Agatoxin
Encyclopedia
Polyamine
A polyamine is an organic compound having two or more primary amino groups .This class of compounds includes several synthetic substances that are important feedstocks for the chemical industry, such as ethylene diamine , 1,3-diaminopropane , and hexamethylenediamine...
and peptide
Peptide
Peptides are short polymers of amino acid monomers linked by peptide bonds. They are distinguished from proteins on the basis of size, typically containing less than 50 monomer units. The shortest peptides are dipeptides, consisting of two amino acids joined by a single peptide bond...
toxins which are isolated from the venom
Venom
Venom is the general term referring to any variety of toxins used by certain types of animals that inject it into their victims by the means of a bite or a sting...
of various spiders. Their mechanism of action includes blockade of glutamate
Glutamate receptor
Glutamate receptors are synaptic receptors located primarily on the membranes of neuronal cells. Glutamate is one of the 20 amino acids used to assemble proteins and as a result is abundant in many areas of the body, but it also functions as a neurotransmitter and is particularly abundant in the...
-gated ion channels
Ligand-gated ion channel
Ligand-gated ion channels are one type of ionotropic receptor or channel-linked receptor. They are a group of transmembrane ion channels that are opened or closed in response to the binding of a chemical messenger , such as a neurotransmitter.The binding site of endogenous ligands on LGICs...
, voltage-gated sodium channels, or voltage-dependent calcium channel
Voltage-dependent calcium channel
Voltage-dependent calcium channels are a group of voltage-gated ion channels found in excitable cells with a permeability to the ion Ca2+...
s. Agatoxin is named after the funnel web spider (Agelenopsis aperta) which produces a venom containing several agatoxins.
Isolation
The venom of the Agelenopsis aperta is located in two glands, which are located in the two fang bases. Ejection of the venom takes place via contraction of surrounding muscles. To obtain this venom, the spider is milked by electrical stimulation. The crude venom is dissolved in an EDTAEDTA
Ethylenediaminetetraacetic acid, widely abbreviated as EDTA , is a polyamino carboxylic acid and a colourless, water-soluble solid. Its conjugate base is named ethylenediaminetetraacetate. It is widely used to dissolve limescale. Its usefulness arises because of its role as a hexadentate ligand...
plasma to avoid proteolysis. Purification of the agatoxin is accomplished by a HPLC
High-performance liquid chromatography
High-performance liquid chromatography , HPLC, is a chromatographic technique that can separate a mixture of compounds and is used in biochemistry and analytical chemistry to identify, quantify and purify the individual components of the mixture.HPLC typically utilizes different types of stationary...
procedure.
Alpha-agatoxins
Alpha-agatoxins are composed of polyamines which are attached to an aromatic moiety (see for example AG 489AG 489
AG 489 is a component of the venom produced by Agelenopsis aperta. It inhibits the ligand gated ion channel TRPV1 through a pore blocking mechanism....
).
Mu-agatoxins
Mu-agatoxins are C-terminally amidated peptides, consisting of 35-37 amino acids and are constrained by four intramolecularIntramolecular
Intramolecular in chemistry describes a process or characteristic limited within the structure of a single molecule, a property or phenomenon limited to the extent of a single molecule.- Examples :...
disulfide bonds.
| Subtype | Amino acid length | MW (kDa) | UniProt UniProt UniProt is a comprehensive, high-quality and freely accessible database of protein sequence and functional information, many of which are derived from genome sequencing projects... |
|---|---|---|---|
| 1 | 36 | 4273 | |
| 2 | 37 | 4110 | |
| 3 | 38 | 4197 | |
| 4 | 37 | 4208 | |
| 5 | 37 | 4208 | |
| 6 | 37 | 4168 |
Omega-agatoxins
Omega-agatoxins in turn are subdivided in four classes based on their primary structures, biochemical properties and calcium channels specifity.| Subtype | Amino acid length | MW (kDa) | UniProt UniProt UniProt is a comprehensive, high-quality and freely accessible database of protein sequence and functional information, many of which are derived from genome sequencing projects... |
|---|---|---|---|
| IA | 112 | 12808 | |
| IB | |||
| IIA | |||
| IIIA | 76 | 8518 | |
| IIIB | 76 | 8620 | |
| IIIC | |||
| IIID | |||
| IVA | 48 | 5210 | |
| IVA | 83 | 9167 |
In several of the omega-agatoxins contain one or more D-amino acids which are produced from L-amino acids through the action of peptide isomerases.
Molecular targets
- Alpha-agatoxin: blocks the glutamate-activated receptor channelsGlutamate receptorGlutamate receptors are synaptic receptors located primarily on the membranes of neuronal cells. Glutamate is one of the 20 amino acids used to assemble proteins and as a result is abundant in many areas of the body, but it also functions as a neurotransmitter and is particularly abundant in the...
in the neuronal postsynaptic terminals insects and mammalian. alpha-agatoxin has an antagonistic function in mammalian, including both NMDANMDA receptorThe NMDA receptor , a glutamate receptor, is the predominant molecular device for controlling synaptic plasticity and memory function....
and AMPAAMPA receptorThe α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor is a non-NMDA-type ionotropic transmembrane receptor for glutamate that mediates fast synaptic transmission in the central nervous system . Its name is derived from its ability to be activated by the artificial glutamate analog AMPA...
receptors. - Mu-agatoxin: is a specific modifier for sodium channels (presynaptic voltage-activated sodium channels), in the neuromuscular joint of an insect. Mu-agatoxin will have no effect in other species.
- Omega-agatoxin: in general type IA and type IIA affect the calcium channelCalcium channelA Calcium channel is an ion channel which displays selective permeability to calcium ions. It is sometimes synonymous as voltage-dependent calcium channel, although there are also ligand-gated calcium channels.-Comparison tables:...
s of insects, while type IIIA and IVA affect the calcium channels in vertebrates. There are two major groups within the voltage activated calcium channels; the high voltage activated calcium channel and the low voltage activated calcium channel. The low activated calcium channels are activated by a smaller depolarisation and they show a rapid voltage-dependent inactivation. High voltage activated channels are activated by a large depolarisation and inactivate more slowly. ω-agatoxin only blocks the PP-type calcium channelThe p-type calcium channel is a type of voltage-dependent calcium channel. Like the others of this class, the α1 subunit is the one that determines most of the channel's properties....
/Q typeQ-type calcium channelThe Q-type calcium channel is a type of voltage-dependent calcium channel. Like the others of this class, the α1 subunit is the one that determines most of the channel's properties....
calcium channelCalcium channelA Calcium channel is an ion channel which displays selective permeability to calcium ions. It is sometimes synonymous as voltage-dependent calcium channel, although there are also ligand-gated calcium channels.-Comparison tables:...
s which are a voltage activated.- Type IA and IIA block the presynaptic calcium channels in the presynaptic terminals of the neuromuscular junction of insects. Thereby, type IIA can also block the presynaptic calcium channels in neuromuscular junction of vertebrates.
- Type IIIA blocks ionic L-typeL-type calcium channelThe L-type calcium channel is a type of voltage-dependent calcium channel. "L" stands for long-lasting referring to the length of activation. Like the others of this class, the α1 subunit is the one that determines most of the channel's properties....
current in myocardial cells. It also blocks other neuronal calcium channels, including N-N-type calcium channelThe N-type calcium channel is a type of voltage-dependent calcium channel. Like the others of this class, the α1 subunit forms the pore through which calcium enters the cell and determines most of the channel's properties...
, P/Q, and R-type calcium channelR-type calcium channelThe R-type calcium channel is a type of voltage-dependent calcium channel. Like the others of this class, the α1 subunit forms the pore through which calcium enters the cell and determines most of the channel's properties...
s. - Type IVA has a high affinity and specificity for P- and Q-type calcium channels.
Mechanism of action
- Alpha-agatoxin - By injecting alpha-agatoxin at the neuromuscular junction the post-junctional glutamate activated channel is blocked and therefore the EJP (Excitatory junctional potential). This will only take place if the synapse is activated during exposure to the toxin. When there already is an EJP it will be reduced rapidly. If the toxin is applied without any synaptical activity there will not be a block. The rate of EJP recovery will be slower when the neurotransmitterNeurotransmitterNeurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to...
glutamate is present. - Mu-agatoxin - Modifying sodium channels leads to an increased sensitivity of these channels, and so the excitation threshold will be shifted downwards. This results in an elevated probability for sodium channels to open, leading to depolarisation. The calcium influx will take place and because of the increased frequency of spontaneous excitatory postsynaptic currents, neurotransmitter release will take place. Repetitive action potentials of motor neurons will be established.
- Omega-agatoxin - In general ω-agatoxin blocks the presynaptic calcium channels, so that the calcium influx will reduce. This results in a decreased release of neurotransmitter in the synpatic cleft. There are several subtypes which can interfere with each other and make the blocking a dynamic process. When ω-agatoxin-IA and ω-agatoxin-IIA are injected separately, they partially block transmitter release. But when they will be injected together, this leads to a complete block of the EJP.
Toxicity
Alpha-agatoxin causes a rapid reversible paralysis in insects, while mu-agatoxin cause a slow long-lasting paralysis. When the two toxins will be injected at the same time, they are synergistic. So co-injection of these toxins leads to a paralysis for a very long, possible everlasting, period of time.Omega-agatoxin injection causes spasms leading to a progressive paralysis which will eventually lead to death in insects.
Because insects have a much smaller repertoire of voltage-gated calcium channels and have a different pharmacology than vertebrates the effects can vary between species.