xa and trypsin, but not the Campylobacter jejuni serine protease HtrA or its ortholog in E. coli, DegP. To further evaluate ecotin function in vivo, an E. coli ecotin-deficient mutant was complemented with the C. rectus and C. showae homologs. Using a neutrophil killing assay, we demonstrate that the low survival rate of the E. coli ecotin-deficient mutant can be rescued upon expression of ecotins from C. rectus and C. showae. In addition, the C. rectus and C. showae ecotins partially compensate for loss of N-glycosylation and increased protease susceptibility in the related pathogen, Campylobacter jejuni, thus implicating a similar role for these proteins in the native host to cope with the protease-rich environment of the oral cavity.

Ecotin, a microbial inhibitor of serine proteases, blocks multiple complement dependent and independent microbicidal activities of human serum

Ecotin is a serine protease inhibitor produced by hundreds of microbial species, including pathogens. Here we show, that ecotin orthologs from Escherichia coli, Yersinia pestis, Pseudomonas aeruginosa and Leishmania major are potent inhibitors of MASP-1 and MASP-2, the two key activator proteases of the complement lectin pathway. Factor D is the key activator protease of another complement activation route, the alternative pathway. We show that ecotin inhibits MASP-3, which is the sole factor D activator in resting human blood. In pathway-specific ELISA tests, we found that all ecotin orthologs are potent lectin pathway inhibitors, and at high concentration, they block the alternative pathway as well. In flow cytometry experiments, we compared the extent of complement-mediated opsonization and lysis of wild-type and ecotin-knockout variants of two E. coli strains carrying different surface lipopolysaccharides. We show, that endogenous ecotin provides significant protections against these microbicidal activities for both bacteria. By using pathway specific complement inhibitors, we detected classical-, lectin- and alternative pathway-driven complement attack from normal serum, with the relative contributions of the activation routes depending on the lipopolysaccharide type. Moreover, in cell proliferation experiments we observed an additional, complement-unrelated antimicrobial activity exerted by heat-inactivated serum. While ecotin-knockout cells are highly vulnerable to these activities, endogenous ecotin of wild-type bacteria provides complete protection against the lectin pathway-related and the complement-unrelated attack, and partial protection against the alternative pathway-related damage. In all, ecotin emerges as a potent, versatile self-defense tool that blocks multiple antimicrobial activities of the serum. These findings suggest that ecotin might be a relevant antimicrobial drug target.

Bloodstream infections are major cause of morbidity and mortality in many countries around the globe. As the number of multi-drug resistant pathogenic strains is growing, it is urgent to identify their virulence factors and unveil the corresponding mechanisms of action that enable the pathogen to avoid potent immune response. A microbial inhibitor of serine proteases, ecotin was previously implicated in protecting various pathogenic bacteria and eukaryotic Leishmania species against the host immune system by inhibiting leukocyte elastase. However, the interaction of ecotin with the complement system, which provides a first line defense against pathogens, remained unexplored. We found that ecotin blocks activation of the complement lectin pathway by inhibiting its key activator enzymes, MASP-1 and MASP-2. Furthermore, by inhibiting MASP-3, ecotin also disrupts a fundamental link between the lectin- and the alternative pathways. We provide evidence that E. coli cells devoid of ecotin are extremely vulnerable to complement-mediated lysis and they are also potently killed by some complement-independent antimicrobial factors of human serum. These findings could explain the observations of other research groups reporting that ecotin is crucial for the survival of pathogenic microbes in the host. Our results therefore also highlight ecotin as a potential target of future antimicrobial therapies.

Protein surface charge of trypsinogen changes its activation pattern

Background

Trypsinogen is the inactive precursor of trypsin, a serine protease that cleaves proteins and peptides after arginine and lysine residues. In this study, human trypsinogen was used as a model protein to study the influence of electrostatic forces on protein–protein interactions. Trypsinogen is active only after its eight-amino-acid-long activation peptide has been cleaved off by another protease, enteropeptidase. Trypsinogen can also be autoactivated without the involvement of enteropeptidase. This autoactivation process can occur if a trypsinogen molecule is activated by another trypsin molecule and therefore is based on a protein–protein interaction.

Results

Based on a rational protein design based on autoactivation-defective guinea pig trypsinogen, several amino acid residues, all located far away from the active site, were changed to modify the surface charge of human trypsinogen. The influence of the surface charge on the activation pattern of trypsinogen was investigated. The autoactivation properties of mutant trypsinogen were characterized in comparison to the recombinant wild-type enzyme. Surface-charged trypsinogen showed practically no autoactivation compared to the wild-type but could still be activated by enteropeptidase to the fully active trypsin. The kinetic parameters of surface-charged trypsinogen were comparable to the recombinant wild-type enzyme.

Conclusion

The variant with a modified surface charge compared to the wild-type enzyme showed a complete different activation pattern. Our study provides an example how directed modification of the protein surface charge can be utilized for the regulation of functional protein–protein interactions, as shown here for human trypsinogen.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-014-0109-5) contains supplementary material, which is available to authorized users.

Autoactivation of mouse trypsinogens is regulated by chymotrypsin C via cleavage of the autolysis loop

Chymotrypsin C (CTRC) is a proteolytic regulator of trypsinogen autoactivation in humans. CTRC cleavage of the trypsinogen activation peptide stimulates autoactivation, whereas cleavage of the calcium binding loop promotes trypsinogen degradation. Trypsinogen mutations that alter these regulatory cleavages lead to increased intrapancreatic trypsinogen activation and cause hereditary pancreatitis. The aim of this study was to characterize the regulation of autoactivation of mouse trypsinogens by mouse Ctrc. We found that the mouse pancreas expresses four trypsinogen isoforms to high levels, T7, T8, T9, and T20. Only the T7 activation peptide was cleaved by mouse Ctrc, causing negligible stimulation of autoactivation. Surprisingly, mouse Ctrc poorly cleaved the calcium binding loop in all mouse trypsinogens. In contrast, mouse Ctrc readily cleaved the Phe-150-Gly-151 peptide bond in the autolysis loop of T8 and T9 and inhibited autoactivation. Mouse chymotrypsin B also cleaved the same peptide bond but was 7-fold slower. T7 was less sensitive to chymotryptic regulation, which involved slow cleavage of the Leu-149-Ser-150 peptide bond in the autolysis loop. Modeling indicated steric proximity of the autolysis loop and the activation peptide in trypsinogen, suggesting the cleaved autolysis loop may directly interfere with activation. We conclude that autoactivation of mouse trypsinogens is under the control of mouse Ctrc with some notable differences from the human situation. Thus, cleavage of the trypsinogen activation peptide or the calcium binding loop by Ctrc is unimportant. Instead, inhibition of autoactivation via cleavage of the autolysis loop is the dominant mechanism that can mitigate intrapancreatic trypsinogen activation.

Intragenic duplication: a novel mutational mechanism in hereditary pancreatitis

OBJECTIVES

In a hereditary pancreatitis family from Denmark, we identified a novel intragenic duplication of 9 nucleotides in exon-2 of the human cationic trypsinogen (PRSS1) gene (c.63_71dup) which at the amino-acid level resulted in the insertion of 3 amino acids within the activation peptide of cationic trypsinogen (p.K23_I24insIDK). The aim of the present study was to characterize the effect of this unique genetic alteration on the function of human cationic trypsinogen.

METHODS

Wild-type and mutant cationic trypsinogens were produced recombinantly and purified to homogeneity. Trypsinogen activation was followed by enzymatic assays and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Trypsinogen secretion was measured from transfected HEK 293T cells.

RESULTS

Recombinant cationic trypsinogen carrying the p.K23_I24insIDK mutation exhibited greater than 10-fold increased autoactivation. Activation by human cathepsin B also was accelerated by 10-fold. Secretion of the p.K23_I24insIDK mutant from transfected cells was diminished, consistent with intracellular autoactivation.

CONCLUSIONS

This is the first report of an intragenic duplication within the PRSS1 gene causing hereditary pancreatitis. The accelerated activation of p.K23_I24insIDK by cathepsin B is a unique biochemical property not found in any other pancreatitis-associated trypsinogen mutant. In contrast, the robust autoactivation of the novel mutant confirms the notion that increased autoactivation is a disease-relevant mechanism in hereditary pancreatitis.

High affinity small protein inhibitors of human chymotrypsin C (CTRC) selected by phage display reveal unusual preference for P4' acidic residues

Human chymotrypsin C (CTRC) is a pancreatic protease that participates in the regulation of intestinal digestive enzyme activity. Other chymotrypsins and elastases are inactive on the regulatory sites cleaved by CTRC, suggesting that CTRC recognizes unique sequence patterns. To characterize the molecular determinants underlying CTRC specificity, we selected high affinity substrate-like small protein inhibitors against CTRC from a phage library displaying variants of SGPI-2, a natural chymotrypsin inhibitor from Schistocerca gregaria. On the basis of the sequence pattern selected, we designed eight inhibitor variants in which amino acid residues in the reactive loop at P1 (Met or Leu), P2' (Leu or Asp), and P4' (Glu, Asp, or Ala) were varied. Binding experiments with CTRC revealed that (i) inhibitors with Leu at P1 bind 10-fold stronger than those with P1 Met; (ii) Asp at P2' (versus Leu) decreases affinity but increases selectivity, and (iii) Glu or Asp at P4' (versus Ala) increase affinity 10-fold. The highest affinity SGPI-2 variant (K(D) 20 pm) bound to CTRC 575-fold tighter than the parent molecule. The most selective inhibitor variant exhibited a K(D) of 110 pm and a selectivity ranging from 225- to 112,664-fold against other human chymotrypsins and elastases. Homology modeling and mutagenesis identified a cluster of basic amino acid residues (Lys(51), Arg(56), and Arg(80)) on the surface of human CTRC that interact with the P4' acidic residue of the inhibitor. The acidic preference of CTRC at P4' is unique among pancreatic proteases and might contribute to the high specificity of CTRC-mediated digestive enzyme regulation.

Expression of recombinant proteins with uniform N-termini

Heterologously expressed proteins in Escherichia coli may undergo unwanted N-terminal processing by methionine and proline aminopeptidases. To overcome this problem, we present a system where the gene of interest is cloned as a fusion to a self-splicing mini-intein. This fusion construct is expressed in an engineered E. coli strain from which the pepP gene coding for aminopeptidase P has been deleted. We describe a protocol using human cationic trypsinogen as an example to demonstrate that recombinant proteins produced in this expression system contain homogeneous, unprocessed N-termini.

Chymotrypsin C is a co-activator of human pancreatic procarboxypeptidases A1 and A2

Human digestive carboxypeptidases CPA1, CPA2, and CPB1 are secreted by the pancreas as inactive proenzymes containing a 94-96-amino acid-long propeptide. Activation of procarboxypeptidases is initiated by proteolytic cleavage at the C-terminal end of the propeptide by trypsin. Here, we demonstrate that subsequent cleavage of the propeptide by chymotrypsin C (CTRC) induces a nearly 10-fold increase in the activity of trypsin-activated CPA1 and CPA2, whereas CPB1 activity is unaffected. Other human pancreatic proteases such as chymotrypsin B1, chymotrypsin B2, chymotrypsin-like enzyme-1, elastase 2A, elastase 3A, or elastase 3B are inactive or markedly less effective at promoting procarboxypeptidase activation. On the basis of these observations, we propose that CTRC is a physiological co-activator of proCPA1 and proCPA2. Furthermore, the results confirm and extend the notion that CTRC is a key regulator of digestive zymogen activation.

Analysis of binding properties and specificity through identification of the interface forming residues (IFR) for serine proteases in silico docked to different inhibitors

Background

Enzymes belonging to the same super family of proteins in general operate on variety of substrates and are inhibited by wide selection of inhibitors. In this work our main objective was to expand the scope of studies that consider only the catalytic and binding pocket amino acids while analyzing enzyme specificity and instead, include a wider category which we have named the Interface Forming Residues (IFR). We were motivated to identify those amino acids with decreased accessibility to solvent after docking of different types of inhibitors to sub classes of serine proteases and then create a table (matrix) of all amino acid positions at the interface as well as their respective occupancies. Our goal is to establish a platform for analysis of the relationship between IFR characteristics and binding properties/specificity for bi-molecular complexes.

Results

We propose a novel method for describing binding properties and delineating serine proteases specificity by compiling an exhaustive table of interface forming residues (IFR) for serine proteases and their inhibitors. Currently, the Protein Data Bank (PDB) does not contain all the data that our analysis would require. Therefore, an in silico approach was designed for building corresponding complexes

The IFRs are obtained by "rigid body docking" among 70 structurally aligned, sequence wise non-redundant, serine protease structures with 3 inhibitors: bovine pancreatic trypsin inhibitor (BPTI), ecotine and ovomucoid third domain inhibitor. The table (matrix) of all amino acid positions at the interface and their respective occupancy is created. We also developed a new computational protocol for predicting IFRs for those complexes which were not deciphered experimentally so far, achieving accuracy of at least 0.97.

Conclusions

The serine proteases interfaces prefer polar (including glycine) residues (with some exceptions). Charged residues were found to be uniquely prevalent at the interfaces between the "miscellaneous-virus" subfamily and the three inhibitors. This prompts speculation about how important this difference in IFR characteristics is for maintaining virulence of those organisms.

Our work here provides a unique tool for both structure/function relationship analysis as well as a compilation of indicators detailing how the specificity of various serine proteases may have been achieved and/or could be altered. It also indicates that the interface forming residues which also determine specificity of serine protease subfamily can not be presented in a canonical way but rather as a matrix of alternative populations of amino acids occupying variety of IFR positions.

The contribution of key hydrophobic residues in ecotin to enzyme-inhibitor complex stability

The Escherichia coli protease inhibitor ecotin is believed to be implicated in the evasion of host defenses during infection. The protein has also attracted attention as a scaffold for the design of novel, specific protease inhibitors. Ecotin interacts with its targets through two sites. Key hydrophobic residues in both sites (Leu-64, Trp-67, Tyr-69, Met-84, and Met-85) were mutated to alanine and the effects on the inhibition of trypsin, chymotrypsin, and elastase were assessed. Each of these mutant ecotin proteins tested in kinetic assays with these enzymes exerted less inhibitory potency compared to wild-type ecotin. However, these effects were relatively small and not additive.

Intracellular autoactivation of human cationic trypsinogen mutants causes reduced trypsinogen secretion and acinar cell death

Mutations in the activation peptide of human cationic trypsinogen have been found in patients with chronic pancreatitis. Previous biochemical studies demonstrated that mutations p.D19A, p.D22G, and p.K23R strongly stimulate trypsinogen autoactivation. In the present study, we characterized the cell biological effects of these mutants using human embryonic kidney 293T and AR42J rat acinar cells. We found that relative to wild-type trypsinogen, secretion of the mutants from transfected cells was markedly decreased. This apparent secretion defect was completely rescued by inhibition of autoactivation via (1) inclusion of the small molecule trypsin inhibitor benzamidine in the growth medium; or (2) cotransfection with the physiological trypsin inhibitor SPINK1; or (3) by mutation of the catalytic Ser(200) residue in trypsinogen. In contrast, extracellularly added SPINK1 or other nonpermeable proteinaceous trypsin inhibitors did not restore normal secretion of the mutants, indicating that intracellular autoactivation is responsible for the observed secretion loss. Acinar cells expressing the p.D22G mutant detached from the culture plate over time, became terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling-positive, and exhibited elevated levels of the proapoptotic transcription factor CHOP. The observations indicate that activation peptide mutants of human cationic trypsinogen undergo autoactivation intracellularly, which leads to decreased trypsinogen secretion and eventual acinar cell death. The results thus define a novel pathological pathway for parenchymal injury in hereditary chronic pancreatitis.

Hereditary pancreatitis caused by a novel PRSS1 mutation (Arg-122 --> Cys) that alters autoactivation and autodegradation of cationic trypsinogen

Hereditary pancreatitis has been found to be associated with germline mutations in the cationic trypsinogen (PRSS1) gene. Here we report a family with hereditary pancreatitis that carries a novel PRSS1 mutation (R122C). This mutation cannot be diagnosed with the conventional screening method using AflIII restriction enzyme digest. We therefore propose a new assay based on restriction enzyme digest with BstUI, a technique that permits detection of the novel R122C mutation in addition to the most common R122H mutation, and even in the presence of a recently reported neutral polymorphism that prevents its detection by the AflIII method. Recombinantly expressed R122C mutant human trypsinogen was found to undergo greatly reduced autoactivation and cathepsin B-induced activation, which is most likely caused by misfolding or disulfide mismatches of the mutant zymogen. The K(m) of R122C trypsin was found to be unchanged, but its k(cat) was reduced to 37% of the wild type. After correction for enterokinase activatable activity, and specifically in the absence of calcium, the R122C mutant was more resistant to autolysis than the wild type and autoactivated more rapidly at pH 8. Molecular modeling of the R122C mutant trypsin predicted an unimpaired active site but an altered stability of the calcium binding loop. This previously unknown trypsinogen mutation is associated with hereditary pancreatitis, requires a novel diagnostic screening method, and, for the first time, raises the question whether a gain or a loss of trypsin function participates in the onset of pancreatitis.

The role of ecotin dimerization in protease inhibition

Ecotin is a homodimeric protein from Escherichia coli that inhibits many serine proteases of the chymotrypsin fold, often with little effect from the character or extent of enzyme substrate specificity. This pan-specificity of inhibition is believed to derive from formation of a heterotetrameric complex with target proteases involving three types of interface: the dimerization interface, a primary substrate-like interaction, and a smaller secondary interaction between the partner ecotin subunit and the protease. A monomeric ecotin variant (mEcotin) and a single-chain ecotin dimer (scEcotin) were constructed to study the effect of a network of protein interactions on binding affinity and the role of dimerization in broad inhibitor specificity. mEcotin was produced by inserting a beta-turn into the C-terminal arm, which normally exchanges with the other subunit. While the dimerization constant (K(dim)) of wild-type (WT) ecotin was found to be picomolar by subunit exchange experiments using FRET and by association kinetics, mEcotin was monomeric up to 1 mM as judged by gel filtration and analytical centrifugation. A crystal structure of uncomplexed mEcotin to 2.0 A resolution verifies the design, showing a monomeric protein in which the C-terminal arm folds back onto itself to form a beta-barrel structure nearly identical to its dimeric counterpart. The kinetic rate constants and equilibrium dissociation constants for monomeric and dimeric ecotin variants were determined with both trypsin and chymotrypsin. The effect of the secondary binding site on affinity was found to vary inversely with the strength of the interaction at the primary site. This compensatory effect yields a nonadditivity of up to 5 kcal/mol and can be explained in terms of the optimization of binding orientation. Such a mechanism of adaptability allows femtomolar affinities for two proteases with very different specificities.

Human cationic trypsinogen. Role of Asn-21 in zymogen activation and implications in hereditary pancreatitis

Mutation Asn-21 --> Ile in human cationic trypsinogen (Tg-1) has been associated with hereditary pancreatitis. Recent studies with rat anionic Tg (Tg-2) indicated that the analogous Thr-21 --> Ile mutation stabilizes the zymogen against autoactivation, whereas it has no effect on catalytic properties or autolytic stability of trypsin (Sahin-Tóth, M. (1999) J. Biol. Chem. 274, 29699-29704). In the present paper, human cationic Tg (Asn-21-Tg) and mutants Asn-21 --> Ile (Ile-21-Tg) and Asn-21 --> Thr (Thr-21-Tg) were expressed in Escherichia coli, and zymogen activation, zymogen degradation, and trypsin autolysis were studied. Enterokinase activated Asn-21-Tg approximately 2-fold better than Ile-21-Tg or Thr-21-Tg, and catalytic parameters of trypsins were comparable. At 37 degrees C, in 5 mm Ca(2+), all three trypsins were highly stable. In the absence of Ca(2+), Asn-21- and Ile-21-trypsins suffered autolysis in an indistinguishable manner, whereas Thr-21-trypsin exhibited significantly increased stability. In sharp contrast to observations with the rat proenzyme, at pH 8.0, 37 degrees C, autoactivation kinetics of Asn-21-Tg and Ile-21-Tg were identical; however, at pH 5. 0, Ile-21-Tg autoactivated at an enhanced rate relative to Asn-21-Tg. Remarkably, at both pH values, Thr-21-Tg showed markedly higher autoactivation rates than the two other zymogens. Finally, autocatalytic proteolysis of human zymogens was limited to cleavage at Arg-117, and no digestion at Lys-188 was detected. The observations indicate that zymogen stabilization by Ile-21 as observed in rat Tg-2 is not characteristic of human Tg-1. Instead, an increased propensity to autoactivation under acidic conditions might be relevant to the pathomechanism of the Asn-21 --> Ile mutation in hereditary pancreatitis. In the same context, faster autoactivation and increased trypsin stability caused by the Asn-21 --> Thr mutation in human Tg-1 might provide a rationale for the evolutionary divergence from Thr-21 found in other mammalian trypsinogens.

Trypsinogen stabilization by mutation Arg117-->His: a unifying pathomechanism for hereditary pancreatitis?

Mutations Arg117-->His and Asn21-->Ile of the human cationic trypsinogen have been recently identified in patients affected by hereditary pancreatitis (HP). The Arg117-->His substitution is believed to cause pancreatitis by eliminating an essential autolytic cleavage site in trypsin, thereby rendering the protease resistant to inactivation through autolysis. Here we demonstrate that the Arg117-->His mutation also significantly inhibits autocatalytic trypsinogen breakdown under Ca(2+)-free conditions and stabilizes the zymogen form of rat trypsin. Taken together with recent findings demonstrating that the Asn21-->Ile mutation stabilizes rat trypsinogen against autoactivation and consequent autocatalytic degradation, the observations suggest a unifying molecular pathomechanism for HP in which zymogen stabilization plays a central role.

Hereditary pancreatitis-associated mutation asn(21) --> ile stabilizes rat trypsinogen in vitro

Mutations Arg(117) --> His and Asn(21) --> Ile in human trypsinogen-I have been recently associated with hereditary pancreatitis (HP). The Arg(117) --> His substitution is believed to cause pancreatitis by stabilizing trypsin against autolytic degradation, while the mechanism of action of Asn(21) --> Ile has been unknown. In an effort to understand the effect(s) of this mutation, Thr(21) in the highly homologous rat trypsinogen-II was replaced with Asn or Ile, and the recombinant zymogens and their active trypsin forms were studied. Kinetic parameters of all three trypsins were comparable, and the active enzymes suffered autolysis at similar rates, indicating that neither catalytic properties nor proteolytic stability of trypsin are influenced by mutations at position 21. When incubated at pH 8.0, 37 degrees C, pure zymogens underwent autoactivation with concomitant trypsinolytic degradation in a Ca(2+)-dependent fashion. Thus, in the presence of 5 mM Ca(2+), autoactivation and digestion of the zymogens after Arg(117) and Lys(188) were observed, while in the presence of 1 mM EDTA autoactivation and cleavage at Lys(188) were reduced, and zymogenolysis at the Arg(117) site was enhanced. Overall rates of zymogen degradation in [Asn(21)]- and [Ile(21)]trypsinogens were higher in Ca(2+) than in EDTA, while [Thr(21)]trypsinogen demonstrated inverse characteristics. Remarkably, both in the presence and absence of Ca(2+), [Ile(21)]trypsinogen exhibited significantly higher stability against autoactivation and proteolysis than zymogens with Asn(21) or Thr(21). The observations suggest that autocatalytic trypsinogen degradation may be an important defense mechanism against excessive trypsin generation in the pancreas, and trypsinogen stabilization by the Asn(21) --> Ile mutation plays a role in the pathogenesis of HP.

Ecotin: a serine protease inhibitor with two distinct and interacting binding sites

The interaction between ecotin and target proteases with trypsin-like specificity has been systematically dissected to understand the structural basis of ecotin's broad inhibitory specificity and the role of the secondary binding site. Site-directed and region-specific mutagenesis were preformed at ecotin's primary site P1 residue (84), the C-terminal dimer interface (133 to 142), and two surface loops of the secondary binding site (67 to 70, 108 to 113). Substitutions at the P1 position resulted in less than fivefold difference in the potency of ecotin binding to rat trypsin, suggesting that the extended binding site is important in binding. A ten amino acid C-terminal truncation variant showed threefold weaker self-association but remained a dimer. The interactions of the secondary binding site of ecotin with bovine trypsin, rat trypsin and human urokinase-type plasminogen activator (uPA) were investigated with alanine substitutions in ecotin at Trp67, Gly68, Tyr69, Asp70, Arg108, Asn110, Lys112 and Leu113, which formed contacts between the inhibitor and protease. By combining these mutations at the secondary binding site with mutations in the primary binding site the molecular recognition between ecotin and its target serine proteases was probed. The contrast in the Ki values of the various ecotin variants towards bovine trypsin, rat trypsin and human uPA established the role of ecotin's secondary binding site in recognizing these homologous serine proteases. Ecotin binds to proteases with a chymotrypsin fold through a combination of primary and secondary site surface loops and is amenable to redesign of its potency and specificity for this class of enzymes.

Affinity purification of recombinant trypsinogen using immobilized ecotin

Affinity purification of inactive precursors (zymogens) of serine proteases on protease inhibitor columns is not feasible, due to the weak interaction between canonical protease inhibitors and protease zymogens. In this study we demonstrate that immobilized ecotin, a unique protease inhibitor from Escherichia coli, provides a superior affinity matrix for the purification of trypsinogen and possibly other serine protease zymogens as well.

Stable monomeric form of an originally dimeric serine proteinase inhibitor, ecotin, was constructed via site directed mutagenesis

Ecotin, a homodimer protein of E. coli, is a unique member of canonical serine proteinase inhibitors, since it is a potent agent against a variety of serine proteinases having different substrate specificity. Monomers of ecotin are held together mostly by their long C-terminal strands that are arranged as a two-stranded antiparallel beta-sheet in the functional dimer. One ecotin dimer can chelate two proteinase molecules, each of them bound to both subunits of ecotin at two different sites, namely the specific primary and the non-specific secondary binding sites. In this study the genes of wild type ecotin and its Met84Arg P1 site mutant were truncated resulting in new forms of ecotin that lack 10 amino acid residues at their C-terminus. These mutants do not dimerize spontaneously, though in combination with trypsin they assemble into the familiar heterotetramer. Our data suggest that this heterotetramer exists even in extremely diluted solutions, and the interaction, which is responsible for the dimerization of ecotin, contributes to the stability of the heterotetrameric complex.

Ecotin is a potent inhibitor of the contact system proteases factor XIIa and plasma kallikrein

Ecotin, a serine protease inhibitor found in the periplasm of Escherichia coli, has been characterized as a potent reversible tight-binding inhibitor of the human contact activation proteases factor XIIa (FXIIa) and plasma kallikrein, having Ki values of 89 pM and 163 pM, respectively. Ecotin also inhibited human leukocyte elastase (HLE) with high affinity (Ki = 55 pM). The association rate constants kon for FXIIa and kallikrein were 5.3 x 10(5) M-1.s-1 and 2.9 x 10(5) M-1.s-1, respectively. The dissociation rate constant koff for kallikrein, measured in the presence of HLE to prevent reassociation, was 6.3 x 10(-5) s-1; the koff for ecotin with FXIIa was 4.7 x 10(-5) s-1. Both FXIIa and kallikrein cleaved ecotin slowly at pH 5.0, identifying Met-84 as the P1 residue. The potent anticoagulant effect by ecotin is explained by the coincident inhibition of FXIIa, kallikrein, and FXa and suggests that it may be useful in the study of inflammatory or thrombotic disorders such as sepsis or cardiopulmonary bypass.

Ecotin: lessons on survival in a protease-filled world

Ecotin, an Escherichia coli periplasmic protein of 142 amino acids, has been shown to be a potent inhibitor of a group of homologous serine proteases with widely differing substrate recognition. It is highly effective against a number of enzymes, including both pancreatic and neutrophil-derived elastases, chymotrypsin, trypsin, factor Xa, and kallikrein. Recent structural and functional studies on ecotin and its interactions with different serine proteases have clarified these initial observations and revealed the remarkable features of this protein in inhibiting a strikingly large subset of the chymotrypsin family of serine proteases. The structures of the ecotin:serine protease complexes provide the first examples of protein-protein recognition where the concept of specificity of interactions needs to be reexamined. The binding sites show a fluidity of protein contacts derived from ecotin's innate flexibility in fitting itself to proteases while strongly interfering with their function.