A classification of sea anemone polypeptide neurotoxins has been proposed based on their amino acid sequence, dividing the group into three classes of sodium channel toxins. Halcurin is structurally homologous with type 2 toxins, but also has sequence homology to type 1 toxins. Type 1 and 2 toxins are composed of 46 to 49 amino acid residues, and cross-linked by three disulfide bridges. Ten residues including six Cysteine (Cys) residues are completely conserved between type 1 and 2 toxins. Therefore, it is possible that type 1 and 2 toxins have evolved from Halcurin as a common ancestor.
