Role of Arginine Residues for the Activity of Fasciculin

Abstract

The West African green mamba, Dendroaspis angusticeps, has two toxins, fasciculins, that are noncompetitive inhibitors of acetylcholinesterase. Arginine residues of fasciculin 2 were modified with 1,2-cyclohexanedione. Two of these residues, Arg24 and Arg37, reacted very slowly or not at all. Modification of Arg28 reduced the activity only by 13%. Argll and Arg27 are unique for fasciculins; a comparison of the sequences of 175 snake toxins homologous to fasciculins showed that no other toxin has arginine in the corresponding positions. Modification of the two unique arginines had a large effect and decreased the activity by 73% (Argll) and 85% (Arg27). This was apparently not due to structural perturbations, since the modification did not change the circular dichroic spectra. The two arginine residues probably participate in the binding to acetylcholinesterase. They are located on the same side of the toxin molecule and the distance between their a-carbons is 2.7 nm. This may indicate binding to sites that are far apart and suggests that fasciculin covers a large area of the enzyme.Keywords. Snake toxin ; fasciculin; arginine modification; acetylcholinesterase.The African snakes of the genus Dendroaspis (mambas) have neurotoxic venoms. A gel-filtration-purified fraction of Dendroaspis angusticeps (Western green mamba) venom produced hypersensitivity to touch and later long-lasting fasciculations in mice, i.e. uncontrolled muscle twitches, salivation, lacrimation, and secretion from the nose [l]. The symptoms indicated an elevated level of acetylcholine at the neuromuscular junction. It had been established that hypersensitivity was due to dendrotoxin, which had been isolated some years earlier [2]. Several dendrotoxins have been subsequently isolated from various mamba venoms. These toxins block voltage-gated potassium channels, which leads to increased release of acetylcholine [3]. The other symptoms observed in mice were typical for poisoning with anticholinesterases (AChE). This observation led to the isolation of two potent inhibitors of AChE. These inhibitors were called fasciculin 1 and 2, because of the symptoms theyThree fasciculins are known, fasciculin 1 and 2 from D. an- values in the range 10~"-10-'2 M [4, 6-11]. Other A C E are more than one million times less sensitive, e.g. AChE from chick biventer cervicis muscle and brain, cobra venom, and insects (heads of Musca dornestica, common house-fly) are inhibited with K, values greater than 10 pM. Butyrylcholinesterases are inhibited with K, values in the micromolar range, 0,5 pM for human serum cholinesterase [6-81.Fasciculins have many lysine and arginine residues giving the molecules a positive net charge at pH 7. Since many AChE inhibitors, e.g. neostigmine and physostigmine, are cationic molecules, it seems logical to investigate the role of amino and guanidino groups for the activity of fasciculin. Earlier [12], we acetylated amino groups of fasciculin 2 with acetic anhydride and isolated the mono acetyl derivatives. Modification of the a-amino g...

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

“…Karlsson and coworkers neutralized positive charges on Fas by chemical modification of single lysines or arginines, and reported a substantial decrease in Fas activity (Cervenansky et al ., 1994; Cervenansky et al ., 1995). Marchot et al .…”

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

“…The fact that the 3D structure of AChE [12] became available at about the same time as that of FAS-I [8] stimulated a wave of experimental studies aimed at elucidating the molecular basis for inhibition, using conventional enzyme kinetics [27][28][29], as well as chemical modification [25,30,31] and site-directed mutagenesis [29,32]. A theoretical study, in which docking of FAS onto the AChE structure was attempted, has also been published [33].…”

“…A theoretical study, in which docking of FAS onto the AChE structure was attempted, has also been published [33]. Chemical modification suggested participation of a number of conserved basic residues of FAS in the interaction with AChE [25,30], and further suggested that the interaction took place at the peripheral anionic site of AChE, which is located at the mouth of the active-site gorge [31]. Site-directed mutagenesis has provided evidence for participation of aromatic residues from the peripheral site of mouse AChE (Tyr70, Tyrl21 and Trp279) in the interaction with FAS [32].…”

Synthetic peptides mimicking the binding site of human acetylcholinesterase for its inhibitor fasciculin 2

“…These structures are generally very similar, revealing that Fas binds to a peripheral anionic site of AChE, thus sealing a gorge that leads to the active site. Fas inhibits h AChE with K i of 10 −11 M . This extraordinarily high affinity has been attributed to a remarkable surface complimentarity between the two proteins, as well as to a considerable burial of the hydrophobic surface area upon binding .…”