Phoratoxins are a group of peptide toxins that belong to the family of thionins, a subdivision of small plant toxins (5 kD MW). Phoratoxins are proteins present in the leaves and branches of the Phoradendron, commonly known as the American variant of the mistletoe, a plant commonly used as decoration during the festive season. The berries of the mistletoe do not contain phoratoxins, making them less toxic compared to other parts of the plant. The toxicity of the mistletoe is dependent on the host tree, since mistletoe is known to be a semi-parasite. The host tree provides fixed inorganic nitrogen compounds necessary for the mistletoe to synthesize phoratoxins. Viscotoxins are similar plant thionins produced from the leaves and stems of the European mistletoe (Viscum album). It also contains the unrelated toxic lectin, viscumin. This protein functions by the same mechanism as ricin on ribosomal rRNA which permanently inactivates protein synthesis and is responsible for the massively higher toxicity of V. album compared to the North American variety which does not appear to contain this class of toxins.
History The history of phoratoxin is filled with myths, legends, and other magical stories. Mistletoe is a semi-parasitic plant occasionally using oak trees as their host. To historic peoples such as the Gauls and the Druids, oak trees were sacred. This led to beliefs that the mistletoe containing viscotoxin was a cure all drug for illnesses. These stories lived on for many years and were spread through the Americas by European settlers who came to the United States and mistook the American mistletoe (Phoradendron) for the more toxic European mistletoe (Viscum album). This made sure the mysterious past belonging to viscotoxin is shared by phoratoxin.
Structure and reactivity The folding motif and overall topology of phoratoxins are identical to that of crambin. Phoratoxin proteins fold into two anti-parallel amphipathic helices that are perpendicular to a double stranded beta sheet and a C-terminal coil region. The general configuration of the protein resembles the shape of the Greek capital letter gamma. Phoratoxin contains three disulfide bridges. Phoratoxin expresses different features that are typical for membrane active proteins, it is compact, contains many basic amino acid residues and it contains one weakly polar flat face. It is likely that phoratoxins have multiple aggregation states and can either exist as a monomer, a dimer, or a tetramer consisting of a dimer of dimers. The state of aggregation depends on the presence of inorganic phosphate or phospholipids. The aggregates are hydrophobic and hydrophilic dimers, joined by intermolecular interaction. Bridging of the hydrophilic dimer is only possible when inorganic phosphate ion is present, making inorganic phosphate ion presence necessary for lattice formation. This inorganic phosphate ion increases the toxin stability by neutralizing the positively charged basic amino acids of the monomers, creating possibilities for more Van Der Waals interactions. Phoratoxin is an amphipathic molecule, a feature that is frequently found in membrane binding proteins. Phoratoxin has a positive membrane binding site that can bind negatively charged phospholipids. This phospholipid binding site is located in between the alpha helix and the beta sheet. The phosphate fits in a binding pocket near Lys-1, while the glycerol part as well as the two termini of the bound phospholipid move towards the random coil region (residue 36–44).
Available forms
There are six forms of phoratoxin known to exist. Five out of six forms are 46 amino acids long; only phoratoxin D is 41 amino acids long. The differences between these six forms are mainly centered in the random coil region of the protein (residue 36-44/41). In phoratoxin F, Phe-18 and Gly-19 are mutated into a leucine and an alanine. Phenylalanine and leucine are both hydrophobic, however phenylalanine is large, aromatic and strand-preferring, while leucine is medium in size and prefers to be in a helix. Glycine and alanine are both small amino acids, but glycine prefers turns while alanine prefers helices, and glycine is extremely flexible and intermediate while alanine is hydrophobic. Assuming phoratoxin folds in the exact same manner as crambin residues 7-19 are in an alpha helix. This mutation makes the helix of phoratoxin F more stable than helix in the other phoratoxin forms. A second difference in the various forms of phoratoxin is Val-25/Ile-25, in the second alpha-helix coding region. The fact that valine and isoleucine are both beta branched, the beta sheet is their secondary structure of preference. Since both of these amino acids are also hydrophobic this mutation is thought to have little interference with the folding and stability of the protein. The last differentiation in amino acid configuration is at Asn-45/Asp-45/Thr-45. This mutation is not in a region coding for a specific secondary protein structure. Asparagine and aspartic acid are both hydrophilic, while threonine has intermediate hydrophilicity. Asparagine and aspartic acid are both larger than threonine, and both asparagine and aspartic acid like to be in a turn while threonine prefers a beta strand. Aspartic acid and asparagine carry a formal charge. This charge however is negative for aspartic acid and positive for asparagine.
Synthesis Phoratoxin has not been synthesized in a lab yet. It can, however, be extracted from the Phoradendron. Phoratoxin is located most in the leaves and berries of the plant. It can be isolated from the plant by means of chromatography or extracted with an acid, and thereafter purified with an amide.
Metabolism There is not a lot of available information about the mechanism of action of phoratoxin. It is however known that phoratoxin has a high affinity for phospholipids, and therefore able to disrupt cell membranes. There is no metabolic activity as the protein already expresses its toxic trait, and stays intact the way it was synthesized in the Phoradendron.
… excerpt ends here. Continue reading the full article.
