The trypanothione synthetase (TryS) catalyses the two-step biosynthesis of trypanothione from spermidine and glutathione and is an attractive new drug target for the development of trypanocidal and antileishmanial drugs, especially since the structural information of TryS from Leishmania major has become available. Unfortunately, the TryS structure was solved without any of the substrates and lacks loop regions that are mechanistically important. This contribution describes docking and molecular dynamics simulations that led to further insights into trypanothione biosynthesis and, in particular, explains the binding modes of substrates for the second catalytic step. The structural model essentially confirm previously proposed binding sites for glutathione, ATP and two Mg2+ ions, which appear identical for both catalytic steps. The analysis of an unsolved loop region near the proposed spermidine binding site revealed a new pocket that was demonstrated to bind glutathionylspermidine in an inverted orientation. For the second step of trypanothione synthesis glutathionylspermidine is bound in a way that preferentially allows N1-glutathionylation of N8-glutathionylspermidine, classifying N8-glutathionylspermidine as the favoured substrate. By inhibitor docking, the binding site for N8-glutathionylspermidine was characterised as druggable.

The large superfamily of nuclear receptors is a family of ligand-activated transcription factors involved in numerous fundamental processes and shows many common characteristics and behaviors. The comprehension of these roles is of fundamental importance to select the right target for receptor structure-based screening. Recently, during the last ten years, several crystallographic structures of nuclear receptors complexed with ligands have been registered in the Protein Data Bank, supplying a structural basis for computational simulations. The macroscopic flexibility of helix12 and local flexibility of some amino acids sidechains within cavities of the Ligand Binding Domain suggest a reason for the behavior of these receptors toward different ligands. Several approaches have been applied in trying to explain this flexibility and to predict how ligand binding can influence complex conformations. In this short review, we present an introduction to the structure and function of nuclear receptors, specifically the estrogen, androgen, glucorticoid, peroxisome proliferator, steroid, thyroid and vitamin D receptors, and a discussion of the state-of-the-art of induced fit approaches for the nuclear receptor family.

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