Phalloidin synthetic analogues: structural requirements in the interaction with F-actin

Synthetic derivatives of phalloidin have been investigated in solution by circular dichroism (CD) and NMR spectroscopy. They differ from natural phalloidin (PHD). bicyclo(Ala1-D-Thr2-Cys3-cis-4-hydroxy-Pro4-Ala5-2-mercapto-Trp6-(OH)2Leu7)(S-3 --> 6), in that they are modified at positions 2, 3, and 7. Among these synthetic analogues, structural differences and varying degrees of atropisomerism are found. By comparing the respective molecular models obtained by restrained molecular dynamics (RMD) simulations based on experimental NMR data, structural features that may be responsible for the different biological behavior become apparent. Our results indicate that the structural changes that result from an inversion of chirality of residue 3 lead to a complete loss of toxicity. Conversely, toxicity is less affected by the structural changes that stem from an inversion of chirality of residue 2. Moreover, unlike the other phallotoxins, when the thioether unit bridges to the opposite face of the main peptide ring, in contrast to the situation in other phallotoxins, large structural changes are observed as well as a total loss of activity. Molecular models of the synthetic phalloidin analogues have been used to investigate the necessary structural requirements for the interaction with F-actin. To this end, the F-actin/PHD model of M. Lorenz et al. was employed; docking experiments of our molecular models in the PHD binding site are presented.

Solid state and solution conformation of [Ala7]-phalloidin: a synthetic phallotoxin analogue

Phallotoxins are toxic compounds produced by poisonous mushroom Amanita phalloides and belong to the class of bicyclic peptides with a transannular thioether bridge. Their intoxication mechanism in the liver involves a specific binding of the toxins to F-actin that, consequently, prevents the depolymerization equilibrium with G-actin. Even though the conformational features of phallotoxins have been worked out in solution, the exact mechanism of interaction with F-actin is still unknown. In this study a toxic phalloidin synthetic derivative, bicyclo(Ala1-D-Thr2-Cys3-cis-4-hydroxy-Pro4-Ala5-2-mercapto-Trp6-Ala7)(S-3-->6) has been synthesized. A substitution at position 7. with an Ala residue replaces the 4,5-dihydroxy-Leu present in the natural phalloidin. This analogue has formed crystals suitable for X-ray analysis, and represents the first case for such a class of compounds. The solid-state structure as well as the solution conformation have been evaluated. NMR techniques have been used to extract interproton distances as restraints in subsequent molecular dynamics calculations. Finally, a direct comparison between structures in solution and in the solid state is presented.

Conformationally restricted peptides through short-range cyclizations

The various types of conformationally restricted peptides obtained by short-range cyclizations, from residue i to residue i + 1, are presented. Relevant examples of N in equilibrium C alpha, C' in equilibrium C alpha, N in equilibrium C', C alpha in equilibrium C alpha, C' in equilibrium C', and N in equilibrium N cyclizations are reported and the pertinent literature listed. In the discussion emphasis is place on the conformational consequences for peptides from the incorporation of such ring structures.

Synthesis of analogues of amaninamide, an amatoxin from the white Amanita virosa mushroom

Analogs of amaninamide, due to the absence of a 6-hydroxy group in the tryptophan moiety, are more easily accessible by synthesis than derivatives of alpha-amanitin. Syntheses of bicyclic octapeptide thioethers 1f-1m have been carried out starting from linear Hpi-S-trityl-octapeptides (3), cyclization by intramolecular 2'-indolylthioether formation yielding monocyclic peptides (2) and final cyclization by DCCI. One of the bicyclic thioethers was oxidized to yield the corresponding chromatographically separated (R)- and (S)-sulfoxides, respectively. The products were characterized by RF-values, u.v. and CD spectra as well as by mass (FAB) spectroscopy. The widely differing inhibitory activities on RNA polymerase II (or B) from calf thymus are listed.

Synthesis of cyclic tryptathionine peptides

The helicity of the tryptathionine moiety of the phallotoxins has been recognized by comparison with cyclic tryptathionine tripeptides. In order to investigate the influence of the configuration of the component amino acids on the conformation of the cyclic peptides, six analogue thioether tripeptides containing L- and D-alanine and L- and D-cysteine, respectively, have been synthesized. The CD spectra of the peptides are very similar to each other, showing mirror images of the CD of phalloidin and, therefore, negative helicity. The spectra of the D-cysteine containing compounds differ from the L-cysteine containing compounds by their weakly positive ellipticity values around 270 nm. The cyclization reaction of Boc-Hpi-D-Ala-D-Cys(STrt)OCH3, along with the cyclic tripeptide, afforded a cyclic hexapeptide by dimerization. The CD spectrum of the dimer is very similar to that of phalloidin, thus pointing to a positive helicity of its two tryptathionine moieties. The dimeric thioether peptide forms a rather strong complex with Cu2+ ions.

[50 years of phalloidine: its discovery, characterization and current and future applications in cell research]

Phalloidin, like the later-detected phallotoxins, consists of a cyclic heptapeptide backbone, the ring being crosslinked by a 2'-indolylthioether moiety (tryptathionine). After intraperitoneal administration--not per os--it will, after a short time, damage the liver specifically, presumably in consequence of its very tight binding to F-actin preventing its dissociation. This affinity can be utilized for a sensitive visual identification of F-actin by using fluorescent derivatives.

The toxic peptides from Amanita mushrooms

The results of 50 years of effort in the chemistry of Amanita toxins are reviewed. The phallotoxins, fast acting components, but not responsible for fatal intoxications after ingestion, are bicyclic heptapeptides. They combine with F-actin, stabilizing this protein against several destabilizing influences. The virotoxins likewise fast acting are monocyclic heptapeptides. The amatoxins which are the real toxins lead to death within several days by inhibiting the enzymatic synthesis of m-RNA. They are bicyclic octapeptides. The structures of all of these compounds are described, as well as conformations, chemical reactions and modification, syntheses and correlations between structures and biological activities.