Crystal and molecular structure of the bovine alpha-chymotrypsin-eglin c complex at 2.0 A resolution.

The crystal structure of the complex between bovine alpha-chymotrypsin and the leech (Hirudo medicinalis) protein proteinase inhibitor eglin c has been refined at 2.0 A resolution to a crystallographic R-factor of 0.167. The structure of the complex includes 2290 protein and 143 solvent atoms. Eglin c is bound to the cognate enzyme through interactions involving 11 residues of the inhibitor (sites P5-P4' in the reactive site loop, P10' and P23') and 17 residues from chymotrypsin. Binding of eglin c to the enzyme causes a contained hinge-bending movement around residues P4 and P4' of the inhibitor. The tertiary structure of chymotrypsin is little affected, with the exception of the 10-13 region, where an ordered structure for the polypeptide chain is observed. The overall binding mode is consistent with those found in other serine proteinase-protein-inhibitor complexes, including those from different inhibition families. Contained, but significant differences are observed in the establishment of intramolecular hydrogen bonds and polar interactions stabilizing the structure of the intact inhibitor, if the structure of eglin c in its complex with chymotrypsin is compared with that of other eglin c-serine proteinase complexes.     

Preliminary crystallographic data on the complex of bovine alpha-chymotrypsin with the recombinant proteinase inhibitor eglin c from Hirudo medicinalis

The molecular complex built by bovine alpha-chymotrypsin and the recombinant proteinase inhibitor eglin c from Hirudo medicinalis has been crystallized from polyethylene glycol solutions, using a twofold molar excess of the inhibitor with respect to the serine proteinase. The optimum pH for crystal growth is 6.5. The crystals belong to the monoclinic space group P2(1), with unit cell constant: a = 55.3 A, b = 59.4 A, c = 42.5 A, beta = 99.0 degrees; one complex moiety is present per asymmetric unit. The crystals diffract to 2.0 A resolution and are suitable for detailed X-ray crystallographic investigations.     

Binding of the recombinant proteinase inhibitor eglin c from leech Hirudo medicinalis to serine (pro)enzymes: a comparative thermodynamic study.

The binding of the recombinant proteinase inhibitor eglin c from the leech Hirudo medicinalis to serine (pro)enzymes belonging to the chymotrypsin and subtilisin families has been investigated from the thermodynamic viewpoint, between pH 4.5 and 9.5 and from 10 degrees C to 40 degrees C. The affinity of eglin c for the serine (pro)enzymes considered shows the following trend: Leu-proteinase [the leucine specific serine proteinase from spinach (Spinacia oleracea L.) leaves] greater than human leucocyte elastase congruent to human cathepsin G congruent to subtilisin Carlsberg congruent to bovine alpha-chymotrypsin greater than bovine alpha-chymotrypsinogen A congruent to porcine pancreatic elastase congruent to bovine beta-trypsin. The serine (pro)enzyme-inhibitor complex formation is an entropy-driven process. On increasing the pH from 4.5 to 9.5, the affinity of eglin c for the serine (pro)enzymes considered increases thus reflecting the acid pK shift of the invariant hystidyl catalytic residue from approximately to 6.9 in the free serine proteinases and bovine alpha-chymotrypsinogen A to congruent to 5.1 in the serine (pro)enzyme-inhibitor complexes. Considering the known molecular models, the observed binding behaviour of eglin c was related to the inferred stereochemistry of the serine (pro)enzyme-inhibitor contact regions.     

The 2.5 A X-ray crystal structure of the acid-stable proteinase inhibitor from human mucous secretions analysed in its complex with bovine alpha-chymotrypsin.

Orthorhombic crystals of the complex formed between bovine alpha-chymotrypsin and a recombinant human mucous proteinase inhibitor (SLPI) were grown. Data to 2.3 A resolution were collected on the area-detector diffractometer FAST. The crystal structure of the complex was solved by Patterson search techniques using chymotrypsin as a search model. A cyclic procedure of modeling and crystallographic refinement enabled the determination of the SLPI structure. The current crystallographic R-value is 0.19. SLPI has a boomerang-like shape with both wings comprising two well separated domains of similar architecture. In each domain the polypeptide chain is arranged like a stretched spiral. Two internal strands form a regular beta-hairpin loop which is accompanied by two external strands linked by the proteinase binding segment. The polypeptide segment of each domain is interconnected by four disulfide bridges with a connectivity pattern hitherto unobserved. The reactive site loop of the second domain has elastase and chymotrypsin binding properties. It contains the scissile peptide bond between Leu72I and Met73I and has a similar conformation to that observed in other serine proteinase protein inhibitors. Eight residues of this loop, two of the adjacent hairpin loop, the C-terminal segment and Trp30I are in direct contact with the cognate enzyme. The binding loop of the first domain (probably with anti-trypsin activity) is disordered due to proteolytic cleavage occurring in the course of crystallization.     

Crystal and molecular structure of the inhibitor eglin from leeches in complex with subtilisin Carlsberg

The crystal structure of the molecular complex of eglin, a serine proteinase inhibitor from leeches, with subtilisin Carlsberg has been determined at 2.0 A resolution by the molecular replacement method. The complex has been refined by restrained-parameter least-squares. The present crystallographic R factor (Formula: see text) is 0.183. Eglin is a member of the potato inhibitor 1 family, a group of serine proteinase inhibitors lacking disulfide bonds. Eglin shows strong structural homology to CI-2, a related inhibitor from barley seeds. The structure of subtilisin Carlsberg in this complex is very similar to the known structure from barley seeds. The structure of subtilisin Carlsberg in this complex is very similar to the known structure of subtilisin novo, despite changes of 84 out of 274 amino acids.     

Binding of the recombinant proteinase inhibitor eglin c from leech Hirudo medicinalis to human leukocyte elastase, bovine alpha-chymotrypsin and subtilisin Carlsberg: thermodynamic study

The effect of pH and temperature on the apparent association equilibrium constant (Ka) for the binding of the recombinant proteinase inhibitor eglin c from leech Hirudo medicinalis to human leukocyte elastase (EC 3.4.21.37), bovine alpha-chymotrypsin (EC 3.4.21.1) and subtilisin Carlsberg (EC 3.4.21.14) has been investigated. On lowering the pH from 9.5 to 4.5, values of Ka for eglin c binding to the serine proteinases considered decrease thus reflecting the acid-pK shift of the invariant histidyl catalytic residue (His57 in human leukocyte elastase and bovine alpha-chymotrypsin, and His64 in subtilisin Carlsberg) from congruent to 6.9, in the free enzymes, to congruent to 5.1, in the enzyme:inhibitor adducts. At pH 8.0, values of the apparent thermodynamic parameters for eglin c binding are: human leukocyte elastase - Ka = 1.0 x 10(10) M-1, delta G phi = -13.4 kcal/mol, delta H phi = +1.8 kcal/mol, and delta S phi = +52 entropy units; bovine alpha-chymotrypsin -Ka = 5.0 x 10(9) M-1, delta G phi = -13.0 kcal/mol, delta H phi = +2.0 kcal/mol, and delta S phi = +51 entropy units; and subtilisin Carlsberg - Ka = 6.6 x 10(9) M-1, delta G phi = -13.1 kcal/mol, delta H phi = +2.0 kcal/mol, and delta S phi = +51 entropy units (values of Ka, delta G phi and delta S phi were obtained at 21 degrees C; values of delta H phi were temperature independent over the range explored, i.e. between 10 degrees C and 40 degrees C; 1 kcal = 4184J).(ABSTRACT TRUNCATED AT 250 WORDS)     

X-ray crystal structure of the serine proteinase inhibitor eglin c at 1.95 A resolution.

The crystal structure of eglin c, naturally occurring in the leech Hirudo medicinalis, is known from its complexes with various serine proteinases, but the crystallization of free eglin c has not yet been reported. A method is described for growing well-diffracting crystals of free eglin c from highly concentrated protein solutions (approximately 200 mg/ml). The space group of the orthorhombic crystals was determined to be P2(1)2(1)2(1) with unit cell parameters a = 32.6, b = 42.0, c = 44.1 A. The structure of free eglin c was resolved at 1.95 A resolution by Patterson search methods. The final model contains all 70 amino acids of eglin c and 125 water molecules. In comparison to the eglin structure known from its complexes with proteinases, only small differences have been observed in free eglin c. However, the reactive site-binding loop and a few residues on the surface of eglin have been found in different conformations due to crystal contacts. In contrast to the complex structures, the first seven amino acids of the highly flexible amino terminus can be located. Crystallographic refinement comprised molecular dynamics refinement, classical restrained least-squares refinement and individual isotropic atomic temperature refinement. The final R-factor is 15.8%.     

The refined 2.4 A X-ray crystal structure of recombinant human stefin B in complex with the cysteine proteinase papain: a novel type of proteinase inhibitor interaction.

A stoichiometric complex of human stefin B and carboxymethylated papain has been crystallized in a trigonal crystal form. Data to 2.37 A resolution were collected using the area detector diffractometer FAST. The crystal structure of the complex has been solved by Patterson search techniques using papain as search model. Starting from the structure of chicken cystatin, the stefin structure was elucidated through cycles of model building and crystallographic refinement. The current crystallographic R factor is 0.19. Like cystatin, the stefin molecule consists of a five stranded beta-sheet wrapped around a five turn alpha-helix, but with an additional carboxy terminal strand running along the convex side of the sheet. Topological equivalence of stefin and cystatin reveal the previous sequence alignment to be incorrect in part, through deletion of the intermediate helix. The conserved residues form a tripartite wedge, which slots into the papain active site as proposed through consideration of the tertiary structures of the individual components (Bode et al., 1988). The main interactions are provided by the amino terminal 'trunk' (occupying the 'unprimed' subsites of the enzyme), and by the first hairpin loop, containing the highly conserved QVVAG sequence, with minor contributions from the second hairpin loop. The carboxyl terminus of stefin provides an additional interaction region with respect to cystatin. The interaction is dominated by hydrophobic contacts. Inhibition by the cysteine proteinase inhibitors is fundamentally different to that observed for the serine proteinase inhibitors.     

Crystal and molecular structures of the complex of alpha-chymotrypsin with its inhibitor turkey ovomucoid third domain at 1.8 A resolution

The molecular structure of the complex between bovine pancreatic alpha-chymotrypsin (EC 3.4.4.5) and the third domain of the Kazal-type ovomucoid from Turkey (OMTKY3) has been determined crystallographically by the molecular replacement method. Restrained-parameter least-squares refinement of the molecular model of the complex has led to a conventional agreement factor R of 0.168 for the 19,466 reflections in the 1.8 A (1 A = 0.1 nm) resolution shell [I greater than or equal to sigma (I)]. The reactive site loop of OMTKY3, from Lys13I to Arg21I (I indicates inhibitor), is highly complementary to the surface of alpha-chymotrypsin in the complex. A total of 13 residues on the inhibitor make 113 contacts of less than 4.0 A with 21 residues of the enzyme. A short contact (2.95 A) from O gamma of Ser195 to the carbonyl-carbon atom of the scissile bond between Leu18I and Glu19I is present; in spite of it, this peptide remains planar and undistorted. Analysis of the interactions of the inhibitor with chymotrypsin explains the enhanced specificity that chymotrypsin has for P'3 arginine residues. There is a water-mediated ion pair between the guanidinium group on this residue and the carboxylate of Asp64. Comparison of the structure of the alpha-chymotrypsin portion of this complex with the several structures of alpha and gamma-chymotrypsin in the uncomplexed form shows a high degree of structural equivalence (root-mean-square deviation of the 234 common alpha-carbon atoms averages 0.38 A). Significant differences occur mainly in two regions Lys36 to Phe39 and Ser75 to Lys79. Among the 21 residues that are in contact with the ovomucoid domain, only Phe39 and Tyr146 change their conformations significantly as a result of forming the complex. Comparison of the structure of the OMTKY3 domain in this complex to that of the same inhibitor bound to a serine proteinase from Streptomyces griseus (SGPB) shows a central core of 44 amino acids (the central alpha-helix and flanking small 3-stranded beta-sheet) that have alpha-carbon atoms fitting to within 1.0 A (root-mean-square deviation of 0.45 A) whereas the residues of the reactive-site loop differ in position by up to 1.9 A (C alpha of Leu18I). The ovomucoid domain has a built-in conformational flexibility that allows it to adapt to the active sites of different enzymes. A comparison of the SGPB and alpha-chymotrypsin molecules is made and the water molecules bound at the inhibitor-enzyme interface in both complexes are analysed for similarities and differences.     

Refined 1.2 A crystal structure of the complex formed between subtilisin Carlsberg and the inhibitor eglin c. Molecular structure of eglin and its detailed interaction with subtilisin.

The crystal structure of the complex formed between eglin c, an elastase inhibitor from the medical leech, and subtilisin Carlsberg has been determined at 1.2 A resolution by a combination of Patterson search methods and isomorphous replacement techniques. The structure has been refined to a crystallographic R-value of 0.18 (8-1.2 A). Eglin consists of a four-stranded beta-sheet with an alpha-helical segment and the protease-binding loop fixed on opposite sides. This loop, which contains the reactive site Leu45I--Asp46I, is mainly held in its conformation by unique electrostatic/hydrogen bond interactions of Thr44I and Asp46I with the side chains of Arg53I and Arg51I which protrude from the hydrophobic core of the molecule. The conformation around the reactive site is similar to that found in other proteinase inhibitors. The nine residues of the binding loop Gly40I--Arg48I are involved in direct contacts with subtilisin. In this interaction, eglin segment Pro42I--Thr44I forms a three-stranded anti-parallel beta-sheet with subtilisin segments Gly100--Gly102 and Ser125--Gly127. The reactive site peptide bond of eglin is intact, and Ser221 OG of the enzyme is 2.81 A apart from the carbonyl carbon.     

Characterization of low-molecular-mass trypsin isoinhibitors from oil-rape (Brassica napus var. oleifera) seed.

A new low-molecular-mass (6767.8 Da) serine proteinase isoinhibitor has been isolated from oil-rape (Brassica napus var. oleifera) seed, designated 5-oxoPro1-Gly62-RTI-III. The 5-oxoPro1-Gly62-RTI-III isoinhibitor is longer than the Asp2-Pro61-RTI-III and the Ser3-Pro61-RTI-III forms, all the other amino acid residues being identical. In RTI-III isoinhibitors, the P1-P1' reactive site bond (where residues forming the reactive site have been identified as PnellipsisP1 and P1'ellipsisPn', where P1-P1' is the inhibitor scissile bond) has been identified at position Arg21-Ile22. The inhibitor disulphide bridges pattern has been determined as Cys5-Cys27, Cys18-Cys31, Cys42-Cys52 and Cys54-Cys57. The disulphide bridge arrangement observed in the RTI-III isoinhibitors is reminiscent of that found in a number of toxins (e.g. erabutoxin b). Moreover, the organization of the three disulphide bridges subset Cys5-Cys27, Cys18-Cys31 and Cys42-Cys52 is reminiscent of that found in epidermal growth factor domains. Preliminary 1H-NMR data indicates the presence of alphaalphaNOEs and 3JalphaNH coupling constants, typical of the beta-structure(s). These data suggest that the three-dimensional structure of the RTI-III isoinhibitors may be reminiscent of that of toxins and epidermal growth factor domains, consisting of three-finger shaped loops extending from the crossover region. Values of the apparent association equilibrium constant for RTI-III isoinhibitors binding to bovine beta-trypsin and bovine alpha-chymotrypsin are 3.3 x 109 m-1 and 2.4 x 106 m-1, respectively, at pH 8.0 and 21.0 degrees C. The serine proteinase : inhibitor complex formation is a pH-dependent entropy-driven process. RTI-III isoinhibitors do not show any similarity to other serine proteinase inhibitors except the low molecular mass white mustard trypsin isoinhibitor, isolated from Sinapis alba L. seed (MTI-2). Therefore, RTI-III and MTI-2 isoinhibitors could be members of a new class of plant serine proteinase inhibitors.     

Changing the inhibitory specificity and function of the proteinase inhibitor eglin c by site-directed mutagenesis: functional and structural investigation.

Amino acids in the serine proteinase inhibitor eglin c important for its inhibitory specificity and activity have been investigated by site-directed mutagenesis. The specificity of eglin c could be changed from elastase to trypsin inhibition by the point mutation Leu45----Arg (L45R) in position P1 [nomenclature according to Schechter and Berger (1967) Biochem. Biophys. Res. Commun. 27, 157-162]. Model building studies based on the crystal structure of mutant L45R [Heinz et al. (1991) J. Mol. Biol. 217, 353-371] were used to rationalize this specificity change. Surprisingly, the double mutant L45R/D46S was found to be a substrate of trypsin and various other serine proteinases. Multidimensional NMR studies show that wild-type eglin c and the double mutant have virtually identical conformations. In the double mutant L45R/D46S, however, the N-H bond vector of the scissile peptide bond shows a much higher mobility, indicating that the internal rigidity of the binding loop is significantly weakened due to the loss or destabilization of the internal hydrogen bond of the P1' residue. Mutant T44P was constructed to examine the role of a proline in position P2, which is frequently found in serine proteinase inhibitors [Laskowski and Kato (1980) Annu. Rev. Biochem. 49, 593-626]. The mutant remains a potent elastase inhibitor but no longer inhibits subtilisin, which could be explained by model building. Both Arg51 and Arg53, located in the core of the molecule and participating in the hydrogen bonding network with residues in the binding loop to maintain rigidity around the scissile bond, were individually replaced with the shorter but equally charged amino acid lysine. Both mutants showed a decrease in their inhibitory potential. The crystal structure of mutant R53K revealed the loss of two hydrogen bonds between the core and the binding loop of the inhibitor, which are partially restored by a solvent molecule, leading to a decrease in inhibition of elastase by 2 orders of magnitude.     

Complexation of two proteic insect inhibitors to the active site of chymotrypsin suggests decoupled roles for binding and selectivity

The crystal structures of two homologous inhibitors (PMP-C and PMP-D2v) from the insect Locusta migratoria have been determined in complex with bovine alpha-chymotrypsin at 2.1- and 3.0-A resolution, respectively. PMP-C is a potent bovine alpha-chymotrypsin inhibitor whereas native PMP-D2 is a weak inhibitor of bovine trypsin. One unique mutation at the P1 position converts PMP-D2 into a potent bovine alpha-chymotrypsin inhibitor. The two peptides have a similar overall conformation, which consists of a triple-stranded antiparallel beta-sheet connected by three disulfide bridges, thus defining a novel family of serine protease inhibitors. They have in common the protease interaction site, which is composed of the classical protease binding loop (position P5 to P'4, corresponding to residues 26-34) and of an internal segment (residues 15-18), held together by two disulfide bridges. Structural divergences between the two inhibitors result in an additional interaction site between PMP-D2v (position P10 to P6, residues 21-25) and the residues 172-175 of alpha-chymotrypsin. This unusual interaction may be responsible for species selectivity. A careful comparison of data on bound and free inhibitors (from this study and previous NMR studies, respectively) suggests that complexation to the protease stabilizes the flexible binding loop (from P5 to P'4).     

Despite having a common P1 Leu, eglin C inhibits alpha-lytic proteinase a million-fold more strongly than does turkey ovomucoid third domain

Results of the inhibition of alpha-lytic proteinase by two standard mechanism serine proteinase inhibitors, turkey ovomucoid third domain (OMTKY3) and eglin C, and many of their variants are presented. Despite similarities, including an identical P1 residue (Leu) in their primary contact regions, OMTKY3 and eglin C have vastly different association equilibrium constants toward alpha-lytic proteinase, with Ka values of 1.8 x 10(3) and 1.2 x 10(9) M(-1), respectively. Although 12 of the 13 serine proteinases tested in our laboratory for inhibition by OMTKY3 and eglin C are more strongly inhibited by the latter, the million-fold difference observed here with alpha-lytic proteinase is the largest we have seen. The million-fold stronger inhibition by eglin C is retained when the Ka values of the P1 Gly, Ala, Ser, and Ile variants of OMTKY3 and eglin C are compared. Despite the small size of the S1 pocket in alpha-lytic proteinase, interscaffolding additivity for OMTKY3 and eglin C holds well for the four P1 residues tested here. To better understand this difference, we measured Ka values for other OMTKY3 variants, including some that had residues elsewhere in their contact region that corresponded to those of eglin C. Assuming intrascaffolding additivity and using the Ka values obtained for OMTKY3 variants, we designed an OMTKY3-based inhibitor of alpha-lytic proteinase that was predicted to inhibit 10,000-fold more strongly than wild-type OMTKY3. This variant (K13A/P14E/L18A/R21T/N36D OMTKY3) was prepared, and its Ka value was measured against alpha-lytic proteinase. The measured Ka value was in excellent agreement with the predicted one (1.1 x 10(7) and 2.0 x 10(7) M(-1), respectively). Computational protein docking results are consistent with the view that the backbone conformation of eglin C is not significantly altered in the complex with alpha-lytic proteinase. They also show that the strong binding for eglin C correlates well with more favorable atomic contact energy and desolvation energy contributions as compared to OMTKY3.     

X-ray diffraction analysis of the inhibition of porcine pancreatic elastase by a peptidyl trifluoromethylketone

X-ray crystallographic data to 2.57 A resolution (1 A = 0.1 nm) have been measured for the complex of a peptidyl trifluoromethylketone inhibitor with porcine pancreatic elastase (PPE); R = 0.14. The inhibitor forms a stable complex with the enzyme by means of a covalent attachment to active site Ser195O gamma, resulting in a hemiketal moiety with tetrahedral geometry. The tripeptide protion binds as an antiparallel beta-sheet, with four hydrogen bonds augmenting the active-site covalent linkage, Ki = 9.5 microM. His57 exhibits a bifurcated H-bond to both Ser195O gamma and an F atom of the inhibitor. This study is one of a series which explores the binding geometry of a variety of small substrates and inhibitors to PPE. This peptidyl-PPE complex affords insight into the binding geometry of a novel trifluoromethylketone moiety to a serine proteinase.     

Characterization of a peptide released during the reaction of human alpha 1-antitrypsin and bovine alpha-chymotrypsin.

The release of a peptide (molecular weight: about 3,600) was observed during complex formation between human alpha 1-antitrypsin (alpha 1-AT) and bovine alpha-chymotrypsin, when monitored by gel-electrophoresis in the presence of sodium lauryl sulfate. Release of the peptide was proportional to the extent of complex formation. Peptides of the same molecular weight were also released during the complex formation of alpha 1-AT with bovine trypsin or porcine elastase. The peptide released from the complex with bovine alpha-chymotrypsin was composed of 32 amino acid residues, which did not correspond to the composition of any 32 amino acid segment in the bovine alpha-chymotrypsin sequence. The N- and C-terminal sequences of the peptide were determined to be H-(Ser)-Ile-Pro-Pro-Glu- and -Gln-Lys-OH, respectively. Though there was some uncertainty as to the N-terminal sequence, it is quite different from that of the original alpha-AT molecule, and showed a similarity to the sequences of the leaving group sides of the reactive sites in some legume proteinase inhibitors. The C-terminal 2 residues were identical with those of native alpha 1-AT. These results suggest that the peptide was released from the C-terminal region of alpha 1-AT uon interaction with alpha-chymotrypsin. It is tempting to suggest that alpha 1-AT inhibits a serine proteinase by the acyl enzyme mechanism at a residue adjacent to the amino group of the N-terminus of this peptide and that this peptide is liberated as a leaving group in the enzymic process.     

Crystal and molecular structure of the serine proteinase inhibitor CI-2 from barley seeds

Chymotrypsin inhibitor 2 (CI-2), a serine proteinase inhibitor from barley seeds, has been crystallized and its three-dimensional structure determined at 2.0-A resolution by the molecular replacement method. The structure has been refined by restrained-parameter least-squares methods to a crystallographic R factor (= sigma parallel Fo magnitude of-Fo parallel/sigma magnitude of Fo) o of 0.198. CI-2 is a member of the potato inhibitor 1 family. It lacks the characteristic stabilizing disulfide bonds of most other members of serine proteinase inhibitor families. The body of CI-2 shows few conformational changes between the free inhibitor and the previously reported structure of CI-2 in complex with subtilisin Novo [McPhalen, C.A., Svendsen, I., Jonassen, I., & James, M.N.G. (1985) Proc. Natl. Acad. Sci. U.S.A. 82, 7242-7246]. However, the reactive site loop has some significant conformational differences between the free inhibitor and its complexed form. The residues in this segment of polypeptide exhibit relatively large thermal motion parameters and some disorder in the uncomplexed form of the inhibitor. The reactive site bond is between Met-59I and Glu-60I in the consecutive sequential numbering of CI-2 (Met-60-Glu-61 according to the alignment of Svendsen et al. [Svendsen, I., Hejgaard, J., & Chavan, J.K. (1984) Carlsberg Res. Commun. 49, 493-502]). The network of hydrogen bonds and electrostatic interactions stabilizing the conformation of the reactive site loop is much less extensive in the free than in the complexed inhibitor.     

The inhibitory properties and primary structure of a novel serine proteinase inhibitor from the fruiting body of the basidiomycete, Lentinus edodes.

A novel proteinase inhibitor, Lentinus proteinase inhibitor, has been purified from the fruiting bodies of the edible mushroom, Lentinus edodes, by buffer extraction and affinity chromatography on immobilized anhydrotrypsin. The protein simultaneously inhibits bovine beta-trypsin and alpha-chymotrypsin at independent sites, with apparent dissociation constants of 3.5 x 10(-10) M and 4 x 10(-8) M, respectively. The purified protein is eluted as two well-separated peaks on reversed-phase HPLC, one of which is inhibitory-active and the other inactive, and they are interconvertible under folding/unfolding conditions. Among the mammalian and microbial serine proteinases examined, including human enzymes of blood coagulation and fibrinolysis, activated factor XI was inhibited by the Lentinus proteinase inhibitor. Chemical modification studies suggest involvement of one or more arginine residues in the inhibition of trypsin. The complete primary structure composed of 142 amino acids with an acetylated N-terminus was determined by protein analysis. The theoretical molecular mass (15999.2) from the sequence is close to the experimental value of 15999.61 +/- 0.61 determined by mass spectrometry. Although there are no apparently homologous proteinase inhibitors in the protein database, there is a rather striking similarity to the propeptide segment of a microbial serine proteinase, as well as to the N-terminal region of the mature enzyme.     

Binding of the recombinant proteinase inhibitor eglin c, of the soybean Bowman-Birk proteinase inhibitor and of its chymotrypsin and trypsin inhibiting fragments to Leu-proteinase, the leucine specific serine proteinase from spinach (Spinacia oleracea L.) leaves: thermodynamic study.

The effect of pH and temperature on the apparent association equilibrium constant (Ka) for the binding of the recombinant proteinase inhibitor eglin c (eglin c), of the soybean Bowman-Birk proteinase inhibitor (BBI) and of its chymotrypsin and trypsin inhibiting fragments (F-C and F-T, respectively) to Leu-proteinase, the leucine specific serine proteinase from spinach (Spinacia oleracea L.) leaves, has been investigated. On lowering the pH from 9.5 to 4.5, values of Ka (at 21 degrees C) for complex formation decrease thus reflecting the acidic pK-shift of the hystidyl catalytic residue from approximately 6.9, in the free Leu-proteinase, to approximately 5.1, in the enzyme: inhibitor adducts. At pH 8.0, values of the apparent thermodynamic parameters for the proteinase:inhibitor complex formation are: Leu-proteinase:eglin c-Ka = 2.2 x 10(11) M-1, delta G degree = -64 kJ/mol, delta H degree = +5.9 kJ/mol, and delta S degree = +240 kJ/molK; Leu-proteinase:BBI-Ka = 3.2 x 10(10) M-1, delta G degree = -59 kJ/mol, delta H degree = +8.8 kJ/mol, and delta S degree = +230 J/molK; and Leu-proteinase:F-C-Ka = 1.1 x 10(6) M-1, delta G degree = -34 kJ/mol, delta H degree = +18 J/mol, and delta S degree = +180 J/molK (values of Ka, delta G degree and delta S degree were obtained at 21.0 degrees C; values of delta H degree were temperature-independent over the range explored, i.e. between 10.0 degrees C and 40.0 degrees C).(ABSTRACT TRUNCATED AT 250 WORDS)     

Crystal structure of a complex formed between proteolytically-generated lactoferrin fragment and proteinase K

Lactoferrin is an iron binding glycoprotein with a molecular weight of 80 kDa. The molecule is divided into two lobes representing the N-terminal and C-terminal halves of the polypeptide chain, each containing an iron binding site. The serine proteinases such as trypsin, chymotrypsin, and pepsin hydrolyze lactoferrin into two unequal halves while proteinase K divides this protein into two equal halves. In the first step of hydrolysis by proteinase K, the C- and N-lobes, each having a molecular weight of approximately 40 kDa, are generated. In the next step, the lobes are further hydrolyzed into small molecular weight peptides. The proteinase K isolated from the hydrolyzed product does not show enzymatic activity suggesting that the enzyme is inhibited. Furthermore, the hydrolysis experiments on N-lobe and C-lobe showed that the inhibitory fragment came from the C-lobe. The purified lactoferrin fragment was found to be a decapeptide with an amino acid sequence of H₂N-Val-Ala-Gln-Gly-Ala-Ala-Gly-Leu-Ala-COOH. The complex formed between proteinase K and lactoferrin fragment was crystallized by microdialysis. The crystals belonged to the monoclinic space group P2₁with cell dimensions a = 44.4 Å, b = 38.6 Å, c = 79.2 Å, β = 105.8^o and Z = 2. The crystal structure has been determined at 2.4 Å resolution. It has been refined to an R factor of 0.163 for 9044 reflections. The Lf-fragment forms several intermolecular interactions with proteinase K. The Ser-224 Oγ and His-57 Nϵ2 move away to a distance of 3.68 Å in the complex. In the crystal structure, Gln-3I (I indicates inhibitor i.e., lactoferrin fragment) is involved in a direct intermolecular interaction with a symmetry related proteinase K molecule through a strong hydrogen bond with Asp-254. The mode of intermolecular interactions in the complex conformational features of the enzyme and placement of the fragment with respect to the enzyme resemble with the molecular complex of proteinase K with its natural inhibitor PKI3 from wheat. Proteins 33:30–38, 1998. © 1998 Wiley-Liss, Inc.