Poneratoxin is a paralyzing

neurotoxic Neurotoxicity is a form of toxicity in which a biological, chemical, or physical agent produces an adverse effect on the structure or function of the central and/or peripheral nervous system. It occurs when exposure to a substance – specifical ...

peptide Peptides are short chains of amino acids linked by peptide bonds. A polypeptide is a longer, continuous, unbranched peptide chain. Polypeptides that have a molecular mass of 10,000 Da or more are called proteins. Chains of fewer than twenty am ...

made by the bullet ant ''

Paraponera clavata ''Paraponera clavata'', commonly known as the bullet ant, is a species of ant known for its extremely painful Insect bites and stings, sting. It inhabits humid lowland rainforests in Central America, Central and South America. Etymology The Spe ...

''. It prevents inactivation of voltage gated sodium channels and therefore blocks

synaptic transmission Neurotransmission (Latin: ''transmissio'' "passage, crossing" from ''transmittere'' "send, let through") is the process by which signaling molecules called neurotransmitters are released by the axon terminal of a neuron (the presynaptic neuron) ...

in the

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain, spinal cord and retina. The CNS is so named because the brain integrates the received information and coordinates and influences the activity o ...

. Specifically, poneratoxin acts on voltage gated sodium channels in

skeletal muscle Skeletal muscle (commonly referred to as muscle) is one of the three types of vertebrate muscle tissue, the others being cardiac muscle and smooth muscle. They are part of the somatic nervous system, voluntary muscular system and typically are a ...

fibers, causing paralysis, and

nociceptive In physiology, nociception , also nocioception; ) is the sensory nervous system's process of encoding noxious stimuli. It deals with a series of events and processes required for an organism to receive a painful stimulus, convert it to a molecular ...

fibers, causing pain. It is rated as a 4 plus on the

Schmidt sting pain index The Schmidt sting pain index is a pain scale rating the relative pain caused by different hymenopteran stings. It is mainly the work of Justin O. Schmidt, who was an entomologist at the Carl Hayden Bee Research Center in Arizona. Schmidt's or ...

, the highest possible rating with that system, and its effects can cause waves of pain up to twelve hours after a single sting. It is additionally being studied for its uses in biological insecticides.

Mechanism of action

Overall, poneratoxin disrupts normal function of

voltage-gated sodium channels Sodium channels are integral membrane proteins that form ion channels, conducting sodium ions (Na+) through a cell's membrane. They belong to the superfamily of cation channels. Classification They are classified into 2 types: Function In exc ...

in both

vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

s and

invertebrate Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordata, chordate s ...

s. It causes repetitive firing and prolongation of

action potential An action potential (also known as a nerve impulse or "spike" when in a neuron) is a series of quick changes in voltage across a cell membrane. An action potential occurs when the membrane potential of a specific Cell (biology), cell rapidly ri ...

s, particularly in the

of insects. The increase in signaling is what causes the intense pain from bullet ant stings. Poneratoxin is in an inactive state when stored in the ant venom reservoir due to the reservoir's acidic conditions, but it becomes toxic when activated via a multistep process. The combination of poneratoxin binding to a

cell membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...

(in order to act upon a voltage-gated sodium channel) and the movement from acidic conditions in the ant venom reservoir to basic conditions at the target site leads to poneratoxin undergoing a

conformational change In biochemistry, a conformational change is a change in the shape of a macromolecule, often induced by environmental factors. A macromolecule is usually flexible and dynamic. Its shape can change in response to changes in its environment or othe ...

that activates it. Catterall ''et al.'' hypothesized that some

polypeptide Peptides are short chains of amino acids linked by peptide bonds. A polypeptide is a longer, continuous, unbranched peptide chain. Polypeptides that have a molecular mass of 10,000 Da or more are called proteins. Chains of fewer than twenty ...

neurotoxin Neurotoxins are toxins that are destructive to nervous tissue, nerve tissue (causing neurotoxicity). Neurotoxins are an extensive class of exogenous chemical neurological insult (medical), insultsSpencer 2000 that can adversely affect function ...

s modify voltage-gated channels function via a "voltage-sensor trapping" mechanism. The hypothesis states that neurotoxins similar to poneratoxin, such as alpha-scorpion toxins, act upon

sodium channel Sodium channels are integral membrane proteins that form ion channels, conducting sodium ions (Na+) through a cell (biology), cell's cell membrane, membrane. They belong to the Cation channel superfamily, superfamily of cation channels. Classific ...

s via binding to the channels'

receptor Receptor may refer to: * Sensory receptor, in physiology, any neurite structure that, on receiving environmental stimuli, produces an informative nerve impulse *Receptor (biochemistry), in biochemistry, a protein molecule that receives and respond ...

site 3, which normally affects the channels' ability to inactivate. Therefore, receptor site 3 neurotoxins often affect sodium channels by slowing or blocking inactivation. Normally, the region of the channel where neurotoxin receptor site 3 is undergoes a conformational change of an outward movement to lead to inactivation. Receptor site 3 neurotoxins are proposed to prevent this conformational change via interaction with

acid An acid is a molecule or ion capable of either donating a proton (i.e. Hydron, hydrogen cation, H+), known as a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis ...

ic and

hydrophobic In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, thu ...

amino acid residues Protein structure is the three-dimensional arrangement of atoms in an amino acid-chain molecule. Proteins are polymers specifically polypeptides formed from sequences of amino acids, which are the monomers of the polymer. A single amino acid m ...

at that site. When frog skeletal muscle fibers were exposed to poneratoxin, it was found that poneratoxin primarily affected voltage-dependent sodium channels by decreasing the peak sodium current and also inducing a slow sodium current. This combination resulted in the sodium channels activating at very negative potentials and deactivating very slowly, a phenomenon commonly seen in excitable tissues. Poneratoxin is considered as a slow-acting

agonist An agonist is a chemical that activates a Receptor (biochemistry), receptor to produce a biological response. Receptors are Cell (biology), cellular proteins whose activation causes the cell to modify what it is currently doing. In contrast, an R ...

for smooth muscles.

Structure

The poneratoxin peptide is stored in an inactive 25-residue peptide (amino acid sequence FLPLLILGSLLMTPPVIQAIHDAQR) in the venom reservoir of ''

''. The

secondary structure Protein secondary structure is the local spatial conformation of the polypeptide backbone excluding the side chains. The two most common Protein structure#Secondary structure, secondary structural elements are alpha helix, alpha helices and beta ...

is characterized by a

helix-turn-helix Helix-turn-helix is a DNA-binding domain (DBD). The helix-turn-helix (HTH) is a major structural motif capable of binding DNA. Each monomer incorporates two alpha helix, α helices, joined by a short strand of amino acids, that bind to the majo ...

motif: two

alpha helices An alpha helix (or α-helix) is a sequence of amino acids in a protein that are twisted into a coil (a helix). The alpha helix is the most common structural arrangement in the secondary structure of proteins. It is also the most extreme type of l ...

connected by a beta-turn. The two

are formed by residues 3–9 at the N-terminus, and residues 17–24 at the C-terminus, and they are connected by the beta-turn at residues 11–16. From a three-dimensional perspective, this structure forms a preferential V-shape with the two helices undergoing loose non-covalent interactions with each other. This is notable because of its structural similarity to other peptides that interact with the membrane, and indicates that poneratoxin will also interact with the membrane and thereby affect embedded voltage gated sodium channels. Furthermore, the structure of the peptide shifts from a

random coil In polymer chemistry, a random coil is a conformation of polymers where the monomer subunits are oriented randomly while still being bonded to adjacent units. It is not one specific shape, but a statistical distribution of shapes for all the cha ...

to the structured

when introduced to a

lipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes form a continuous barrier around all cell (biology), cells. The cell membranes of almost all organisms and many viruses a ...

environment, which indicates that this motif is important for interacting with the membrane. The two alpha helices, however, have markedly different characteristics. The N-terminal alpha helix is apolar, containing a central

core with

hydrophilic A hydrophile is a molecule or other molecular entity that is attracted to water molecules and tends to be dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Clarendon Press. In contrast, hydrophobes are n ...

residues at either end, and is uncharged. It is similar in structure to a transmembrane

signal peptide A signal peptide (sometimes referred to as signal sequence, targeting signal, localization signal, localization sequence, transit peptide, leader sequence or leader peptide) is a short peptide (usually 16–30 amino acids long) present at the ...

, which implies that it will anchor at the membrane by burying the hydrophobic core within the bilayer. In particular, the bulky and very hydrophobic

phenylalanine Phenylalanine (symbol Phe or F) is an essential α-amino acid with the chemical formula, formula . It can be viewed as a benzyl group substituent, substituted for the methyl group of alanine, or a phenyl group in place of a terminal hydrogen of ...

residue is important for interacting with uncharged lipid bilayers, such as those composed of

phosphatidylcholine Phosphatidylcholines (PC) are a class of phospholipids that incorporate choline as a headgroup. They are a major component of biological membranes and can easily be obtained from a variety of readily available sources, such as egg yolk or soyb ...

. The C-terminal alpha helix is amphipathic with one side displaying polar and charged residues, and the other displaying non-polar residues, which drives insertion into the plasma membrane. Specifically, the positively charged

arginine Arginine is the amino acid with the formula (H2N)(HN)CN(H)(CH2)3CH(NH2)CO2H. The molecule features a guanidinium, guanidino group appended to a standard amino acid framework. At physiological pH, the carboxylic acid is deprotonated (−CO2−) a ...

and the non-polar

alanine Alanine (symbol Ala or A), or α-alanine, is an α-amino acid that is used in the biosynthesis of proteins. It contains an amine group and a carboxylic acid group, both attached to the central carbon atom which also carries a methyl group sid ...

residues were both shown to be essential for poneratoxin potency. See figure, where the hydrophobic (red) and hydrophilic (blue) regions of poneratoxin and the lipid bilayer align, demonstrating that the structure is evolved to insert into the membrane, which will promote interaction with the voltage gated sodium channels.

Toxicology

Many people consider a sting from a bullet ant to resemble the sensation of getting shot. Justin Schmidt, an

entomologist Entomology (from Ancient Greek ἔντομον (''éntomon''), meaning "insect", and -logy from λόγος (''lógos''), meaning "study") is the branch of zoology that focuses on insects. Those who study entomology are known as entomologists. In ...

who developed the

, described it as "pure, intense, brilliant pain...like walking over flaming charcoal with a three-inch nail embedded in your heel," and considers the sting from a bullet ant to be the most painful insect sting he has experienced. The pain from bullet ant stings can last for many hours, even up to 24 hours. Both the immense pain and the duration of the sting are due to the effects of poneratoxin. In addition to the notorious pain, symptoms of stings from bullet ants (as well as stings from other ants of the genus '' Paraponera'' as well as the genus '' Dinoponera'') include fever, cold sweats, nausea, vomiting,

lymphadenopathy Lymphadenopathy or adenopathy is a disease of the lymph nodes, in which they are abnormal in size or consistency. Lymphadenopathy of an inflammatory type (the most common type) is lymphadenitis, producing swollen or enlarged lymph nodes. In c ...

and

cardiac arrhythmia Arrhythmias, also known as cardiac arrhythmias, are irregularities in the heartbeat, including when it is too fast or too slow. Essentially, this is anything but normal sinus rhythm. A resting heart rate that is too fast – above 100 beat ...

s. Toxicity assays have found that the LT₅₀ of poneratoxin, delivered via injections of genetically engineered viruses, to ''S. frugiperda'' larvae, was at 131 hours post-injection. A dose of 10⁵ pfu of poneratoxin was sufficient to kill the ''S. frugiperda'' larvae, and a dose of 10 ng could paralyze them. Based on these experiments, scientists believe poneratoxin can make a good candidate as a bio-

insecticide Insecticides are pesticides used to kill insects. They include ovicides and larvicides used against insect eggs and larvae, respectively. The major use of insecticides is in agriculture, but they are also used in home and garden settings, i ...

because of its

neurotoxicity Neurotoxicity is a form of toxicity in which a biological, chemical, or physical agent produces an adverse effect on the structure or function of the central and/or peripheral nervous system. It occurs when exposure to a substance – specifical ...

to other insects, making it capable of immobilizing or even killing insects infected with it. The making of a

recombinant virus A recombinant virus may occur naturally or be produced by recombining pieces of DNA or RNA in a laboratory. Synthetic recombination This may be used to produce viral vaccines or gene therapy vectors. Natural recombination The term is also u ...

by engineering a

baculovirus ''Baculoviridae'' is a family of viruses. Arthropods, among the most studied being Lepidoptera, Hymenoptera and Diptera, serve as natural hosts. Currently, 85 species are placed in this family, assigned to four genera. Baculoviruses are known ...

that expresses poneratoxin has been proposed.

References

External links

Antinociceptive effect of poneratoxin (PoTX) in rats. Pestycydy, 2008, (1-2), 135-141. ISSN 0208-8703
* {{Ion channel modulators Ion channel toxins Neurotoxins Peripheral membrane proteins Myrmecology Sodium channel openers

Mechanism of action

Structure

Toxicology

See also

References

External links