Convulxin is a

snake venom Snake venom is a highly toxic saliva containing zootoxins that facilitates in the immobilization and digestion of prey. This also provides defense against threats. Snake venom is injected by unique fangs during a bite, whereas some species are a ...

toxin found in a tropical

rattlesnake Rattlesnakes are venomous snakes that form the genera '' Crotalus'' and '' Sistrurus'' of the subfamily Crotalinae (the pit vipers). All rattlesnakes are vipers. Rattlesnakes are predators that live in a wide array of habitats, hunting small ...

known as ''

Crotalus durissus terrificus ''Crotalus'' is a genus of venomous pit vipers in the family Viperidae, known as rattlesnakes or rattlers. Wright AH, Wright AA (1957). ''Handbook of Snakes of the United States and Canada''. Ithaca and London: Comstock Publishing Associates (7 ...

''. It belongs to the family of

hemotoxins Hemotoxins, haemotoxins or hematotoxins are toxins that destroy red blood cells, disrupt blood clotting, and/or cause organ degeneration and generalized tissue damage. The term ''hemotoxin'' is to some degree a misnomer since toxins that damage t ...

, which destroy red blood cells or, as is the case with convulxin, induce blood coagulation. It causes

platelet Platelets, also called thrombocytes (from Greek θρόμβος, "clot" and κύτος, "cell"), are a component of blood whose function (along with the coagulation factors) is to react to bleeding from blood vessel injury by clumping, thereby i ...

activation in the blood, forming clots and buildup of pressure. Convulxin acts as an

agonist An agonist is a chemical that activates a receptor to produce a biological response. Receptors are cellular proteins whose activation causes the cell to modify what it is currently doing. In contrast, an antagonist blocks the action of the ago ...

to the

GPVI Glycoprotein VI (platelet), also known as GPVI, is a glycoprotein receptor for collagen which is expressed in platelets. In humans, glycoprotein VI is encoded by the ''GPVI'' gene. GPVI was first cloned in 2000 by several groups including that o ...

receptor, the major signalling receptor for collagen. This can cause the blood stream to burst, or the heart or brain to lose blood, thus resulting in death. It is a

tetramer A tetramer () (''tetra-'', "four" + '' -mer'', "parts") is an oligomer formed from four monomers or subunits. The associated property is called ''tetramery''. An example from inorganic chemistry is titanium methoxide with the empirical formula T ...

C-type

lectin Lectins are carbohydrate-binding proteins that are highly specific for sugar groups that are part of other molecules, so cause agglutination of particular cells or precipitation of glycoconjugates and polysaccharides. Lectins have a role in rec ...

with an oligomeric structure, made up of heterodimeric subunits.

Family

Convulxin is part of the snake venom

C-type lectin A C-type lectin (CLEC) is a type of carbohydrate-binding protein known as a lectin. The C-type designation is from their requirement for calcium for binding. Proteins that contain C-type lectin domains have a diverse range of functions including c ...

family, a group of hemorrhagic toxins that disrupt body

homeostasis In biology, homeostasis (British English, British also homoeostasis) Help:IPA/English, (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physics, physical, and chemistry, chemical conditions maintained by organism, living systems. Thi ...

. The name describes their similarity in structure to C-type lectins from other animals, proteins that bind calcium to induce various

signalling pathways Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a series of molecular events, most commonly protein phosphorylation catalyzed by protein kinases, which ultimately results in a cellular ...

. Proteins of about 130 amino acids in length, C-type lectins contain at least a carbohydrate recognition domain (CRD), which mediates sugar and calcium binding. They are involved in various biological activities, ranging from

cell-cell adhesion Cell adhesion is the process by which cells interact and attach to neighbouring cells through specialised molecules of the cell surface. This process can occur either through direct contact between cell surfaces such as cell junctions or indir ...

, serum glycoprotein turnover, to immune responses and

cell apoptosis Apoptosis (from grc, wikt:ἀπόπτωσις, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemistry, Biochemical events lead to characteristic cell changes (Morp ...

History

The toxin was first described in detail in 1969 by two Brazilian researchers from

University of Campinas The State University of Campinas ( pt, Universidade Estadual de Campinas), commonly called Unicamp, is a public research university in the state of São Paulo, Brazil. Unicamp is consistently ranked among the top universities in Brazil and ...

, Júlia Prado-Franceschi and Oswaldo Vital-Brazil. The snake C-type lectin convulxin was reported to activate platelets in a similar way to collagen in the late 1970s, but this was only announced after the discovery of the association with the FcR γ-chain and after it was recognized to mediate activation through GPVI. Now this toxin is widely used to study mammalian platelet receptors.

Structure

Convulxin is a

heterodimer In biochemistry, a protein dimer is a macromolecular complex formed by two protein monomers, or single proteins, which are usually non-covalently bound. Many macromolecules, such as proteins or nucleic acids, form dimers. The word ''dimer'' has ...

made up of α-(13.9 kDa) and β- (12.6 kDa) subunits, with 38% sequence identity and homologous structures. The subunits are connected by

disulfide bridges In biochemistry, a disulfide (or disulphide in British English) refers to a functional group with the structure . The linkage is also called an SS-bond or sometimes a disulfide bridge and is usually derived by the coupling of two thiol groups. In ...

to form a cyclic, ring-like α4β4 structure . Its function arises from its ability to bind with high affinity to the platelet receptor for collagen, glycoprotein (GP) VI. It is, therefore, an important task to determine the

binding site In biochemistry and molecular biology, a binding site is a region on a macromolecule such as a protein that binds to another molecule with specificity. The binding partner of the macromolecule is often referred to as a ligand. Ligands may inclu ...

on the heterodimer to the GPVI. The heterodimer structure presents a concave surface, predicted to be the ligand binding site. Furthermore, general research into the C-type lectin family describes the binding site as being formed by loop regions, falling between the second α-helix and the second β-strand on both the α- and β-subunits. Investigating the particular sequences of these structures showed high variability, suggesting that it is indeed these variable, concave loops that offer specificity in

ligand binding In biochemistry and pharmacology, a ligand is a substance that forms a complex with a biomolecule to serve a biological purpose. The etymology stems from ''ligare'', which means 'to bind'. In protein-ligand binding, the ligand is usually a mole ...

. Analysis on the specific Cvx structure has revealed 3 possible sites of interaction with GPVI. Firstly, two adjacent patches of positive and negative charge on the α-subunit; secondly, a cavity in the same subunit lines with the following residues:Trp23, Ser67, Leu104, Ala117, Gly121 and Ile123; finally, a negatively charged patch on the β-subunit.

Mechanism of action

Normally, upon injury to the

endothelium The endothelium is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and the rest of the ve ...

, collagen mediated GPVI signalling increases

formation by

thromboxane A2 Thromboxane A2 (TXA2) is a type of thromboxane that is produced by activated platelets during hemostasis and has prothrombotic properties: it stimulates activation of new platelets as well as increases platelet aggregation. This is achieved by act ...

, therefore creating a blood clot. In case of blood vessel damage, collagen on the

extracellular matrix In biology, the extracellular matrix (ECM), also called intercellular matrix, is a three-dimensional network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, glycoproteins and hydroxyapatite that provide struc ...

is exposed. As platelets interact with it, an activation signal is sent for aggregation. Platelets interact indirectly with collagen, via the von Willebrand Factor (vWF), which connects the collagen to the platelet GPIb receptor, forcing them close to the site of vessel damage. There, they can interact with receptors on the extracellular matrix, which stimulate adhesion through integrins (heterodimer α₂β₁), and downstream signalling. GPVI is present as a complex with the Fc receptor (FcR) γ-chain, which gets phosphorylated by

SYK Tyrosine-protein kinase SYK, also known as spleen tyrosine kinase, is an enzyme which in humans is encoded by the ''SYK'' gene. Function SYK, along with ZAP70, is a member of the Syk family of tyrosine kinases. These cytoplasmic non-receptor ty ...

as a result of activation by a stimulus. This generates a downstream signal, leading to platelet activation. While this is important in case of injury, inappropriate activation of platelets can lead to the formation of clots within the circulation. Such is the case with Convulxin, which can induce a signalling cascade similar to that of collagen. Due to its high affinity, convulxin bind to GPVI and causes clustering of the glycoproteins. Research has proved that GPlb is not involved in convulxin-induced activation, but that the p62/GPVI collagen receptor is the unique binding site, and protein phosphorylation happens more rapidly and more intensely than in the case of collagen. Furthermore, denatured samples of the toxin containing α, β, or both subunits have been shown not to induce platelet aggregation or tyrosine phosphorylation suggesting that pellet formation requires the native conformation of the protein. However, reduced convulxin subunits still inhibit the effect of collagen since they bind to a common receptor necessary for collagen activation of platelets. . The free toxin might be evacuated through opsonization via the reticuloendothelial system (for the most part the liver and kidneys) or it might degrade through the

lysosomes A lysosome () is a membrane-bound organelle found in many animal cells. They are spherical vesicles that contain hydrolytic enzymes that can break down many kinds of biomolecules. A lysosome has a specific composition, of both its membrane ...

. Therefore, Convulxin acts as an agonist to collagen, inducing platelet activation via GPVI binding, ultimately causing blood clots accumulation in the absence of a homeostatic signal.

Toxicity

Studies carried out on the

toxicity Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subs ...

of convulxin show that symptoms are a function of the dose. The effects of the toxin stand out through the sudden and brief duration of the symptoms after exposure. In mice, low doses (5 μg/animal) administered intravenously (I.V) elicited

tachypnea Tachypnea, also spelt tachypnoea, is a respiratory rate greater than normal, resulting in abnormally rapid and shallow breathing. In adult humans at rest, any respiratory rate of 1220 per minute is considered clinically normal, with tachypnea be ...

, followed by

apnea Apnea, BrE: apnoea, is the temporal cessation of breathing. During apnea, there is no movement of the muscles of inhalation, and the volume of the lungs initially remains unchanged. Depending on how blocked the airways are (patency), there may ...

, within 20 seconds. The ED₅₀ for brief duration apnea was determined to be 180 μg/kg. Higher doses (10 μg/animal) evoked intense convulsive crisis, and usually ended with the death of the animal. LD₅₀ was determined to be 524 μg/kg. Intraperitoneal (I.P.) administration of up to 200 μg/animal proved to be ineffective. In cats, I.V. injections of 100 μg/kg showed respiratory disturbances,

miosis Miosis, or myosis (), is excessive constriction of the pupil.Farlex medical dictionary
citing: ...

, salivation, abdominal cramps,

nystagmus Nystagmus is a condition of involuntary (or voluntary, in some cases) eye movement. Infants can be born with it but more commonly acquire it in infancy or later in life. In many cases it may result in reduced or limited vision. Due to the invol ...

, loss of equilibrium, convulsions and sometimes a brief phase of

hypotonia Hypotonia is a state of low muscle tone (the amount of tension or resistance to stretch in a muscle), often involving reduced muscle strength. Hypotonia is not a specific medical disorder, but a potential manifestation of many different diseases ...

. The ED₅₀ dose for convulsions is 80 μg/kg. The majority of animals recovered within 30 minutes. In dogs, the effect of convulxin showed two stages. After I.V. injection of 100-125 μg/kg they became excited, barked and exhibited loss of equilibrium, respiratory disturbances,

, urination, defecation and vomiting. After recovery, two out of five animals had intermittent crisis of

clonic Clonus is a set of involuntary and rhythmic muscular contractions and relaxations. Clonus is a sign of certain neurological conditions, particularly associated with upper motor neuron lesions involving descending motor pathways, and in many cas ...

convulsions that appeared after 24 hours and lasted until their death. The other three dogs showed periods of vivid agitation alternating with

torpor Torpor is a state of decreased physiological activity in an animal, usually marked by a reduced body temperature and metabolic rate. Torpor enables animals to survive periods of reduced food availability. The term "torpor" can refer to the tim ...

. While the lethal dose in humans is not yet known, what has been discovered is that the level of toxic effects depends on the origin of the snake. As for a cure, so far a polyvalent snake

antivenom Antivenom, also known as antivenin, venom antiserum, and antivenom immunoglobulin, is a specific treatment for envenomation. It is composed of antibodies and used to treat certain venomous bites and stings. Antivenoms are recommended only if ...

is being used.

References

{{reflist, 32em, refs= {{cite journal , vauthors = Hermans C, Wittevrongel C, Thys C, Smethurst PA, Van Geet C, Freson K , title = A compound heterozygous mutation in glycoprotein VI in a patient with a bleeding disorder , journal = Journal of Thrombosis and Haemostasis , volume = 7 , issue = 8 , pages = 1356–63 , date = August 2009 , pmid = 19552682 , doi = 10.1111/j.1538-7836.2009.03520.x , doi-access = free {{open access {{cite journal , vauthors = Polgár J, Clemetson JM, Kehrel BE, Wiedemann M, Magnenat EM, Wells TN, Clemetson KJ , title = Platelet activation and signal transduction by convulxin, a C-type lectin from Crotalus durissus terrificus (tropical rattlesnake) venom via the p62/GPVI collagen receptor , journal = The Journal of Biological Chemistry , volume = 272 , issue = 21 , pages = 13576–83 , date = May 1997 , pmid = 9153205 , doi = 10.1074/jbc.272.21.13576 , doi-access = free {{open access {{cite book , last1=Michelson , first1=Alan , name-list-style = vanc , title=Platelets , date=9 January 2013 , publisher=Elsevier , isbn=9780123878373 , pages=215–231 , edition=third {{cite web , title=Convulxin (T3DB2544) , url=http://www.t3db.ca/toxins/T3D2544 , website=The toxin and Toxin Targer Database (T3DB) , accessdate=20 March 2020 Vertebrate toxins Proteins __FORCETOC__