Postsynaptic neurotoxins Structure

Before discussing the structure of the neurotoxins, it is necessary to define the types of neurotoxins. Three types of neurotoxins have been found so far in snake venoms. The first one is a postsynaptic neurotoxin, the second is a presynaptic neurotoxin, and the last is a cholinesterase inhibiting neurotoxin. Most sea snake venoms seem to contain only the postsynaptic neurotoxin. Only in Enhydrina... 

In order to understand the exact mechanism of the neurotoxic action, it is important to know the secondary structure of the neurotoxins as well. It is now known that postsynaptic neurotoxins attach to the a-subunits of acetylcholine receptor (AChR). 

The similarity of the primary structure of different sea snake venoms has already been discussed. Postsynaptic neurotoxins from Elapidae venom have been extensively studied. Elapidae include well-known snakes such as cobra, krait, mambas, coral snakes, and all Australian snakes. Like sea snake toxins, Elapidae toxins can also be grouped into short-chain (Type I) and long-chain (Type II) toxins. Moreover, two types of neurotoxins are also similar to cardiotoxins, especially in the positions of disulfide bonds. However, amino acid sequences between cardiotoxins and sea snake and Elapidae neurotoxins are quite different. In comparing the sequence of sea snake and Elapidae neurotoxins, there is a considerable conservation in amino acid sequence, but the difference is greater than among the various sea snake toxins. 

Figure 3. Examples of neurotoxins (A, C) and a cardiotoxin (B). (A) Primary structure of lapemis toxin, a short-chain postsynaptic neurotoxin. (B) Cardiotoxin from Naja naja venom. (C) Toxin B from Naja naja venom.

The structure of postsynaptic neurotoxins is well studied. There are actually two types of these neurotoxins (Fig. 3A,C). One type has four disulfide bonds (called type I or short-chain neurotoxins). The short-chain neurotoxin has one or two amino acids at segment 8, whereas the long-chain neurotoxins have a longer segment 8 (see Fig. 3). Another difference is that there is only one amino acid within segment 5 of the short-chain neurotoxin, whereas the long-chain neurotoxin has three amino residues within the segment (see Fig. 3). 

Postsynaptic neurotoxins are composed mainly of an antiparallel (3-sheet and a p-tum structure, with only a small amount of a-helical structure (Yu et al., 1975 Tu, 1990 Betzel et al., 1991 LeGoas et al., 1992 Yu et al., 1990 Tu et al., 1976). The toxin is comprised of three loops. A, B, and C (Fig. 4). Loop B is considered most important, and it is believed that this loop is attached to the acetylcholine-binding site of the AChR. Loop B is also the antigenic determinant. 

The amino acid sequences of over 100 postsynaptic neurotoxins have been determined by many investigators therefore, we will not discuss the sequence of all the toxins. However, one should be aware of the incorrect sequence of a-btx, as originally reported earlier (Mebs et al., 1971). The correct primary structure of a-bungarotoxin was later established (Ohta et al., 1987). The original paper (Mebs et al., 1971) reported the sequences of Ile-Pro-Ser (9-11), His-Pro (67-68), and Arg-Gln (71-72). However, these sequences are incorrect, and the correct sequences have now been established as Ser-Pro-Ile (9-11), Pro-His (67-68), and Gln-Arg (71-72) by Ohta et al. (1987). 

The primary structure of postsynaptic toxins is unique to snakes, and there are no homologies with the toxins of scorpions, spiders, or bees. However, there is an interesting report that significant homologous sequences to snake postsynaptic neurotoxins are found in visna virus and HIV-I tat proteins (Gourdou et al., 1990). 

Snake venoms also contain nonneurotoxic proteins, with structures very similar to a postsynaptic neurotoxin. For instance, mambia is a platelet aggregation inhibitor isolated from the venom of Dendroaspis Jamesonii. It has 59-amino acid residues, with four disulfide bonds and a high homology to postsynaptic neurotoxins (McDowell et al., 1992). 

The acetylcholine receptor and a neurotoxin form a noncovalent bond-type complex. The most important question is what portion of a neurotoxin is really involved in the receptor binding. Is it a particular residue, or are several residues involved Figure 4 is a two-dimensional structure of a postsynaptic neurotoxin— lapemis toxin from Lapemis hardwickii— that is based on the x-ray diffraction study of another similar toxin. 

A detailed NMR study of cardiotoxin indicated that there is a major structural difference that exists in all loops between cardiotoxins and postsynaptic neurotoxins (Bhaskaran et al., 1994). The difference in the secondary structure makes cardiotoxins bind to heart membranes, whereas neurotoxins do not bind. 

The homology and common mode of action of postsynaptic neurotoxins imply similar three-dimensional structure. X-ray crystallography has provided models for erabu-toxin b (structural type, 62-4) (Low et al, 1976 Tsernoglou and Petsko, 1976 Low and Corfield, 1986), cobratoxin (71-5) (Walkinshaw et al, 1980), and a-bungarotoxin (74-5)... 

Love and Stroud, 1986). All postsynaptic neurotoxins are similar in their overall folding, but differ in details such as the extent of secondary structure and the position of an invariant side-chain. Recently, the NMR three dimensional structure of cobrotoxin in solution has been determined (Yu et al, 1993). The mean solution structure was compared with the X-ray crystal structure of homologous protein erabutoxin b which has been solved to a resolution of 1.4 A (Low and Corifield, 1986) (Fig. 3). This yielded information that both structures resemble each other except at the exposed loops and surface regions, where the solution structure reveals the higher flexibility in its conformation. 

It seems necessary to distinguish between two types of important groups. The structurally important groups of one type are required for a molecule to attain its active conformation. Disulfide bonds and tyrosine residue at position 25 are structurally important to maintain the active conformation of the toxin. The second type is functionally important groups. For the postsynaptic neurotoxins, groups directly involved in binding the toxins to an AChR of the muscle motor endplate thus prevent transmission across the cholinergic synapse. 

Endo, T. and Tamiya, N. (1991) Structure-function relationships of postsynaptic neurotoxins from snake venoms, in Snake Toxins " (Harvey, A.L., ed.) Pergamon Press, New York, pp. 165-222. 

The structure and toxic symptoms of pinnamine resemble those of anatoxin-a (11) [19,20], a potent postsynaptic depolarizing neurotoxin known as very fast death factor (VFDF), and atropine [21], a representative suppressor of the... 

The mechanism of neurotoxin action on the nerve systems involves blocking of the nerve impulse transmission at the site of the neuromuscular junction by strong specific binding to the acetylcholine receptor in the postsynaptic membrane, thus interrupting the pathway whereby the neurotransmitter released in the synaptic cleft could affect the excitability of the postsynaptic neuron. The availability of highly purified acethylcholine receptor protein(24) allows a direct study of the structure-activity relationship of neurotoxins and their bindings with the receptor. 

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