Structural characterization of hellethionins from Helleborus purpurascens

Thionins are relatively small-sized multiple-cystine peptides that are probably involved in the plant defense against pathogens. As such, these peptides constitute promising candidates for engineered plant resistance in the agricultural industry. More recently, thionins have been proposed as potential immunotoxins in tumor therapy. In the search for pharmacologically active natural products, a new family of thionins was recently discovered in the roots of Helleborus purpurascens that accordingly were termed hellethionins. The structural characterization by NMR of one representative member of this family, i.e., of hellethionin D, clearly reveals that thionins from different sources share a highly conserved overall fold. In fact, the well-defined 3D structure of hellethionin D is very similar to those reported so far for viscotoxins, purothionins, or crambin, although distinct differences could be detected in the C-terminal portion, especially for loop 36-39. These differences may derive from the unusual distribution of charged residues in the C-terminal half of the peptide sequence compared to other thionins and from the uncommon occurrence of four contiguous threonine residues in loop 36-39. As expected, reduction of the disulfide bonds in hellethionin D leads to complete unfolding, but upon oxidative refolding by air oxygen in the presence of glutathione the correct isomer is recovered in high yields, confirming the very robust fold of this class of bioactive cystine peptides.