Structural features of a snake venom thrombin-like enzyme: thrombin and trypsin on a single catalytic platform?

Enhancing snakebite management: The role of small molecule therapeutics in complementing antivenom strategies

The variability in snake composition presents a significant challenge in accessing an effective broad-spectrum antivenom. These highly complex mixtures can result in numerous deleterious effects affecting thousands of individuals worldwide, particularly in Asia, sub-Saharan Africa, and Latin America. While the administration of antivenom remains a recommended treatment for snakebite envenomation and is the primary means to prevent systemic damage, there are limitations concerning specificity, reversal of local effects, and economic factors that hinder the availability of these antibodies. In this review, we have compiled information on the use of small molecule therapeutics in initial first-aid treatments before antivenom administration. These enzyme inhibitors have shown promise as viable candidates to broaden our treatment approaches, simplify procedures, reduce costs, and improve the clinical outcomes of affected patients.

Small Angle X-ray Scattering, Molecular Modeling, and Chemometric Studies from a Thrombin-Like (Lmr-47) Enzyme of Lachesis m. rhombeata Venom

INTRODUCTION

Snakebite envenomation is considered a neglected tropical disease, and SVTLEs critical elements are involved in serious coagulopathies that occur on envenoming. Although some enzymes of this group have been structurally investigated, it is essential to characterize other proteins to better understand their unique properties such as the Lachesis muta rhombeata 47 kDa (Lmr-47) venom serine protease.

METHODS

The structure of Lmr-47 was studied in solution, using SAXS, DLS, CD, and in silico by homology modeling. Molecular docking experiments simulated 21 competitive inhibitors.

RESULTS

At pH 8.0, Lmr-47 has an Rg of 34.5 ± 0.6 Å, Dmax of 130 Å, and SR of 50 Å, according to DLS data. Kratky plot analysis indicates a rigid shape at pH 8.0. Conversely, the pH variation does not change the center of mass's intrinsic fluorescence, possibly indicating the absence of fluorescent amino acids in the regions affected by pH variation. CD experiments show a substantially random coiled secondary structure not affected by pH. The low-resolution model of Lmr-47 presented a prolate elongated shape at pH 8.0. Using the 3D structure obtained by molecular modeling, docking experiments identified five good and three suitable competitive inhibitors.

CONCLUSION

Together, our work provided insights into the structure of the Lmr-47 and identified inhibitors that may enhance our understanding of thrombin-like family proteins.

Edema Induced by a Crotalus durissus terrificus Venom Serine Protease (Cdtsp 2) Involves the PAR Pathway and PKC and PLC Activation

Snake venom serine proteases (SVSPs) represent an essential group of enzymatic toxins involved in several pathophysiological effects on blood homeostasis. Some findings suggest the involvement of this class of enzymatic toxins in inflammation. In this paper, we purified and isolated a new gyroxin isoform from the Crotalus durissus terrificus (Cdt) venom, designated as Cdtsp 2, which showed significant proinflammatory effects in a murine model. In addition, we performed several studies to elucidate the main pathway underlying the edematogenic effect induced by Cdtsp 2. Enzymatic assays and structural analysis (primary structure analysis and three-dimensional modeling) were closely performed with pharmacological assays. The determination of edematogenic activity was performed using Cdtsp 2 isolated from snake venom, and was applied to mice treated with protein kinase C (PKC) inhibitor, phospholipase C (PLC) inhibitor, dexamethasone (Dexa), antagonists for protease-activated receptors (PARs), or saline (negative control). Additionally, we measured the levels of cyclooxygenase 2 (COX-2), malondialdehyde (MDA), and prostaglandin E2 (PGE2). Cdtsp 2 is characterized by an approximate molecular mass of 27 kDa, an isoelectric point (pI) of 4.5, and significant fibrinolytic activity, as well as the ability to hydrolyze Nα-benzoyl-l-arginine 4-nitroanilide (BAPNA). Its primary and three-dimensional structures revealed Cdtsp 2 as a typical snake venom serine protease that induces significant edema via the metabolism of arachidonic acid (AA), involving PARs, PKC, PLC, and COX-2 receptors, as well as inducing a significant increase in MDA levels. Our results showed that Cdtsp 2 is a serine protease with significant enzymatic activity, and it may be involved in the degradation of PAR1 and PAR2, which activate PLC and PKC to mobilize AA, while increasing oxidative stress. In this article, we provide a new perspective for the role of SVSPs beyond their effects on blood homeostasis.

Ecotin: Exploring a feasible antithrombotic profile

Ecotin is an Escherichia coli-derived protein that can inhibit serine proteases. It has been suggested that this protein (ecotin-WT) and some of its variants could be used to develop a prototype to treat thrombosis. In this work, the effect of ecotin-WT and a variant of this protein harboring two mutations (Met84Arg and Met85Arg, ecotin-RR) were analyzed to determine their ability to prevent thrombus formation using in vivo models. Ecotins were analyzed in vitro using the coagulation tests. An in vivo venous thrombosis rat model and a pulmonary thromboembolism mouse model were used to investigate the antithrombotic activity. The bleeding time in rats using a tail-transection model was evaluated as a possible side effect caused by the administration of these proteins. Ecotin-RR was more effective in inhibiting thrombin than ecotin-WT. Both ecotins presented similar mechanisms of anticoagulation activity and were able to decrease thrombus formation. In contrast, only ecotin-RR increased survival rates in the in vivo pulmonary thromboembolism model, reinforcing the antithrombotic activity of ecotin-RR. Ecotin-WT and more so ecotin-RR showed potent antithrombotic effects, not associated with bleeding. The presented results indicate that ecotin-WT and ecotin-RR may be new scaffolds that could be used to develop anticoagulation molecules.

Effects of single N-glycosylation site knockout on folding and defibrinogenating activities of acutobin recombinants from HEK293T

Acutobin, the α-fibrinogenase from Deinagkistrodon acutus venom, contains four N-glycosylation sites with disialylated complex-typed glycans. Here, we explore the functional roles of each of the N-glycan by site-directed mutagenesis. The wild-type (ATB-wt) and single glycan-knockout mutants of recombinant acutobin were prepared from HEK293T, demonstrating that mutations at Asn(77), Asn(81) and Asn(100) impaired the folding while the S79A mutant and various Asn(229)-deglycosylated mutants were correctly folded. Based on homology modeling of acutobin and multiple sequence alignment with various venom thrombin-like enzymes, the importance of a hydrophilic environment at each glycosylation site to the enzyme folding could be rationalized. Remarkably, all the mutants showed similar catalytic activities for the chromogenic substrate and similar thermal stabilities as ATB-wt, suggesting that the glycan knockout did not affect the gross conformation and stability of the active sites. Although SDS-PAGE analyses revealed that ATB-wt and the D229-mutant degraded all human fibrinogen subunits faster but less specifically in vitro as compared with other mutants that cleaved only the α-subunit, ATB-wt and D229-mutant were not able to release fibrinogen-peptide A and thus coagulated human plasma slower than the other mutants did. In the mice model, the defibrinogenating effect of ATB-wt was stronger and lasting-longer than those of all the mutants. Taken together, all the glycans contribute to the pharmacokinetics of acutobin and ATB-wt in vivo, and the microenvironment around the Asn(229)-glycan appears to regulate the fibrinogen-chain specificity of acutobin while the N-glycans at positions 77, 81 and 100 are crucial for its folding.

Isolation and molecular characterization of a major hemolymph serpin from the triatomine, Panstrongylus megistus

Background

Chagas disease kills 2.5 thousand people per year of 15 million persons infected in Latin America. The disease is caused by the protozoan, Trypanosome cruzi, and vectored by triatomine insects, including Panstrongylus megistus, an important vector in Brazil. Medicines treating Chagas disease have unpleasant side effects and may be ineffective, therefore, alternative control techniques are required. Knowledge of the T. cruzi interactions with the triatomine host needs extending and new targets/strategies for control identified. Serine and cysteine peptidases play vital roles in protozoan life cycles including invasion and entry of T. cruzi into host cells. Peptidase inhibitors are, therefore, promising targets for disease control.

Methods

SDS PAGE and chromatograpy detected and isolated a P. megistus serpin which was peptide sequenced by mass spectrometry. A full amino acid sequence was obtained from the cDNA and compared with other insect serpins. Reverse transcription PCR analysis measured serpin transcripts of P. megistus tissues with and without T. cruzi infection. Serpin homology modeling used the Swiss Model and Swiss-PDB viewer programmes.

Results

The P. megistus serpin (PMSRP1) has a ca. 40 kDa molecular mass with 404 amino acid residues. A reactive site loop contains a highly conserved hinge region but, based on sequence alignment, the normal cleavage site for serine proteases at P1-P1′ was translocated to the putative position P4′-P5′. A small peptide obtained corresponded to the C-terminal 40 amino acid region. The secondary structure of PMSRP1 indicated nine α-helices and three β-sheets, similar to other serpins. PMSRP1 transcripts occurred in all tested tissues but were highest in the fat body and hemocytes. Levels of mRNA encoding PMSRP1 were significantly modulated in the hemocytes and stomach by T. cruzi infection indicating a role for PMSRP1 in the parasite interactions with P. megistus.

Conclusions

For the first time, a constitutively expressed serpin has been characterized from the hemolymph of a triatomine. This opens up new research avenues into the roles of serine peptidases in the T. cruzi/P. megistus association. Initial experiments indicate a role for PMSRP1 in T. cruzi interactions with P. megistus and will lead to further functional studies of this molecule.

New insights on the Golgi complex of Tritrichomonas foetus

Tritrichomonas foetus is a protist that causes bovine trichomoniasis and presents a well-developed Golgi. There are very few studies concerning the Golgi in trichomonads. In this work, monoclonal antibodies were raised against Golgi of T. foetus and used as a tool on morphologic and biochemical studies of this organelle. Among the antibodies produced, one was named mAb anti-Golgi 20.3, which recognized specifically the Golgi complex by fluorescence and electron microscopy. By immunoblotting this antibody recognized two proteins with 60 and 66 kDa that were identified as putative beta-tubulin and adenosine triphosphatase, respectively. The mAb 20.3 also recognized the Golgi complex of the Trichomonas vaginalis, a human parasite. In addition, the nucleotide coding sequences of these proteins were identified and included in the T. foetus database, and the 3D structure of the proteins was predicted. In conclusion, this study indicated: (1) adenosine triphosphatase is present in the Golgi, (2) ATPase is conserved between T. foetus and T. vaginalis, (3) there is new information concerning the nucleic acid sequences and protein structures of adenosine triphosphatase and beta-tubulin from T. foetus and (4) the mAb anti-Golgi 20.3 is a good Golgi marker and can be used in future studies.

Rapid purification and procoagulant and platelet aggregating activities of Rhombeobin: a thrombin-like/gyroxin-like enzyme from Lachesis muta rhombeata snake venom

We report a rapid purification method using one-step chromatography of SVSP Rhombeobin (LMR-47) from Lachesis muta rhombeata venom and its procoagulant activities and effects on platelet aggregation. The venom was fractionated by a single chromatographic step in RP-HPLC on a C8 Discovery BIO Wide Pore, showing high degree of molecular homogeneity with molecular mass of 47035.49 Da. Rhombeobin showed amidolytic activity upon BA ρ NA, with a broad optimum pH (7-10) and was stable in solution up to 60°C. The amidolytic activity was inhibited by serine proteinase inhibitors and reducing agents, but not chelating agents. Rhombeobin showed high coagulant activity on mice plasma and bovine fibrinogen. The deduced amino acid sequence of Rhombeobin showed homology with other SVSPs, especially with LM-TL (L. m. muta) and Gyroxin (C. d. terrificus). Rhombeobin acts, in vitro, as a strong procoagulant enzyme on mice citrated plasma, shortening the APTT and PT tests in adose-dependent manner. The protein showed, "ex vivo", a strong defibrinogenating effect with 1 µg/animal. Lower doses activated the intrinsic and extrinsic coagulation pathways and impaired the platelet aggregation induced by ADP. Thus, this is the first report of a venom component that produces a venom-induced consumptive coagulopathy (VICC).

Structural model of haptoglobin and its complex with the anticoagulant ecotin variants: structure-activity relationship study and analysis of interactions

Recently the literature described the binding of Haptoglobin (HP) with ecotin, a fold-specific serine-proteases inhibitor with an anticoagulant profile and produced by Escherichia coli. In this work, we used some in silico and in vitro techniques to evaluate HP 3D-fold and its interaction with wild-type ecotin and two variants. Our data showed HP models conserved trypsin fold, in agreement to the in vitro immunological recognition of HP by trypsin antibodies. The analysis of the three ecotin-HP complexes using the mutants RR and TSRR/R besides the wild type revealed several hydrogen bonds between HP and ecotin secondary site. These data are in agreement with the in vitro PAGE assays that showed the HP-RR complex in native gel conditions. Interestingly, the ternary complex interactions varied depending on the inhibitor structure and site-directed mutation. The interaction of HP with TSRR/R involved new residues compared to wild type, which infers a binding energy increase caused by the mutation.

Thrombin a-chain: activation remnant or allosteric effector?

Although prothrombin is one of the most widely studied enzymes in biology, the role of the thrombin A-chain has been neglected in comparison to the other domains. This paper summarizes the current data on the prothrombin catalytic domain A-chain region and the subsequent thrombin A-chain. Attention is given to biochemical characterization of naturally occurring prothrombin A-chain mutations and alanine scanning mutants in this region. While originally considered to be simply an activation remnant with little physiologic function, the thrombin A-chain is now thought to play a role as an allosteric effector in enzymatic reactions and may also be a structural scaffold to stabilize the protease domain.

Higher specificity of the activity of low molecular weight fucoidan for thrombin-induced platelet aggregation

Algal fucoidans possess a wide variety of biological activities, including anticoagulation and antithrombosis, making them potential candidates for clinical use. We assessed the antiaggregant, anticoagulant and antithrombotic activities and the underlying mechanism of the low- and high-molecular weight fucoidans from the seaweed Laminaria japonica of Qingdao, China (F-Q and HMWF-Q). In the platelets of rats and humans, HMWF-Q demonstrated a pro-aggregation response, whereas F-Q (like the commercially purchased fucoidan (F-S) and heparin), showed an inhibitory effect on thrombin-induced aggregation with an IC(50) of 8 microg/mL, approximately five times lower than those of F-S and heparin. In the activated partial thromboplastin time test, F-Q (40 microg/mL) demonstrated less potent effect than F-S (40 microg/mL) and heparin (7mug/mL); 162+/-2.4s vs. 250+/-13.2s and >300s, p<0.01, respectively. It was also less effective than F-S on inhibiting thrombin catalyzed fibrinogen cleavage (IC(50) 10 microg/mL vs. 2.8 microg/mL) in vitro and rat thrombosis in vivo at 3mg/kg (i.v.). The inhibitory effects of F-Q and heparin on thrombin activity were strikingly enhanced by either antithrombin (AT) or heparin cofactor II (HCII). A direct interaction of F-S with thrombin, and F-Q or heparin with AT was demonstrated in both fluorescence quenching and PAGE analysis. Additionally, a pro-aggregation effect and an enhancement of thrombin activity were also observed with F-S, but not with F-Q or heparin, treatment. These results indicate that F-Q inhibits thrombin via activation of AT and HCII, whereas F-S mainly interacts directly with thrombin. Importantly, F-Q shows a higher specificity for hypoaggregation and a weaker effect for anticoagulant profiles than heparin and F-S. Therefore, F-Q could be a promising candidate for the treatment of thrombosis-related cardiovascular diseases.

Isolation and characterization of a new serine protease with thrombin-like activity (TLBm) from the venom of the snake Bothrops marajoensis

The thrombin-like serine protease TLBm from Bothrops marajoensis was isolated in one chromatographic step in reverse phase HPLC. Its molecular mass was 33239.95 Da, as based on the determined primary structure and confirmed experimentally by MALDI-TOF mass spectrometry (33332.5 Da) and it contains 12 half-cysteine residues. This TLBm exhibited high specificity for BArhoNA, Michaelis-Menten behavior with K(m) 2.3x10(-1)M and the V(max) 0.52x10(-1) nmoles rho-NA/lt/min for this substrate. TLBm also showed ability to coagulate bovine fibrinogen and was inhibited by soybean trypsin inhibitor, EDTA and S(Dm) from the serum of the species Didelphis marsupialis. The primary structure of TLBm showed the presence of His(45), Asp(103) and Ser(228) residues in the corresponding positions of the catalytic triad established in the serine proteases and Ser(228) are inhibited by phenylmethylsulfonyl fluoride (PMSF). Amino acid analysis showed a high content of Asp, Glu, Gly, Ser, Ala and Pro as well as 12 half-cysteine residues and calculated pI of 6.47; TLBm presented 285 amino acid residues. In this work, we investigated the ability of TLBm to degrade fibrinogen and we observed that it is able to cause alpha- and beta-chain cleavage. Enzymatic as well as the platelet aggregation activities were strongly inhibited when incubated with PMSF, a specific inhibitor of serine protease. Also, TLBm induced platelet aggregation in washed and platelet-rich plasma, and in both cases, PMSF inhibited its activity.

Integrin inhibitors from snake venom: exploring the relationship between the structure and activity of RGD-peptides

alphaIIbbeta3 is an integrin that is involved in platelet adhesion and aggregation. This receptor may be inhibited by cysteine-rich peptides known as disintegrins. We isolated two disintegrins from Bothrops jararaca venom called jarastatin and jararacin. We evaluated the structural characteristics and the effects on human platelet aggregation of these disintegrins. Inhibitory profiles were compared to six distinct peptides synthesized based on their RGD hairpin loop primary sequences. Both jarastatin and jararacin inhibited ADP and thrombin induction. Conversely, none of the cyclic peptides showed high-quality activity in assays induced by ADP or thrombin. We constructed homology models for all of these molecules, and theoretically evaluated their interaction with the alphaIIbbeta3 crystal structure using a molecular modeling approach. These results support the observations that the cyclic peptides had little effects, and also reinforce the observation that residues outside the disintegrin RGD sequence are required for interactions with receptor.

Structural characterization and low-resolution model of BJ-48, a thrombin-like enzyme from Bothrops jararacussu venom

Thrombin-like enzymes (TLEs) are important components of snake venoms due to their involvement in coagulopathies occurring on envenoming. Structural characterization of this group of serine proteases is of utmost importance for better understanding their unique properties. However, the high carbohydrate content of some members of this group prevents successful crystallization for structural determination. Circumventing this difficulty, the structure of BJ-48, a highly glycosylated TLE from Bothrops jararacussu venom, was studied in solution. At pH 8.0, where the enzyme displays maximum activity, BJ-48 has a radius of gyration (Rg) of 37 A and a maximum dimension (D(max)) of 130 A as measured by small-angle X-ray scattering (SAXS) and a Stokes radius (SR) of 50 A according to dynamic light scattering (DLS) data. At the naturally more acidic pH (6.0) of the B. jararacussu venom BJ-48 behaves as a more compact particle as evidenced by SAXS (R(g)=27.9 A and D(max)=82 A) and DLS (SR=30 A) data. In addition, Kratky plot analysis indicates a rigid shape at pH 8.0 and a flexible shape at pH 6.0. On the other hand, the center of mass of intrinsic fluorescence was not changed while varying pH, possibly indicating the absence of fluorescent amino acids in the regions affected by pH variation. Circular dichroism experiments carried out with BJ-48 indicate a substantially random coiled secondary structure that is not affected by pH. Low-resolution model of BJ-48 presented a prolate elongated shape at pH 8.0 and a U-shape at 6.0. BJ-48 tertiary structure at pH 6.0 was maintained on heating up to 52 degrees C and was completely lost at 75 degrees C. The possible existence of two pH-induced folding states for BJ-48 and its importance for the biological role and stability of this enzyme was discussed.

Isolation and characterization of a serine protease, Ba III-4, from Peruvian Bothrops atrox venom

A serine protease from Bothrops atrox (Peruvian specimen's venom) was isolated in two chromatographic steps in LC molecular exclusion and reverse phase-HPLC. This protein was denominated Ba III-4 (33,080.265 Da determinated by MALDI-TOF mass spectrometry) and showed pI of 5.06, Km 0.2 x 10(-1 ) M and the V (máx) 4.1 x 10(-1 )nmoles p-NA/lt/min on the synthetic substrate BapNA. Ba III-4 also showed ability to coagulate bovine fibrinogen. The serine protease was inhibited by soyben trypsin inhibitor and DA2II, which is an anti-hemorrhagic factor isolated from the opossum specie Didelphis albiventris. The primary structure of Ba III-4 showed the presence of His(44), Asp(94) and Ser(193) residues in the corresponding positions to the catalytic triad established in the serine proteases and Ser(193) are inhibited by phenylmethylsulfonylfluoride (PMSF). Amino acid analysis showed a high content of Asp, Glu, Gly, Ser, Ala and Pro, as well as 12 half-cysteine residues. Ba III-4 contained 293 amino acid residues and the primary structure of VIGGDECDIN EHPFLAFMYY SPRYFCGMTL INQEWVLTAA HCRYFCGMTL IHLGVHRESE KANYDEVRRF PKEKYFIFCD NNFTDDEVDK DIMLIRLDKP VSNSEHIAPL SLPSNPPSVG SVCRIMGWGQ TTTSPIDVLS PDEPHCANIN LFDNTVCHTA HPQVANTRTS TDTLCAGDLQ GGRDTCNGDS GGPLICNEQL HGILSWGGDP CAQPNKPAFY TKVYYFDHPW IKSIIAGNKK TVNFTCPPLR SDAKDDSTTY INQEWDWVLT AEHCDRTHMR NSFYDYSSIN SDS. Titration experiments did not show the presence of free sulfhydryl groups after 4 h incubation, nor were differences found in relation to titration kinetics in the presence of nondenaturating buffer. The isolation of this protein, Ba III-4, is of potential interest for the understanding of the pathomechanism of the snake venom action and for the identification of new blood coagulation enzymes of natural sources.

Structure–function inferences based on molecular modeling, sequence-based methods and biological data analysis of snake venom lectins

Lectins are a structurally and functionally diverse group of proteins from different sources, capable to recognize and bind specifically carbohydrates. Several snake venoms contain calcium-dependent true lectins (SVLs) that recognize galactose. Herein, in order to enlighten some of the structure-function relationships of snake venom lectins (SVLs), we constructed theoretical models for 10 SVLs based on the Crotalus atrox lectin (CaL), the only SVL crystal structure available, and compared with other animal and plant lectins, and C-type lectin-like proteins (CLPs) that do not bind carbohydrates. Although these are theoretical structures, we could identify some SVL features, including: (i) a singular intrachain disulfide bond (Cys(38)-Cys(133)) that is not present in CLPs; (ii) a significant reorientation (39-41A) of the 80's loop position that folds back to the globular domain, assists the carbohydrate recognition domain (CRD), and orients the dimer formation, even in BfL-1 and BfL-2, which did not present the Cys(86) interchain; (iii) a CRD presenting a negative and concave surface that allows the interaction with the specific saccharide hydroxyl groups and calcium ion; (iv) the role of water molecules in some interchain interactions, similar to other animal and plant lectins; and (v) the inability of forming oligomers in contrast to CaL and some CLPs, such as convulxin.

Insights into the substrate specificity of a novel snake venom serine peptidase by molecular modeling

The cDNA encoding BthaTL, a serine peptidase from the venom of the snake Bothrops alternatus, was cloned and sequenced. The deduced primary structure shows over 62% of identity with snake venom thrombin-like enzymes (SVTLEs), molecules with high substrate specificity toward different natural substrates. Indeed, a phylogenetic reconstruction by two different methods clustered this enzyme close to other SVTLEs. These enzymes generally affect the hemostatic system in several ways, and therefore are used as tools in pharmacology and clinical diagnosis. A three-dimensional model of BthaTL was built by homology modeling using TSV-PA (Trimeresurus stejnegeri venom plasminogen activator) crystal structure as template. BthaTL model showed that the typical catalytic triad conformation of serine peptidases was preserved. The calcium coordination ligands were absent or adopt an unfavorable conformation, preventing interactions with metals. On the other hand, the Asp97-Arg174 saline bridge of TSV-PA was not found and its specificity determinant Phe193 is replaced by a Gly in BthaTL. The substitution of essential residues in the neighborhoods of the catalytic site cleft of BthaTL indicates that these two proteins do not share the same enzymatic specificity, what means that BthaTL will probably not activate plasminogen. Such observations may be helpful in the understanding of the molecular mechanism for substrate specificity of these enzymes.

Characterization of a New Platelet Aggregating Factor from Crotoxin Crotalus durissus cascavella Venom

In this article we investigated the platelet aggregating activity of whole crotoxin and its subunits isolated from Crotalus durissus cascavella venom. During the purification protocols of the venom, using HPLC molecular exclusion, we detected the presence of two different serine protease activities in the gyroxin fraction, and another in the crotoxin fraction, which induced strong and irreversible platelet aggregation, in addition to blood coagulation. From crotoxin, we isolated PLA2, crotapotin (both fractions corresponding approximately 85% of whole crotoxin) and another minor fraction (F20) that exhibited serine protease activity. After a new fractionation on reverse phase HPLC chromatography, we obtained three other fractions named as F201, F202 and F203. F202 was obtained with high degree of molecular homogeneity with molecular mass of approximately 28 kDa and a high content of acidic amino residues, such as aspartic acid and glutamic acid. Other important amino acids were histidine, cysteine and lysine. This protein exhibited a high specificity for BApNA, a Michaelis-Menten behavior with Vmax estimated in 5.64 microM/min and a Km value of 0.58 mM for this substrate. In this work, we investigated the ability of F202 to degrade fibrinogen and observed alpha and beta chain cleavage. Enzymatic as well as the platelet aggregation activities were strongly inhibited when incubated with TLCK and PMSF, specific inhibitors of serine protease. Also, F202 induced platelet aggregation in washed and platelet-rich plasma, and in both cases, TLCK inhibited its activity. The N-terminal amino acid sequence of F202 presented a high amino acid sequence homology with other thrombin-like proteins, but it was significantly different from gyroxin. These results showed that crotoxin is a highly heterogeneous protein composed of PLA2, thrombin-like and other fractions that might explain the diversity of physiological and pharmacological activities of this protein.

Mechanism of action and determination of the best substrate for a thrombin-like enzyme from Lachesis muta muta venom by regression analysis of the kinetic parameters determined with peptidyl p-nitroanilide substrates

The kinetic behavior of a thrombin-like enzyme from Lachesis muta muta venom has been studied with 13 tripeptidyl p-nitroanilide substrates. Eight substrates were unprotected at the N terminus and were used for the regression analysis of the experimentally determined kinetic parameters 1/Km, kcat and kcat/Km. The individual contribution of each amino acid side chain to the kinetic parameters was calculated. The amino acid sequence of the ideal substrate (D-Pro-Leu-Arg-pNA) was determined from a regression analysis for each kinetic parameter. This result was confirmed experimentally. The structural analysis of the tripeptides showed that the binding to the S3 sub-site had a small effect on Km. The binding of L-Leu to the S2 sub-site increased kcat without changing the value of Km. The analysis of the kinetic parameters revealed that, in the binding of L-Leu to the S2 sub-site, the enzyme bound the transition state configuration of the substrate/product transformation more tightly than that of the substrate.

Ecotin modulates thrombin activity through exosite-2 interactions

Ecotin is a Escherichia coli-derived protein that has been characterized as a potent inhibitor of serine-proteases. This protein is highly effective against several mammalian enzymes, which includes pancreatic and neutrophil-derived elastases, chymotrypsin, trypsin, factor Xa, and kallikrein. In this work we showed that ecotin binds to human alpha-thrombin via its secondary binding site, and modulates thrombin catalytic activity. Formation of wild type ecotin-alpha-thrombin complex was observed by native PAGE and remarkably, gel filtration chromatography showed an unusual 2:1 ecotin:enzyme stoichiometry. Analysis of the protease inhibitor effects on thrombin biological activities showed that (i) it decreases the inhibition of thrombin by heparin/antithrombin complex (IC50=3.2 microM); (ii) it produces a two-fold increase in the thrombin-induced fibrinogen clotting; and (iii) it inhibits thrombin-induced platelet aggregation (IC50=4.5 microM). Allosteric changes on thrombin structure were then evaluated. Complex formation with ecotin caused a three-fold increase in the rate of thrombin inhibition by BPTI, suggesting a displacement of the enzyme's 60-loop. In addition, ecotin modulated the enzyme's catalytic site, as demonstrated by changes in the fluorescence emission of fluorescein-FPRCK-alpha-thrombin (EC50=3.5 microM). Finally, solid phase competition assays demonstrated that heparin and prothrombin fragment 2 prevents thrombin interaction with ecotin. Altogether, these observations strongly support an ecotin interaction with thrombin anion-binding exosite-2, resulting in modulation of its biological activities. At this point, ecotin might be useful as a new tool for studying thrombin allosteric modulation.

Determinants of specificity in coagulation proteases

Proteases play diverse roles in a variety of essential biological processes, both as non-specific catalysts of protein degradation and as highly specific agents that control physiologic events. Here, we review the mechanisms of substrate specificity employed by serine proteases and focus our discussion on coagulation proteases. We dissect the interplay between active site and exosite specificity and how substrate recognition is regulated allosterically by Na+ binding. We also draw attention to a functional polarity that exists in the serine protease fold, which sheds light on the structural linkages between the active site and exosites.

Biological and immunological properties of the venom of Bothrops alcatraz, an endemic species of pitviper from Brazil

Bothrops alcatraz is a new pitviper species derived from the Bothrops jararaca group, whose natural habitat is situated in Alcatrazes Archipelago, a group of marine islands near São Paulo State coast in Brazil. Herein, the biological and biochemical properties of venoms of four adult specimens of B. alcatraz were examined comparatively to a reference pool of Bothrops jararaca venom. Both venoms showed similar activities and electrophoretic patterns, but B. alcatraz venom showed three protein bands of molecular masses of 97, 80 and 38 kDa that were not present in B. jararaca reference venom. The i.p. median lethal dose of B. alcatraz venom ranged from 5.1 to 6.6 mg/kg, while it was 1.5 mg/kg for B. jararaca venom. The minimum hemorrhagic dose of B. jararaca venom was 0.63, whereas 2.28 mug/mouse for B. alcatraz venom. In contrast, B. alcatraz venom was more potent in regard to procoagulant and proteolytic activities. These differences were supported by western blotting and neutralization tests, employing commercial bothropic antivenom, which showed that hemorrhagic and lethal activities of B. alcatraz venom were less effectively inhibited than B. jararaca venom. Such results evidence that B. alcatraz shows quantitative and qualitative differences in venom composition in comparison with its B. jararaca relatives, which might represent an optimization of venom towards a specialized diet.

Snake venom serine proteinases: sequence homology vs. substrate specificity, a paradox to be solved

Snake venom glands synthesize a variety of serine proteinases capable of affecting the haemostatic system. They act on macromolecular substrates of the coagulation, fibrinolytic, and kallikrein-kinin systems, and on platelets to cause an imbalance of the haemostatic system of the prey. In this review we describe their biochemical/biophysical characteristics, biological activities as well as aspects of their evolution and structure-activity relationship.

S1 subsite in snake venom thrombin-like enzymes: can S1 subsite lipophilicity be used to sort binding affinities of trypsin-like enzymes to small-molecule inhibitors?

Thrombin-like enzymes isolated from snake venoms comprise a group of serine proteinases responsible for many important coagulation disorders in the envenomed victims. Besides, these proteinases have great biotechnological interest as antithrombotic agents and as diagnostic tools. However, in spite of the recent overflow of snake venom thrombin-like enzymes (SVTLEs) on protein sequence databases, there is a lack of three-dimensional (3D) structural information on this family. Without such 3D structures available many aspects of the biological function and biochemical properties of these enzymes still remain obscure. Therefore, we have gone through a series of computational techniques, which enabled us to identify the set of residues involved in molecular recognition of inhibitors bound to the S1 subsite of snake venom thrombin-like enzymes (SVTLEs) and ultimately conclude that nonpolar (van der Waals) intermolecular interactions and ligand's hydrophobicity are the most important factors affecting binding affinities to the S1 subsite of a SVTLE isolated from the venom of Lachesis muta muta (Lmm-TLE). Consequently, we have proposed that S1 subsite lipophilicity may be used to sort binding affinities of trypsin-like enzymes to small molecules by showing that the inhibitory potency of several S1-directed compounds follows subsite lipophilicity among Lmm-TLE and other three homologous proteases. Noteworthy, in the course of our analyses we determined that thrombin's S1 subsite should, in fact, be considered less lipophilic than that of trypsin if we account for the presence of the sodium-controlled water channel communicating with the S1 subsite in the coagulant enzyme.

Cloning, characterization, and structural analysis of a C-type lectin from Bothrops insularis (BiL) venom

Lectins are carbohydrate-binding molecules that mediate a variety of biological processes. In this work, we identify and characterize a lectin from Bothrops insularis venom, with respect to its biochemical properties and theoretical structure. Initially, from a venom gland cDNA library, we cloned and sequenced a cDNA encoding a protein with high identity to snake venom lectins. A lectin molecule was purified to homogeneity from the venom by affinity column and gel filtration. This protein named BiL displayed hemagglutinating activity that was inhibited by galactose, lactose, and EDTA. Mass spectrometry analysis and sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that BiL is a disulfide-linked dimeric protein consisting of monomers with 16,206 m/z. The amino acid sequence, deduced from its cDNA sequence, was confirmed by Edman sequencing and by peptide mass fingerprint analysis. BiL shows similarity to other C-type lectin family members. Modeling studies provide insights into BiL dimeric structure and its structural determinants for carbohydrate and calcium binding.

Bitis gabonica (Gaboon viper) snake venom gland: toward a catalog for the full-length transcripts (cDNA) and proteins

The venom gland of the snake Bitis gabonica (Gaboon viper) was used for the first time to construct a unidirectional cDNA phage library followed by high-throughput sequencing and bioinformatic analysis. Hundreds of cDNAs were obtained and clustered into contigs. We found mostly novel full-length cDNA coding for metalloproteases (P-II and P-III classes), Lys49-phospholipase A2, serine proteases with essential mutations in the active site, Kunitz protease inhibitors, several C-type lectins, bradykinin-potentiating peptide, vascular endothelial growth factor, nucleotidases and nucleases, nerve growth factor, and L-amino acid oxidases. Two new members of the recently described short coding region family of disintegrin, displaying RGD and MLD motifs are reported. In addition, we have identified for the first time a cytokine-like molecule and a multi-Kunitz protease inhibitor in snake venoms. The CLUSTAL alignment and the unrooted cladograms for selected families of B. gabonica venom proteins are also presented. A significant number of sequences were devoid of database matches, suggesting that their biologic function remains to be identified. This paper also reports the N-terminus of the 15 most abundant venom proteins and the sequences matching their corresponding transcripts. The electronic version of this manuscript, available on request, contains spreadsheets with hyperlinks to FASTA-formatted files for each contig and the best match to the GenBank and Conserved Domain Databases, in addition to CLUSTAL alignments of each contig. We have thus generated a comprehensive catalog of the B. gabonica venom gland, containing for each secreted protein: (i) the predicted molecular weight, (ii) the predicted isoelectric point, (iii) the accession number, and (iv) the putative function. The role of these molecules is discussed in the context of the envenomation caused by the Gaboon viper.

Functional characterization of recombinant batroxobin, a snake venom thrombin-like enzyme, expressed fromPichia pastoris

A thrombin-like enzyme of Bothrops atrox moojeni venom, batroxobin, specifically cleaves fibrinogen alpha chain, resulting in the formation of non-crosslinked fibrin clots. The cDNA encoding batroxobin was cloned, expressed in Pichia pastoris and the molecular function of purified recombinant protein was also characterized. The recombinant batroxobin had an apparent molecular weight of 33 kDa by SDS-PAGE analysis and biochemical activities similar to those of native batroxobin. The purified recombinant protein strongly converted fibrinogen into fibrin clot in vitro, and shortened bleeding time and whole blood coagulation time in vivo. However, it did not make any considerable alterations on other blood coagulation factors. Several lines of experimental evidence in this study suggest that the recombinant batroxobin is a potent pro-coagulant agent.

Molecular cloning and expression of a functional snake venom serine proteinase, with platelet aggregating activity, from the Cerastes cerastes viper

The venoms of Viperidae snakes contain numerous serine proteinases that have been recognized to possess one or more of the essential activities of thrombin on fibrinogen and platelets. Among them, a platelet proaggregant protein, cerastocytin, has been isolated from the venom of the Tunisian viper Cerastes cerastes. Using the RACE-PCR technique, we isolated and identified the complete nucleotide sequence of a cDNA serine proteinase precursor. The recombinant protein was designated rCC-PPP (for C. cerastes platelet proaggregant protein), since its deduced amino acid sequence is more than 96% identical to the partial polypeptide sequences that have been determined for natural cerastocytin. The structure of the rCC-PPP cDNA is similar to that of snake venom serine proteinases. The expression of rCC-PPP in Escherichia coli system allowed, for the first time, the preparation and purification of an active protein from snake venom with platelet proaggregant and fibrinogenolytic activities. Purified rCC-PPP efficiently activates blood platelets at nanomolar (8 nM) concentrations, as do natural cerastocytin (5 nM) and thrombin (1 nM). It is able to clot purified fibrinogen and to hydrolyze alpha-chains. Thus, rCC-PPP could be therefore considered a cerastocytin isoform. By comparison with other snake venom serine proteinases, a Gly replaces the conserved Cys(42). This implies that rCC-PPP lacks the conserved Cys(42)-Cys(58) disulfide bridge. A structural analysis performed by molecular modeling indicated that the segment of residues Tyr(67)-Arg(80) of rCC-PPP corresponds to anion-binding exosite 1 of thrombin that is involved in its capacity to induce platelet aggregation. Furthermore, the surface of the rCC-PPP molecule is characterized by a hydrophobic pocket, comprising the 90 loop (Phe(90)-Val(99)), Tyr(172), and Trp(215) residues, which might be involved in the fibrinogen clotting activity of rCC-PPP.