Role of arginine residues for the activity of fasciculin

Nature: a substantial source of auspicious substances with acetylcholinesterase inhibitory action

Acetylcholinesterase (AChE) (EC 3.1.1.7) is an important enzyme that breaks down of acetylcholine in synaptic cleft in neuronal junctions. Inhibition of AChE is associated with treatment of several diseases such as Alzheimer's disease (AD), myasthenia gravis, and glaucoma as well as the mechanisms of insecticide and anthelmintic drugs. Several AChE inhibitors are available in clinical use currently for the treatment of AD; however, none of them has ability, yet, to seize progress of the disease. Consequently, an extensive research has been going on finding new AChE inhibitors. In this sense, natural inhibitors have gained great attention due to their encouraging effects toward AChE. In this review, promising candidate molecules with marked AChE inhibition from both plant and animal sources will be underlined.

Design, expression and characterization of mutants of fasciculin optimized for interaction with its target, acetylcholinesterase

Predicting mutations that enhance protein–protein affinity remains a challenging task, especially for high-affinity complexes. To test our capability to improve the affinity of such complexes, we studied interaction of acetylcholinesterase with the snake toxin, fasciculin. Using the program ORBIT, we redesigned fasciculin's sequence to enhance its interactions with Torpedo californica acetylcholinesterase. Mutations were predicted in 5 out of 13 interfacial residues on fasciculin, preserving most of the polar inter-molecular contacts seen in the wild-type toxin/enzyme complex. To experimentally characterize fasciculin mutants, we developed an efficient strategy to over-express the toxin in Escherichia coli, followed by refolding to the native conformation. Despite our predictions, a designed quintuple fasciculin mutant displayed reduced affinity for the enzyme. However, removal of a single mutation in the designed sequence produced a quadruple mutant with improved affinity. Moreover, one designed mutation produced 7-fold enhancement in affinity for acetylcholinesterase. This led us to reassess our criteria for enhancing affinity of the toxin for the enzyme. We observed that the change in the predicted inter-molecular energy, rather than in the total energy, correlates well with the change in the experimental free energy of binding, and hence may serve as a criterion for enhancement of affinity in protein–protein complexes.

Modification of Substrate Inhibition of Synaptosomal Acetylcholinesterase by Cardiotoxins

Different types of cardiotoxin (I-V and n) were isolated and purified from the venom of the Taiwan cobra (Naja naja atra). The effects of these cardiotoxins were studied on membrane-bound acetylcholinesterase, which was isolated from a sheep's brain cortex. The results showed that cardiotoxins I-III, V, and n activated the enzyme by modification of substrate inhibition, but cardiotoxin IV's reaction was different. The inhibition and activation of acetylcholinesterase were linked to the functions of the hydrophobicity index, presence of a cationic cluster, and the accessible arginine residue. Our results indicate that Cardiotoxins have neither a cationic cluster nor an arginine residue in their surface area of loop I; therefore, in contrast to fasciculin, cardiotoxins are attached by loop II to the peripheral site of the enzyme. As a result, fasciculin seems to stabilize nonfunctional conformation, but cardiotoxins seem to stabilize the functional conformation of the enzyme. Based on our experimental and theoretical findings, similar secondary and tertiary structures of cardiotoxins and fasciculin seem to have an opposite function once they interact with acetylcholinesterase.

Neurotoxinas con actividad anticolinesterásica y su posible uso como agentes de guerra

Anatoxin-a(s), onchidal and fasciculins are neurotoxins with anticholinesterase activity. An intoxication by these neurotoxins is characterized by cholinergic syndromes similar to organophosphate insecticide and nerve agent intoxications. Anticholinesterase neurotoxins, as well as other toxins, have some disadvantages if used as weapons of mass destruction. Drawbacks include difficulties to produce them in big quantities and their dissemination in form of aerosols. However, other properties such as high toxicity, improbable identification with common commercial portable detectors for chemical warfare agents and toxic industrial chemicals, as well as the lack of effectiveness of antidotal treatments with oximes may make them attractive in order to be used in military operations or terrorist attacks. For these reasons, it should be necessary to control these neurotoxins through international treaties which have real verification measures such as the Chemical Weapons Convention.

Dynamical properties of fasciculin-2

Fasciculin-2 (FAS2) is a potent protein inhibitor of the hydrolytic enzyme acetylcholinesterase. A 2-ns isobaric-isothermal ensemble molecular dynamics simulation of this toxin was performed to examine the dynamic structural properties which may play a role in this inhibition. Conformational fluctuations of the FAS2 protein were examined by a variety of techniques to identify flexible residues and determine their characteristic motion. The tips of the toxin "finger" loops and the turn connecting loops I and II were found to fluctuate, while the rest of the protein remained fairly rigid throughout the simulation. Finally, the structural fluctuations were compared to NMR data of fluctuations on a similar timescale in a related three-finger toxin. The molecular dynamics results were in good qualitative agreement with the experimental measurements. Proteins 1999;36:447-453.

Interaction of synthetic peptides from fasciculin with acetylcholinesterase

Fasciculins (Fas) are three-looped polypeptides isolated from mamba venom which exert their toxic action by inhibiting noncompetitively acetylcholinesterase (AChE). A peptide (Fas-D) encompassing the first loop sequence was synthesized and characterized chemically, structurally, and functionally. Fas-D possesses an intramolecular disulfide bridge, present in the native toxin. Circular dichroism (CD) indicated the existence of 21.8% beta-sheet content and 24.2% beta-turn in this peptide, compatible with crystallographic data of the native toxin. The peptide showed only low partial AChE inhibition at submillimolar concentrations, much lower than that observed with Fas and a peptide (Fas-B) encompassing the second loop sequence. The simultaneous presence of Fas-D and Fas-B produced an additive inhibitory effect on AChE activity; calculated Ki and alphaKi values (7.3 +/-2.4 microM and 10.0 +/- 1.8 microM, respectively) were not significantly different, thus indicating noncompetitive inhibition. These results are consistent with site-directed mutagenesis studies and analysis of the crystal structure of the Fas-AChE complex, which indicate that residues from loops I and II contribute to Fas binding to the enzyme.

125I-labeled fasciculin 2: a new tool for quantitation of acetylcholinesterase densities at synaptic sites by EM-autoradiography

Radio-iodinated fasciculin 2 (Fas2), a polypeptide anticholinesterase toxin from Mamba venom, was used as a new probe for localizing and quantifying acetylcholinesterase (AChE) at mouse neuromuscular junctions (NMJs) by quantitative electron microscope autoradiography. We demonstrate that 125I-Fas2 binds very specifically to the NMJs of mouse sternomastoid muscles, with very little binding to other regions in the muscles. Junctional AChE-site densities obtained from the autoradiograms were similar to those previously obtained for the same muscles using 3H-DFP. The use of 125I-Fas2 with EM-autoradiography is simpler and provides higher resolution and sensitivity, as well as considerably lower non-specific binding than previously attainable with 3H-DFP. The advantages and limitations of this procedure are discussed.

Synthetic peptides derived from the central loop of fasciculin: structural analysis and evaluation as inhibitors of acetylcholinesterase

Fasciculins are peptides present in the venom of green and black mamba snakes, with potent inhibitory activity towards acetylcholinesterase. In order to determine the role of fasciculin loop II in the acetylcholinesterase inhibition, two fasciculin fragments were synthesized by the solid phase procedure using N-alpha-Boc protected amino acids. The two peptides, Fas-A and Fas-B, span the 26-32 and 22-35 sequences of fasciculin and a disulfide bridge links each peptide end, thus ensuring the formation of a looped structure. Both peptides were characterized chemically, structurally and functionally. Circular dichroism indicated the existence of 19.4 and 24.9% of beta-sheet for Fas-A and Fas-B, respectively; SDS-PAGE patterns and mass spectrometry disclosed the intramolecular disulfide formation in both peptides. An inhibitory effect on eel acetylcholinesterase was observed with the longer peptide (Ki = 15.1 microM), without reaching the affinity level of the parent native toxin (Ki = 0.3 nM). This study confirms that fasciculin central loop residues strongly contribute to toxin interaction with acetylcholinesterase.

Expression and activity of mutants of fasciculin, a peptidic acetylcholinesterase inhibitor from mamba venom

Fasciculin, a selective peptidic inhibitor of acetylcholinesterase, is a member of the three-fingered peptide toxin superfamily isolated from snake venoms. The availability of a crystal structure of a fasciculin 2 (Fas2)-acetylcholinesterase complex affords an opportunity to examine in detail the interaction of this toxin with its target site. To this end, we constructed a synthetic fasciculin gene with an appropriate leader peptide for expression and secretion from mammalian cells. Recombinant wild-type Fas2, expressed and amplified in Chinese hamster ovary cells, was purified to homogeneity and found to be identical in composition and biological activities to the venom-derived toxin. Sixteen mutations at positions where the crystal structure of the complex indicates a significant interfacial contact point or determinant of conformation were generated. Two mutants of loop I, T8A/T9A and R11Q, ten mutants of the longest loop II, R24T, K25L, R27W, R28D, H29D, DeltaPro30, P31R, K32G, M33A, and V34A/L35A, and two mutants of loop III, D45K and K51S, were expressed transiently in human embryonic kidney cells. Inhibitory potencies of the Fas2 mutants toward mouse AChE were established, based on titration of the mutants with a polyclonal anti-Fas2 serum. The Arg27, Pro30, and Pro31 mutants each lost two or more orders of magnitude in Fas2 activity, suggesting that this subset of three residues, at the tip of loop II, dominates the loop conformation and interaction of Fas2 with the enzyme. The Arg24, Lys32, and Met33 mutants lost about one order of magnitude, suggesting that these residues make moderate contributions to the strength of the complex, whereas the Lys25, Arg28, Val34-Leu35, Asp45, and Lys51 mutants appeared as active as Fas2. The Thr8-Thr9, Arg11, and His29 mutants showed greater ratios of inhibitory activity to immunochemical titer than Fas2. This may reflect immunodominant determinants in these regions or intramolecular rearrangements in conformation that enhance the interaction. Of the many Fas2 residues that lie at the interface with acetylcholinesterase, only a few appear to provide substantial energetic contributions to the high affinity of the complex.

Cloning and expression of acetylcholinesterase from Bungarus fasciatus venom. A new type of cooh-terminal domain; involvement of a positively charged residue in the peripheral site

As deduced from cDNA clones, the catalytic domain of Bungarus fasciatus venom acetylcholinesterase (AChE) is highly homologous to those of other AChEs. It is, however, associated with a short hydrophilic carboxyl-terminal region, containing no cysteine, that bears no resemblance to the alternative COOH-terminal peptides of the GPI-anchored molecules (H) or of other homomeric or heteromeric tailed molecules (T). Expression of complete and truncated AChE in COS cells showed that active hydrophilic monomers are produced and secreted in all cases, and that cleavage of a very basic 8-residue carboxyl-terminal fragment occurs upon secretion. The COS cells produced Bungarus AChE about 30 times more efficiently than an equivalent secreted monomeric rat AChE. The recombinant Bungarus AChE, like the natural venom enzyme, showed a distinctive ladder pattern in nondenaturing electrophoresis, probably reflecting a variation in the number of sialic acids. By mutagenesis, we showed that two differences (methionine instead of tyrosine at position 70; lysine instead of aspartate or glutamate at position 285) explain the low sensitivity of Bungarus AChE to peripheral site inhibitors, compared to the Torpedo or mammalian AChEs. These results illustrate the importance of both the aromatic and the charged residues, and the fact that peripheral site ligands (propidium, gallamine, D-tubocurarine, and fasciculin 2) interact with diverse subsets of residues.

Fasciculin: modification of carboxyl groups and discussion of structure-activity relationship

Norleucine methylester was coupled to carboxylates of fasciculin 2, a snake toxin that inhibits acetylcholinesterase (AChE). This neutralized negative charges but had no effect on the activity, suggesting that carboxyls do not participate in binding to AChE. Earlier results are discussed. Modification of three aromatic amino acids in the peripheral site of AChE, the binding site for fasciculin, decreased the affinity 100 to one million times. Neutralizing the charge of cationic groups of fasciculin lowered the affinity only three to seven times. A change in either the toxin or enzyme part of a binding site should have about the same effect. Since this was not so, it suggests that cationic groups of fasciculin do not bind to aromatic rings in the peripheral site.

Crystal structure of an acetylcholinesterase-fasciculin complex: interaction of a three-fingered toxin from snake venom with its target

BACKGROUND

Fasciculin (FAS), a 61-residue polypeptide purified from mamba venom, is a three-fingered toxin which is a powerful reversible inhibitor of acetylcholinesterase (AChE). Solution of the three-dimensional structure of the AChE/FAS complex would provide the first structure of a three-fingered toxin complexed with its target.

RESULTS

The structure of a complex between Torpedo californica AChE and fasciculin-II (FAS-II), from the venom of the green mamba (Dendroaspis angusticeps) was solved by molecular replacement techniques, and refined at 3.0 A resolution to an R-factor of 0.231. The structure reveals a stoichiometric complex with one FAS molecule bound to each AChE subunit. The AChE and FAS conformations in the complex are very similar to those in their isolated structures. FAS is bound at the 'peripheral' anionic site of AChE, sealing the narrow gorge leading to the active site, with the dipole moments of the two molecules roughly aligned. The high affinity of FAS for AChE is due to a remarkable surface complementarity, involving a large contact area (approximately 2000 A2) and many residues either unique to FAS or rare in other three-fingered toxins. The first loop, or finger, of FAS reaches down the outer surface of the thin aspect of the gorge. The second loop inserts into the gorge, with an unusual stacking interaction between Met33 in FAS and Trp279 in AChE. The third loop points away from the gorge, but the C-terminal residue makes contact with the enzyme.

CONCLUSIONS

Two conserved aromatic residues in the AChE peripheral anionic site make important contacts with FAS. The absence of these residues from chicken and insect AChEs and from butyrylcholinesterase explains the very large reduction in the affinity of these enzymes for FAS. Several basic residues in FAS make important contacts with AChE. The complementarity between FAS and AChE is unusual, inasmuch as it involves a number of charged residues, but lacks any intermolecular salt linkages.

Acetylcholinesterase inhibition by fasciculin: crystal structure of the complex

The crystal structure of the snake toxin fasciculin, bound to mouse acetylcholinesterase (mAChE), at 3.2 A resolution reveals a synergistic three-point anchorage consistent with the picomolar dissociation constant of the complex. Loop II of fasciculin contains a cluster of hydrophobic residues that interact with the peripheral anionic site of the enzyme and sterically occlude substrate access to the catalytic site. Loop I fits in a crevice near the lip of the gorge to maximize the surface area of contact of loop II at the gorge entry. The fasciculin core surrounds a protruding loop on the enzyme surface and stabilizes the whole assembly. Upon binding of fasciculin, subtle structural rearrangements of AChE occur that could explain the observed residual catalytic activity of the fasciculin-enzyme complex.