Polyamine-based thiols in Trypanosomatids: evolution, protein structural adaptations and biological functions

<b>Significance.</b> Major pathogenic enterobacteria and protozoan parasites from the phylum Euglenozoa, such as Trypanosomatids, are endowed with glutathione-spermidine derivatives that play important roles in signaling, metal and thiol-redox homeostasis. For some Euglenozoa lineages, the glutathione-spermidine conjugates represent the main redox co-substrates around which entire new redox systems have evolved. Several proteins underwent molecular adaptations to synthesize and utilize the new polyamine-based thiols. <b>Recent Advances. </b>The genomes of several evolutionarily related organisms have recently been sequenced, which allows mining and analysis of gene sequences that belong to these peculiar redox systems. Similarly, the 3D structures of several of these proteins have been solved, which allows for comparison with their counterparts in classical redox systems that rely on glutathione/glutaredoxin and thioredoxin. <b>Critical Issues.</b> The evolutionary and structural aspects related to the emergence and use of glutathione-spermidine conjugates in Euglenozoa are reviewed focusing on unique structural specializations that proteins developed to use N1,N8-bisglutathionylspermidine (trypanothione) as redox co-substrate. An updated overview on the biochemical and biological significance of the major enzymatic activities is also provided. <b>Future Directions.</b> A thiol-redox system strictly dependent on trypanothione is a feature unique to Trypanosomatids. The physicochemical properties of the polyamine-glutathione conjugates were a major driving force for structural adaptation of proteins that use these thiols as ligand and redox cofactor. In fact, the structural differences of indispensable components of this system can be exploited towards selective drug development. Future research should clarify whether additional cellular processes are regulated by the trypanothione-system.