Functional and sequence characterization of coagulation factor IX/factor X-binding protein from the venom of Echis carinatus leucogaster

Modulation of Diverse Procoagulant Venom Activities by Combinations of Platinoid Compounds

Procoagulant snake venoms have been inhibited by the ruthenium containing compounds CORM-2 and RuCl₃ separately, presumably by interacting with critical histidine or other sulfur-containing amino acids on key venom enzymes. However, combinations of these and other platinoid containing compounds could potentially increase, decrease or not affect the procoagulant enzyme function of venom. Thus, the purpose of this investigation was to determine if formulations of platinoid compounds could inhibit venom procoagulant activity and if the formulated compounds interacted to enhance inhibition. Using a human plasma coagulation kinetic model to assess venom activity, six diverse venoms were exposed to various combinations and concentrations of CORM-2, CORM-3, RuCl₃ and carboplatin (a platinum containing compound), with changes in venom activity determined with thrombelastography. The combinations of CORM-2 or CORM-3 with RuCl₃ were found to enhance inhibition significantly, but not in all venoms nor to the same extent. In sharp contrast, carboplatin-antagonized CORM-2 mediated the inhibition of venom activity. These preliminary results support the concept that platinoid compounds may inhibit venom enzymatic activity at the same or different molecular sites and may antagonize inhibition at the same or different sites. Further investigation is warranted to determine if platinoid formulations may serve as potential antivenoms.

Ruthenium, Not Carbon Monoxide, Inhibits the Procoagulant Activity of Atheris, Echis, and Pseudonaja Venoms

The demonstration that carbon monoxide releasing molecules (CORMs) affect experimental systems by the release of carbon monoxide, and not via the interaction of the inactivated CORM, has been an accepted paradigm for decades. However, it has recently been documented that a radical intermediate formed during carbon monoxide release from ruthenium (Ru)-based CORM (CORM-2) interacts with histidine and can inactivate bee phospholipase A₂ activity. Using a thrombelastographic based paradigm to assess procoagulant activity in human plasma, this study tested the hypothesis that a Ru-based radical and not carbon monoxide was responsible for CORM-2 mediated inhibition of Atheris,Echis, and Pseudonaja species snake venoms. Assessment of the inhibitory effects of ruthenium chloride (RuCl₃) on snake venom activity was also determined. CORM-2 mediated inhibition of the three venoms was found to be independent of carbon monoxide release, as the presence of histidine-rich albumin abrogated CORM-2 inhibition. Exposure to RuCl₃ had little effect on Atheris venom activity, but Echis and Pseudonaja venom had procoagulant activity significantly reduced. In conclusion, a Ru-based radical and ion inhibited procoagulant snake venoms, not carbon monoxide. These data continue to add to our mechanistic understanding of how Ru-based molecules can modulate hemotoxic venoms, and these results can serve as a rationale to focus on perhaps other, complementary compounds containing Ru as antivenom agents in vitro and, ultimately, in vivo.

Snake Venoms in Drug Discovery: Valuable Therapeutic Tools for Life Saving

Animal venoms are used as defense mechanisms or to immobilize and digest prey. In fact, venoms are complex mixtures of enzymatic and non-enzymatic components with specific pathophysiological functions. Peptide toxins isolated from animal venoms target mainly ion channels, membrane receptors and components of the hemostatic system with high selectivity and affinity. The present review shows an up-to-date survey on the pharmacology of snake-venom bioactive components and evaluates their therapeutic perspectives against a wide range of pathophysiological conditions. Snake venoms have also been used as medical tools for thousands of years especially in tradition Chinese medicine. Consequently, snake venoms can be considered as mini-drug libraries in which each drug is pharmacologically active. However, less than 0.01% of these toxins have been identified and characterized. For instance, Captopril^® (Enalapril), Integrilin^® (Eptifibatide) and Aggrastat^® (Tirofiban) are drugs based on snake venoms, which have been approved by the FDA. In addition to these approved drugs, many other snake venom components are now involved in preclinical or clinical trials for a variety of therapeutic applications. These examples show that snake venoms can be a valuable source of new principle components in drug discovery.

Structurally Robust and Functionally Highly Versatile-C-Type Lectin (-Related) Proteins in Snake Venoms

Snake venoms contain an astounding variety of different proteins. Among them are numerous C-type lectin family members, which are grouped into classical Ca²⁺- and sugar-binding lectins and the non-sugar-binding snake venom C-type lectin-related proteins (SV-CLRPs), also called snaclecs. Both groups share the robust C-type lectin domain (CTLD) fold but differ in a long loop, which either contributes to a sugar-binding site or is expanded into a loop-swapping heterodimerization domain between two CLRP subunits. Most C-type lectin (-related) proteins assemble in ordered supramolecular complexes with a high versatility of subunit numbers and geometric arrays. Similarly versatile is their ability to inhibit or block their target molecules as well as to agonistically stimulate or antagonistically blunt a cellular reaction triggered by their target receptor. By utilizing distinct interaction sites differentially, SV-CLRPs target a plethora of molecules, such as distinct coagulation factors and receptors of platelets and endothelial cells that are involved in hemostasis, thrombus formation, inflammation and hematogenous metastasis. Because of their robust structure and their high affinity towards their clinically relevant targets, SV-CLRPs are and will potentially be valuable prototypes to develop new diagnostic and therapeutic tools in medicine, provided that the molecular mechanisms underlying their versatility are disclosed.

Snake venom components in medicine: From the symbolic rod of Asclepius to tangible medical research and application

Both mythologically and logically, snakes have always fascinated man. Snakes have attracted both awe and fear not only because of the elegant movement of their limbless bodies, but also because of the potency of their deadly venoms. Practically, in 2017, the world health organization (WHO) listed snake envenomation as a high priority neglected disease, as snakes inflict up to 2.7 million poisonous bites, around 100.000 casualties, and about three times as many invalidities on man. The venoms of poisonous snakes are a cocktail of potent compounds which specifically and avidly target numerous essential molecules with high efficacy. The individual effects of all venom toxins integrate into lethal dysfunctions of almost any organ system. It is this efficacy and specificity of each venom component, which after analysis of its structure and activity may serve as a potential lead structure for chemical imitation. Such toxin mimetics may help in influencing a specific body function pharmaceutically for the sake of man's health. In this review article, we will give some examples of snake venom components which have spurred the development of novel pharmaceutical compounds. Moreover, we will provide examples where such snake toxin-derived mimetics are in clinical use, trials, or consideration for further pharmaceutical exploitation, especially in the fields of hemostasis, thrombosis, coagulation, and metastasis. Thus, it becomes clear why a snake captured its symbolic place at the Asclepius rod with good reason still nowadays.

Biochemistry of envenomation

Venoms and toxins are of significant interest due to their ability to cause a wide range of pathophysiological conditions that can potentially result in death. Despite their wide distribution among plants and animals, the biochemical pathways associated with these pathogenic agents remain largely unexplored. Impoverished and underdeveloped regions appear especially susceptible to increased incidence and severity due to poor socioeconomic conditions and lack of appropriate medical treatment infrastructure. To facilitate better management and treatment of envenomation victims, it is essential that the biochemical mechanisms of their action be elucidated. This review aims to characterize downstream envenomation mechanisms by addressing the major neuro-, cardio-, and hemotoxins as well as ion-channel toxins. Because of their use in folk and traditional medicine, the biochemistry behind venom therapy and possible implications on conventional medicine will also be addressed.

Metal ion binding to anticoagulation factor II from the venom of Agkistrodon acutus: stabilization of the structure and regulation of the binding affinity to activated coagulation factor X

Anticoagulation factor II (ACF II) isolated from the venom of Agkistrodon acutus is an activated coagulation factor X (FXa)-binding protein with both anticoagulant and hypotensive activities. The thermodynamics of the binding of alkaline earth metal ions to ACF II and their effects on the stability of ACF II and the binding of ACF II to FXa were investigated by isothermal titration calorimetry, fluorescence, differential scanning calorimetry, and surface plasmon resonance. The binding of ACF II to FXa does not have an absolute requirement for Ca(2+). Mg(2+), Sr(2+), and Ba(2+) can induce the binding of ACF II to FXa. The radii of the cations bound in ACF II crucially affect the binding affinity of ACF II for cations and the structural stability of ACF II against guanidine hydrochloride and thermal denaturation, whereas the radii of cations bound in FXa markedly affect the binding affinity between ACF II and FXa. The binding affinities of ACF II for cations and the capacities of metal-induced stabilization of ACF II follow the same trend: Ca(2+) > Sr(2+) > Ba(2+). The metal-induced binding affinities of ACF II for FXa follow the trend Mg(2+) > Ca(2+) > Sr(2+) > Ba(2+). Although Mg(2+) shows significantly low binding affinity with ACF II, Mg(2+) is the most effective to induce the binding of ACF II with FXa. Our observations suggest that in blood the bindings of Ca(2+) in two sites of ACF II increase the structural stability of ACF II, but these bindings are not essential for the binding of ACF II with FXa, and that the binding of Mg(2+) and Ca(2+) to FXa may be essential for the recognition between FXa and ACF II. Like Ca(2+), the abundant Mg(2+) in blood also plays an important role in the anticoagulation of ACF II.

Effect of metal ion substitutions in anticoagulation factor I from the venom of Agkistrodon acutus on the binding of activated coagulation factor X and on structural stability

Anticoagulation factor I (ACF I) isolated from the venom of Agkistrodon acutus is an activated coagulation factor X (FXa)-binding protein that binds in a Ca(2+)-dependent fashion with marked anticoagulant activity. The thermodynamics of the binding of alkaline earth metal ions to ACF I and the effects of alkaline earth metal ions on the guanidine hydrochloride (GdnHCl)-induced unfolding of ACF I and the binding of ACF I to FXa were studied by isothermal titration calorimetry, fluorescence, circular dichroism, and surface plasmon resonance, respectively. The results indicate that the ionic radii of the cations occupying Ca(2+)-binding sites in ACF I crucially affect the binding affinity of ACF I for alkaline earth metal ions as well as the structural stability of ACF I against GdnHCl denaturation. Sr(2+) and Ba(2+), with ionic radii larger than the ionic radius of Ca(2+), can bind to Ca(2+)-free ACF I (apo-ACF I), while Mg(2+), with an ionic radius smaller than that of Ca(2+), shows significantly low affinity for the binding to apo-ACF I. All bindings of Ca(2+), Sr(2+), and Ba(2+) ions in two sites of ACF I are mainly enthalpy-driven and the entropy is unfavorable for them. Sr(2+)-stabilized ACF I exhibits slightly lower resistance to GdnHCl denaturation than Ca(2+)-ACF I, while Ba(2+)-stabilized ACF I exhibits much lower resistance to GdnHCl denaturation than Ca(2+)-ACF I. Mg(2+) and Sr(2+), with ionic radii close to that of Ca(2+), can bind to FXa and therefore also induce the binding of ACF I to FXa, whereas Ba(2+), with a much larger ionic radius than Ca(2+), cannot support the binding of ACF I with FXa. Our observations suggest that bindings of Ca(2+), Sr(2+), and Ba(2+) ions in two sites of ACF I increase the structural stability of ACF I, but these bindings are not essential for the binding of ACF I with FXa, and that the binding of Mg(2+), Ca(2+), and Sr(2+) ions to FXa may be essential for the recognition between FXa and ACF I.

Snake-venom-derived Factor IX-binding protein specifically blocks the gamma-carboxyglutamic acid-rich-domain-mediated membrane binding of human Factors IX and X

A potent anticoagulant protein, IX-bp (Factor IX binding protein), has been isolated from the venom of Trimeresurus flavoviridis (habu snake) and is known to bind specifically to the Gla (gamma-carboxyglutamic acid-rich) domain of Factor IX. To evaluate the molecular basis for its anticoagulation activity, we assessed its interactions with various clotting factors. We found that the anticoagulation activity is primarily due to binding to the Gla domains of Factors IX and X, thus preventing these factors from recognizing phosphatidylserine on the plasma membrane. The present study suggests that ligands that bind to the Gla domains of Factors IX and X may have the potential to become novel anticoagulants.

Calcium Ion-Induced Stabilization and Refolding of Agkisacutacin from Agkistrodon Acutus Venom Studied by Fluorescent Spectroscopy

Agkisacutacin isolated from the venom of Agkistrodon acutus is a coagulation factor IX / coagulation factor X-binding protein with marked anticoagulant- and platelet-modulating activities. Ca(2+) ion-induced stabilization and refolding of Agkisacutacin have been studied by following fluorescent measurements. Ca(2+) ions not only increase the structural stability of agkisacutacin against GdnHCl denaturation, but also induce its refolding. The GdnHCl-induced unfolding of the apo-agkisacutacin and the purified agkisacutacin is a single-step process with no detectable intermediate state. Ca(2+) ions play an important role in the stabilization of the structure of agkisacutacin. Ca(2+)-stabilized agkisacutacin exhibits higher resistance to GdnHCl denaturation than the apo-agkisacutacin. It is possible to induce refolding of the unfolded apo-agkisacutacin merely by adding 1 mM Ca(2+) ions without changing the concentration of the denaturant. The kinetic result of Ca(2+)-induced refolding provides evidences for that agkisacutacin consists of at least two refolding phases and the first phase of Ca(2+)-induced refolding should involve the formation of the compact Ca(2+)-binding site regions, and subsequently, the protein undergoes further conformational rearrangements to form the native structure.

Structure-Function Relationships of C-Type Lectin-Related Proteins

The structural and functional studies of the first identified C-type lectin-like protein (CLP), blood coagulation factor IX/factor X-binding protein (IX/X-bp), have been instrumental in defining how new functionally heterodimeric CLPs are generated from monomeric carbohydrate recognition domain in C-type lectins by three-dimensional domain swapping. The crystal structures of gamma-carboxyglutamic acid domains of coagulation factors X and IX have recently been clarified in structural studies of complexes between the gamma-carboxyglutamic acid domain of factors X and X-bp (a venom CLP) and between the gamma-carboxyglutamic acid domain of factors IX and IX-bp (a venom CLP).

Characterization and molecular cloning of dabocetin, a potent antiplatelet C-type lectin-like protein from Daboia russellii siamensis venom

A novel C-type lectin-like protein, dabocetin, was purified from Daboia russellii siamensis venom. On SDS-polyacrylamide gel electrophoresis, it showed a single band with an apparent molecular weight of 28 kDa and two distinct bands with the apparent molecular weights of 15.0 kDa and 14.5 kDa under non-reducing and reducing conditions, respectively. cDNA clones containing the coding sequences for dabocetin alpha and beta subunits were isolated and sequenced. The deduced protein sequences of both subunits were confirmed by N-terminal amino acid sequencing and trypsin-digested peptide mass fingerprinting. Dabocetin did not induce platelet aggregation in platelet-rich plasma. It also had little effect on the platelet aggregation induced by ADP, TMVA or stejnulxin. Whereas, dabocetin inhibited ristocetin-induced platelet agglutination in platelet-rich plasma in a dose-dependent manner with an IC50 value of 0.35 microM. Flow cytometry analysis showed that dabocetin significantly inhibited mAb SZ2 binding to platelet membrane glycoprotein Ib alpha, indicating that platelet membrane glycoprotein Ib is involved in the inhibitory effect of dabocetin on ristocetin-induced platelet agglutination.

Expression of a recombinant anticoagulant C-type lectin-like protein ACFI in Pichia pastoris: heterodimerization of two subunits is required for its function

ACFI is an anticoagulant C-type lectin-like protein (CLP) isolated from Agkistrodon acutus venom. To investigate the function of ACFI and its subunits, the cDNAs of two subunits were transformed and expressed in Pichia pastoris separately or together by a novel strategy using two vectors with different selectable markers. The results showed that recombinant homodimers were secreted when the subunits were expressed alone, while heterodimers (rACFI) were secreted when two subunits were co-expressed. The secreted proteins were purified from culture supernatants in one step by metal-chelating affinity chromatography with the yields of 1-4 mg/L. PAGE and ELISA showed that rACFI competed the binding of native ACFI for human factor X and IX with affinities of 1.6 and 30 nM, respectively. In addition, rACFI prolonged the activated partial thromboplastin time (APTT) in a concentration dependent manner as same as native ACFI. However, recombinant alpha or beta homodimers completely lost these activities, indicating the heterodimerization of two subunits is required for its function. It also suggests that P. pastoris is a promising system for structure-function studies of snake CLPs.

Snake venom C-type lectins interacting with platelet receptors. Structure–function relationships and effects on haemostasis

Snake venoms contain components that affect the prey either by neurotoxic or haemorrhagic effects. The latter category affect haemostasis either by inhibiting or activating platelets or coagulation factors. They fall into several types based upon structure and mode of action. A major class is the snake C-type lectins or C-type lectin-like family which shows a typical folding like that in classic C-type lectins such as the selectins and mannose-binding proteins. Those in snake venoms are mostly based on a heterodimeric structure with two subunits alpha and beta, which are often oligomerized to form larger molecules. Simple heterodimeric members of this family have been shown to inhibit platelet functions by binding to GPIb but others activate platelets via the same receptor. Some that act via GPIb do so by inducing von Willebrand factor to bind to it. Another series of snake C-type lectins activate platelets by binding to GPVI while yet another series uses the integrin alpha(2)beta(1) to affect platelet function. The structure of more and more of these C-type lectins have now been, and are being, determined, often together with their ligands, casting light on binding sites and mechanisms. In addition, it is relatively easy to model the structure of the C-type lectins if the primary structure is known. These studies have shown that these proteins are quite a complex group, often with more than one platelet receptor as ligand and although superficially some appear to act as inhibitors, in fact most function by inducing thrombocytopenia by various routes. The relationship between structure and function in this group of venom proteins will be discussed.

Structures and functions of snake venom CLPs (C-type lectin-like proteins) with anticoagulant-, procoagulant-, and platelet-modulating activities

C-type lectin-like proteins (CLPs) have a variety of biological activities, including anticoagulant- and platelet-modulating activities but have no lectin activity. CLPs are made up of heterodimers or oligomers of heterodimers, while C-type lectins from snake venom are composed exclusively of homodimers or homooligomers. In the last decade, numerous CLPs, such as blood coagulation factor IX/X-binding protein and botrocetin, have been isolated from various snake venoms, sequenced, and characterized. In addition, RVV-X (factor X activator) and carinactivase-1 (prothrombin activator) are metalloproteases composed of two C-type lectin-like domains that recognize the Gla domain of factor X and prothrombin, respectively. The basic structures of these CLPs include two homologous subunits: subunit alpha (A chain) of 14-15 kDa and subunit beta (B chain) of 13-14 kDa. CLPs occur in a variety of oligomeric forms, including alphabeta, (alphabeta)(2), and (alphabeta)(4). The basic homologous dimer (alphabeta) of these CLPs is formed by three-dimensional (3D) domain swapping. The CLPs constitute a new protein family and are useful tools for elucidating the mechanisms involved in clotting and platelet activation as well as the structure-function relationships of both blood clotting factors and platelet glycoproteins.

Molecular diversity and accelerated evolution of C-type lectin-like proteins from snake venom

A number of C-type lectin-like proteins that affect thrombosis and hemostasis by inhibiting or activating specific platelet membrane receptors or blood coagulation factors have been isolated from the venom of various snake species and characterized and more than 80 have been sequenced. Recent data on the primary sequences and 3D structures of C-type lectins and C-type lectin-like proteins from snake venoms have enabled us to analyze their molecular evolution. Statistical analysis of their cDNA sequences shows that C-type lectin-like proteins, with some exceptions, have evolved in an accelerated manner to acquire their diverse functions. Phylogenetic analysis shows that the A and B chains of C-type lectin-like proteins are clearly separated from C-type lectins and that the A and B chains are further divided into a group of platelet receptor-binding proteins and a group of coagulation factor-binding proteins. Elucidation of the tertiary structures of several C-type lectin-like proteins led to the discovery of a unique domain-swapping interaction between heterodimeric subunits, which creates a concave surface for ligand binding.

Structure of rhodocetin reveals noncovalently bound heterodimer interface

Rhodocetin is a unique heterodimer consisting of alpha- and beta-subunits of 133 and 129 residues, respectively. The molecule, purified from the crude venom of the Malayan pit viper, Calloselasma rhodostoma, functions as an inhibitor of collagen-induced aggregation. Rhodocetin has been shown to have activity only when present as a dimer. The dimer is formed without an intersubunit disulfide bridge, unlike all the other Ca(2+)-dependent lectin-like proteins. We report here the 1.9 A resolution structure of rhodocetin, which reveals the compensatory interactions that occur in the absence of the disulfide bridge to preserve activity.

Purpureotin: a novel di-dimeric C-type lectin-like protein from Trimeresurus purpureomaculatus venom is stabilized by noncovalent interactions

Purpureotin, a novel di-dimeric C-type lectin-like protein (CLP) from Trimeresurus purpureomaculatus, was purified and sequenced. While its native molecular mass was determined to be 63kDa, purpureotin showed a single band of 30kDa on nonreducing SDS-PAGE and two polypeptide chains (16.0 and 14.5kDa) under reducing condition. These results were subsequently confirmed by mass spectrometric analyses. Based on these results, we postulate that purpureotin is a dimer of the alpha,beta-heterodimer which is held together by noncovalent interactions. Molecular modeling studies indicate that a dimer of alpha,beta-heterodimers can be formed where the alpha chains are held together by electrostatic charges and beta chains via hydrophobic interactions. Functionally, purpureotin induced platelet aggregation without any cofactor in a dose-dependent manner. However, the platelet aggregation effect was blocked by echicetin. Therefore, purpureotin is assumed to be a GPIb-binding protein which binds to the same or a closely related GPIb site on platelets as echicetin.

Novel sequences encoding venom C-type lectins are conserved in phylogenetically and geographically distinct Echis and Bitis viper species

Envenoming by Echis saw scaled vipers and Bitis arietans puff adders is the leading cause of death and morbidity in Africa due to snake bite. Despite their medical importance, the composition and constituent functionality of venoms from these vipers remains poorly understood. Here, we report the cloning of cDNA sequences encoding seven clusters or isoforms of the haemostasis-disruptive C-type lectin (CTL) proteins from the venom glands of Echis ocellatus, E. pyramidum leakeyi, E. carinatus sochureki and B. arietans. All these CTL sequences encoded the cysteine scaffold that defines the carbohydrate-recognition domain of mammalian CTLs. All but one of the Echis and Bitis CTL sequences showed greater sequence similarity to the beta than alpha CTL subunits in venoms of related Asian and American vipers. Four of the new CTL clusters showed marked inter-cluster sequence conservation across all four viper species which were significantly different from that of previously published viper CTLs. The other three Echis and Bitis CTL clusters showed varying degrees of sequence similarity to published viper venom CTLs. Because viper venom CTLs exhibit a high degree of sequence similarity and yet exert profoundly different effects on the mammalian haemostatic system, no attempt was made to assign functionality to the new Echis and Bitis CTLs on the basis of sequence alone. The extraordinary level of inter-specific and inter-generic sequence conservation exhibited by the Echis and Bitis CTLs leads us to speculate that antibodies to representative molecules should neutralise the biological function of this important group of venom toxins in vipers that are distributed throughout Africa, the Middle East and the Indian subcontinent.

Lebecetin, a potent antiplatelet C-type lectin from Macrovipera lebetina venom

A novel C-type lectin protein (CLP), lebecetin, was purified to homogeneity from the venom of Macrovipera lebetina by gel filtration on a Sephadex G75 column and ion exchange chromatography on Mono S column. Lebecetin is a basic protein with a pHi=9.9 and migrates in SDS-PAGE as a single band or two distinct bands under nonreducing and reducing conditions, respectively. These results are further confirmed by MALDI-TOF mass spectrometry that indicates a molecular mass of 29779 Da for native lebecetin and molecular masses of 15015 and 16296 Da for alpha and beta subunits, respectively. The N-terminal amino acid sequences of lebecetin subunits show a high degree of similarity with those of C-type lectin-like proteins. In addition, functional studies showed that lebecetin has a potent inhibitory effect on platelet aggregation induced by thrombin in a concentration-dependent manner. In contrast, no inhibitory effect is observed when platelets are exposed to thromboxane A2 (TxA2) mimetic (U46619) or arachidonic acid. Moreover, there was no effect either on blood coagulation or A, B and O washed human erythrocytes agglutination. Furthermore, flow cytometric analysis revealed that fluoro-isothiocyanate (FITC)-labelled lebecetin bound to human formalin fixed platelets in a saturable and concentration manner and this binding was specifically prevented by anti-glycoprotein Ib (GPIb) mAb. These observations suggest that lebecetin is a C-type lectin-like protein that selectively binds to platelet GPIb.

Cloning and characterization of a blood coagulation factor IX-binding protein from the venom of Trimeresurus stejnegeri

A blood coagulation factor IX-binding protein (TSV-FIX-BP) was isolated from the snake venom of Trimeresurus stejnegeri. On SDS-polyacrylamide gel electrophoresis, TSV-FIX-BP showed a single band with an apparent molecular weight of 23,000 under non-reducing conditions, and two distinct bands with apparent molecular weights of 14,800 and 14,000 under reducing conditions. cDNA clones containing the coding sequences of TSV-FIX-BP were isolated and sequenced to determine the structure of the precursors of TSV-FIX-BP subunits. The deduced amino acid sequences of two subunits of TSV-FIX-BP were confirmed by N-terminal protein sequencing and trypsin-digested peptide mass fingerprinting. TSV-FIX-BP was a non-enzymatic C-type lectin-like anti-coagulant. The anti-coagulant activity of TSV-FIX-BP was mainly caused by its dose dependent interaction with blood coagulation factor IX but not with blood coagulation factor X.

Calcium-binding analysis and molecular modeling reveal echis coagulation factor IX/factor X-binding protein has the Ca-binding properties and Ca ion-independent folding of other C-type lectin-like proteins

Many biologically active heterodimeric proteins of snake venom consist of two C-type lectin-like subunits. One of these proteins, habu IX/X-bp, is a Gla domain-binding protein whose subunits both bind to a Ca2+ ion, with a total of two Ca2+-binding sites. The molecular modeling and Ca2+-binding analysis of echis IX/X-bp revealed that it lacks one of two Ca2+-binding sites, though the folding of this subunit is conserved. It is concluded that heterodimeric C-type lectin-like proteins function independent of Ca2+ and have essentially a similar folding to habu IX/X-bp.

Characterization and cDNA cloning of halyxin, a heterogeneous three-chain anticoagulant protein from the venom of Agkistrodon halys brevicaudus

We report upon the isolation, characterization, and cDNA cloning of an anticoagulant protein, halyxin from Agkistrodon halys brevicaudus venom. The protein exists as a 29kDa protein, and is separated into three chains on SDS-PAGE under reducing conditions. However, we cloned only two cDNAs encoding halyxin from the cDNA library of the snake venom gland, on the basis of the determined amino acid sequences. The complete amino acid sequences were deduced from their nucleotide sequences and named halyxin A (129 amino acid residues) and B chain (123 amino acid residues). The deduced amino acid sequence of halyxin A chain corresponds to the two smaller chains. Thus, it is considered that halyxin A chain could be synthesized as a single-chain protein that is subsequently cleaved to yield the mature two-chain protein. The amino acid sequence of halyxin is similar to that of other snake venom proteins of the C-type lectin superfamily, and prolongs plasma-clotting time. In the presence of Ca(2+) ions, halyxin binds to coagulation factors IX, X, IXa, and Xa, but not to other vitamin K-dependent coagulation factors. It also inhibits factor Xa in a non-competitive manner but does not affect other activated coagulation factors.

Proteins from Mucuna pruriens and enzymes from Echis carinatus venom: characterization and cross-reactions

Mucuna pruriens seeds have been widely used against snakebite in traditional medicine. The antivenin property of a water extract of seeds was assessed in vivo in mice. The serum of mice treated with extract was tested for its immunological properties. Two proteins of Echis carinatus venom with apparent molecular masses of 25 and 16 kDa were detected by Western blot analysis carried out using IgG of mice immunized with extract or its partially purified protein fractions. By enzymatic in-gel digestion and electrospray ionization-mass spectrometry/mass spectrometry analysis of immunoreactive venom proteins, phospholipase A(2,) the most toxic enzyme of snake venom, was identified. These results demonstrate that the observed antivenin activity has an immune mechanism. Antibodies of mice treated with non-lethal doses of venom reacted against some proteins of M. pruriens extract. Proteins of E. carinatus venom and M. pruriens extract have at least one epitope in common as confirmed by immunodiffusion assay.

Ca(II)- and Tb(III)-induced stabilization and refolding of anticoagulation factor I from the venom of Agkistrodon acutus

Anticoagulation factor I (ACF I) isolated from the venom of Agkistrodon acutus is an activated coagulation factor X-binding protein in a Ca(2+)-dependent fashion with marked anticoagulant activity. The equilibrium unfolding/refolding of apo-ACF I, holo-ACF I, and Tb(3+)-reconstituted ACF I in guanidine hydrochloride (GdnHCl) solutions was studied by following the fluorescence and circular dichroism. Metal ions were found to increase the structural stability of ACF I against GdnHCl and thermal denaturation and, furthermore, influence its unfolding/refolding behavior. The GdnHCl-induced unfolding/refolding of both apo-ACF I and Tb(3+)-ACF I is a two-state process with no detectable intermediate state(s), whereas the GdnHCl-induced unfolding/refolding of holo-ACF I in the presence of 1 mM Ca(2+) follows a three-step transition, with intermediate state a (Ia) and intermediate state b (Ib). Ca(2+) ions play an important role in the stabilization of the Ia and Ib states. The decalcification of holo-ACF I shifts the ending zone of unfolding/refolding curve toward lower GdnHCl concentration, whereas the reconstitution of apo-ACF I with Tb(3+) ions shifts the initial zone of denaturation curve toward higher GdnHCl concentration. Therefore, it is possible to find a denaturant concentration (2.0 M GdnHCl) at which refolding from the fully denatured state of apo-ACF I to the Ib state of holo-ACF I or to the native state of Tb(3+)-ACF I can be initiated merely by adding the 1 mM Ca(2+) ions or 10 microM Tb(3+) ions to the unfolded state of apo-ACF I, respectively, without changing the concentration of the denaturant. Using Tb(3+) as a fluorescence probe of Ca(2+), the kinetic results of metal ions-induced refolding provide evidence that the compact Tb(3+)-binding region forms first, and subsequently, the protein undergoes further conformational rearrangements to form the native structure.

A novel dimer of a C-type lectin-like heterodimer from the venom of Calloselasma rhodostoma (Malayan pit viper)

We have isolated a potent platelet aggregation inducer from the crude venom of Calloselasma rhodostoma (Malayan pit viper), termed rhodoaggretin, with a novel oligomeric structure consisting of a dimer of C-type lectin-like heterodimers. On the basis of its native molecular mass of 66 kDa, and a M(r) of 30 kDa for its disulfide-linked alphabeta-heterodimer, we propose that rhodoaggretin exists as a (alphabeta)2 complex in the native state. We postulate that the di-dimer is stabilized by non-covalent interactions as well as by an intersubunit disulfide bridge between the two alphabeta-heterodimers. This conclusion is based on the following observations: (a) sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of the non-reduced rhodoaggretin gave a major 28 and a minor 52 kDa band. (b) Prior treatment of rhodoaggretin with a limited amount of 2-mercaptoethanol (2-ME; 0.1%) resulted in the complete abolishment of the 52 kDa band in SDS-PAGE. (c) Two-dimensional SDS-PAGE in the presence of 3% 2-ME showed that both the 28 and 52 kDa bands gave two bands each with M(r)s of 18 (alpha-subunit) and 15 (beta-subunit) kDa. (d) Mass spectrometric analyses showed that purified rhodoaggretin had a M(r) of 30155.39+/-3.25 Da while its s-pyridylethylated alpha- and beta-subunits had M(r)s of 16535.62+/-2.98 and 15209.89+/-1.61 Da respectively. These molecular weight data suggested the presence of 15 cysteinyl residues in rhodoaggretin as compared to the 14 that are reported for the heterodimeric C-type lectin-like proteins. This extra cysteinyl residue is a candidate for the formation of the intersubunit disulfide bond in the (alphabeta)2 complex. (e) Homology structural modeling studies showed that the extra cysteinyl residue can indeed form a disulfide bond that covalently links the two alphabeta-heterodimers as proposed above.

Purification and characterization of a variant of rhodocetin from Calloselasma rhodostoma (Malayan pit viper) venom

Rhodocetin is a novel C-type lectin-related protein (CLP) purified from the venom of Calloselasma rhodostoma. Thus far, it is the only reported CLP whose alpha and beta subunits are not linked by an interdisulfide bond. We report here the isolation of a variant of rhodocetin from a different source of venom. This variant of rhodocetin exhibited a different elution profile in reverse-phase HPLC as compared to the rhodocetin reported in our original publication [Wang et al., (1999), Biochemistry 38, 7584-7593]. Specifically, the alpha subunit of the variant was eluted at a considerably lower percentage of acetonitrile, which suggested a less hydrophobic polypeptide chain as compared to the original rhodocetin. Using a combination of microcharacterization techniques such as peptide mapping, mass spectrometry, and amino acid sequencing, we identified an amino acid substitution, 163K, in the polypeptide chain that could account for the difference in elution behavior of the alpha subunit. In addition, we also found a conserved E88D substitution in the beta chain which was not apparent during reverse-phase HPLC. However, neither of these substitutions resulted in the alteration of the functional properties of the rhodocetin variant.

Antithrombotic and thrombolytic activities of Agkisacutacin, a snake venom proteinase, in experimental models

The antithrombotic and thrombolytic activities of Agkisacutacin (Agk), a component isolated from Agkistrodon acutus, were determined in vitro and in vivo. The models employed included Chandler's model, arterio-venous shunt model and pulmonary embolus model. The effects of Agkisacutacin on coagulation, plasma fibrinogen and platelet aggregation induced by collagen, adenosine diphosphate (ADP) and thrombin were also investigated. The results showed that Agkisacutacin can significantly inhibit thrombus formation in Chandler's and arterio-venous shunt models, and accelerate thrombolysis of pulmonary emboli in rats. The data suggested that Agkisacutacin possessed antithrombotic and thrombolytic activities. Agkisacutacin was also partial characterized.

Purification, characterization, and cDNA cloning of a new fibrinogenlytic venom protein, Agkisacutacin, from Agkistrodon acutus venom

Agkisacutacin is a new fibrinogenlytic protein from Agkistrodon acutus venom. It consists of two heterologous subunits linked by an intersubunit disulfide bond. The cDNAs encoding the two chains of Agkisacutacin were cloned from a lambdagt11 cDNA library of the snake venom gland and sequenced, including the leader peptides (23/23 amino acid residues) and mature subunits (129/123 amino acid residues). It is structurally related to the family of IX/X-binding protein (IX/X-bp)-like proteins and shows high similarity (alpha-70%/beta-64%) to habu IX/X-bp from Trimeresurus flavoridis, but displays distinct biological activity with direct action on fibrinogen.

Crystal structure of coagulation factor IX-binding protein from habu snake venom at 2.6 A: implication of central loop swapping based on deletion in the linker region

Coagulation factor IX-binding protein (IX-bp) isolated from the venom of the habu snake (Trimeresurus flavoviridis) is a disulfide-linked heterodimer consisting of homologous subunits A and B. The structure of IX-bp has been solved by X-ray crystallography at 2.6 A resolution to a crystallographic R -value of 0.181. The main-chain fold of each subunit is homologous to the carbohydrate-recognition domain of C-type lectins (C-type CRDs) except for the extended central loop. The structure is almost identical with that of factors IX and X-binding protein (IX/X-bp) as expected from the high level of amino acid sequence homology. The functional difference in ligand recognition from IX/X-bp must reside in the amino acid differences. A continuity of different amino acid residues located from the C-terminal of the second alpha-helix to the following loop forms the local conformational difference in this region between the two proteins. This loop participates in the formation of the concave surface between the two subunits, the putative binding site for the Gla-domain (gamma-carboxyglutamic acid-containing domain) of the coagulation factors. Another difference between the two proteins is in the relative disposition of subunits A and B. When the B subunits are superimposed, about a 6 degrees rotation is required for the superposition of the A subunits. A calcium ion links the second alpha-helix region to the C-terminal tail in each subunit and helps to stabilize the structure for Gla-domain binding. The interface created by the central loop swapping in the dimer IX-bp is almost identical with that seen within the monomeric C-type CRDs. This dimer forms as the result of the amino acid deletion in the linker region of the central loop of the original C-type lectins. Such a dimerization disrupts the lectin active site and creates a Gla-domain binding site, imparting functional diversity.

Coagulation factor X-binding protein from Deinagkistrodon acutus venom is a Gla domain-binding protein

Factor IX/factor X-binding protein (IX/X-bp) is an anticoagulant isolated from the venom of Trimeresurus flavoviridis (habu snake) and binds predominantly to factor IX. In this study, we isolated IX/X-bp-like proteins from the venom of Deinagkistrodon acutus (hundred pace snake) with binding characteristics different from those of IX/X-bp. The complete amino acid sequence and binding characteristics of the main anticoagulant protein, named X-bp, were investigated. The concentrations of X-bp at half-maximal binding to solid-phase factors X and IX were 0.4 and 3 nM, respectively. The binding of X-bp to solid-phase factor X was inhibited by 50% by 6- and 9-fold excess concentrations of factor X and Gla domain (GD) peptide 1-44, respectively, but was not influenced by GD peptide 1-41 and Gla domainless factor X. X-bp bound two Ca2+ ions per molecule with Kd values of 16 +/- 0.7 (mean +/- SE, n = 6) and 103 +/- 10 microM. X-bp was a heterodimer of C-type lectin-like subunits. The 16 kDa chain (A chain) consisted of 129 amino acid residues and was 68% identical to the sequence of the A chain of IX/X-bp. The 15 kDa chain (B chain) consisted of 123 amino acid residues and was 87% identical to IX/X-bp. Three-dimensional model construction from the known fold of IX/X-bp showed that amino acid residues different from those of IX/X-bp are mostly on the molecular surface. Some of these are concentrated on a part of the concave surface which is considered to be the coagulation factor-binding site, presumably acting as a discriminator for ligand binding. These results indicated that X-bp isolated from D. acutus venom was a GD-binding protein, and the C-terminal region of GD peptide was critical for folding of the peptide.

Snake venoms and the hemostatic system

Snake venoms are complex mixtures containing many different biologically active proteins and peptides. A number of these proteins interact with components of the human hemostatic system. This review is focused on those venom constituents which affect the blood coagulation pathway, endothelial cells, and platelets. Only highly purified and well characterized snake venom proteins will be discussed in this review. Hemostatically active components are distributed widely in the venom of many different snake species, particularly from pit viper, viper and elapid venoms. The venom components can be grouped into a number of different categories depending on their hemostatic action. The following groups are discussed in this review: (i) enzymes that clot fibrinogen; (ii) enzymes that degrade fibrin(ogen); (iii) plasminogen activators; (iv) prothrombin activators; (v) factor V activators; (vi) factor X activators; (vii) anticoagulant activities including inhibitors of prothrombinase complex formation, inhibitors of thrombin, phospholipases, and protein C activators; (viii) enzymes with hemorrhagic activity; (ix) enzymes that degrade plasma serine proteinase inhibitors; (x) platelet aggregation inducers including direct acting enzymes, direct acting non-enzymatic components, and agents that require a cofactor; (xi) platelet aggregation inhibitors including: alpha-fibrinogenases, 5'-nucleotidases, phospholipases, and disintegrins. Although many snake venoms contain a number of hemostatically active components, it is safe to say that no single venom contains all the hemostatically active components described here. Several venom enzymes have been used clinically as anticoagulants and other venom components are being used in pre-clinical research to examine their possible therapeutic potential. The disintegrins are an interesting group of peptides that contain a cell adhesion recognition motif, Arg-Gly-Asp (RGD), in the carboxy-terminal half of their amino acid sequence. These agents act as fibrinogen receptor (integrin GPIIb/IIIa) antagonists. Since this integrin is believed to serve as the final common pathway leading to the formation of platelet-platelet bridges and platelet aggregation, blockage of this integrin leads to inhibition of platelet aggregation regardless of the stimulating agent. Clinical trials suggest that platelet GPIIb/IIIa blockade is an effective therapy for the thrombotic events and restenosis frequently accompanying cardiovascular and cerebrovascular disease. Therefore, because of their clinical poten tial, a large number of disintegrins have been isolated and characterized.

Amino acid sequence of the alpha subunit and computer modelling of the alpha and beta subunits of echicetin from the venom of Echis carinatus (saw-scaled viper)

Echicetin, a heterodimeric protein from the venom of Echis carinatus, binds to platelet glycoprotein Ib (GPIb) and so inhibits platelet aggregation or agglutination induced by various platelet agonists acting via GPIb. The amino acid sequence of the beta subunit of echicetin has been reported and found to belong to the recently identified snake venom subclass of the C-type lectin protein family. Echicetin alpha and beta subunits were purified. N-terminal sequence analysis provided direct evidence that the protein purified was echicetin. The paper presents the complete amino acid sequence of the alpha subunit and computer models of the alpha and beta subunits. The sequence of alpha echicetin is highly similar to the alpha and beta chains of various heterodimeric and homodimeric C-type lectins. Neither of the fully reduced and alkylated alpha or beta subunits of echicetin inhibited the platelet agglutination induced by von Willebrand factor-ristocetin or alpha-thrombin. Earlier reports about the inhibitory activity of reduced and alkylated echicetin beta subunit might have been due to partial reduction of the protein.