A conotoxin is one of a group of

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 ...

s isolated from the venom of the marine cone snail, genus '' Conus''. Conotoxins, which are peptides consisting of 10 to 30

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

residues, typically have one or more disulfide bonds. Conotoxins have a variety of mechanisms of actions, most of which have not been determined. However, it appears that many of these peptides modulate the activity of

ion channel Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore. Their functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by Gating (electrophysiol ...

s. Over the last few decades conotoxins have been the subject of pharmacological interest. The LD₅₀ of conotoxin ranges from 5-25 μg/kg.

Hypervariability

Conotoxins are hypervariable even within the same species. They do not act within a body where they are produced (

endogenous Endogeny, in biology, refers to the property of originating or developing from within an organism, tissue, or cell. For example, ''endogenous substances'', and ''endogenous processes'' are those that originate within a living system (e.g. an ...

ly) but act on other organisms. Therefore, conotoxin genes experience less selection against

mutations In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mitosi ...

(like

gene duplication Gene duplication (or chromosomal duplication or gene amplification) is a major mechanism through which new genetic material is generated during molecular evolution. It can be defined as any duplication of a region of DNA that contains a gene ...

and nonsynonymous substitution), and mutations remain in the genome longer, allowing more time for potentially beneficial novel functions to arise. Variability in conotoxin components reduces the likelihood that prey organisms will develop resistance; thus cone snails are under constant selective pressure to maintain polymorphism in these genes because failing to evolve and adapt will lead to extinction ('' Red Queen hypothesis'').

Disulfide connectivities

Types of conotoxins also differ in the number and pattern of disulfide bonds. The disulfide bonding network, as well as specific amino acids in inter-cysteine loops, provide the specificity of conotoxins.

Types and biological activities

As of 2005, five biologically active conotoxins have been identified. Each of the five conotoxins attacks a different target: * α-conotoxin inhibits

nicotinic acetylcholine receptor Nicotinic acetylcholine receptors, or nAChRs, are Receptor (biochemistry), receptor polypeptides that respond to the neurotransmitter acetylcholine. Nicotinic receptors also respond to drugs such as the agonist nicotine. They are found in the c ...

s at

nerve A nerve is an enclosed, cable-like bundle of nerve fibers (called axons). Nerves have historically been considered the basic units of the peripheral nervous system. A nerve provides a common pathway for the Electrochemistry, electrochemical nerv ...

s and

muscle Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...

s. * δ-conotoxin inhibits fast inactivation of voltage-dependent sodium channels. * κ-conotoxin inhibits potassium channels. * μ-conotoxin inhibits voltage-dependent sodium channels in muscles. * ω-conotoxin inhibits N-type voltage-dependent calcium channels. Because N-type voltage-dependent calcium channels are related to algesia (sensitivity to

pain Pain is a distressing feeling often caused by intense or damaging Stimulus (physiology), stimuli. The International Association for the Study of Pain defines pain as "an unpleasant sense, sensory and emotional experience associated with, or res ...

) in the nervous system, ω-conotoxin has an

analgesic An analgesic drug, also called simply an analgesic, antalgic, pain reliever, or painkiller, is any member of the group of drugs used for pain management. Analgesics are conceptually distinct from anesthetics, which temporarily reduce, and in s ...

effect: the effect of ω-conotoxin M VII A is 100 to 1000 times that of

morphine Morphine, formerly also called morphia, is an opiate that is found naturally in opium, a dark brown resin produced by drying the latex of opium poppies (''Papaver somniferum''). It is mainly used as an analgesic (pain medication). There are ...

. Therefore, a synthetic version of ω-conotoxin M VII A has found application as an analgesic drug ziconotide (Prialt).

Alpha

Alpha conotoxins have two types of cysteine arrangements, and are competitive nicotinic acetylcholine receptor antagonists.

Delta, kappa, and omega

Omega, delta and kappa families of conotoxins have a knottin or inhibitor cystine knot scaffold. The knottin scaffold is a very special disulfide-through-disulfide knot, in which the III-VI disulfide bond crosses the macrocycle formed by two other disulfide bonds (I-IV and II-V) and the interconnecting backbone segments, where I-VI indicates the six cysteine residues starting from the N-terminus. The cysteine arrangements are the same for omega, delta and kappa families, even though omega conotoxins are calcium channel blockers, whereas delta conotoxins delay the inactivation of sodium channels, and kappa conotoxins are potassium channel blockers.

Mu

Mu-conotoxins have two types of cysteine arrangements, but the knottin scaffold is not observed. Mu-conotoxins target the muscle-specific voltage-gated sodium channels, and are useful probes for investigating voltage-dependent sodium channels of excitable tissues. Mu-conotoxins target the voltage-gated

sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...

channels, preferentially those of

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 ...

, and are useful probes for investigating voltage-dependent sodium channels of excitable tissues. Different subtypes of voltage-gated sodium channels are found in different tissues in mammals, ''e.g.,'' in muscle and brain, and studies have been carried out to determine the sensitivity and specificity of the mu-conotoxins for the different isoforms.

References

External links

* * * {{Nicotinic acetylcholine receptor modulators Snail toxins Ion channel toxins Neurotoxins Nicotinic antagonists Peripheral membrane proteins