Tetramerization and cooperativity in Plasmodium falciparum glutathione transferase are mediated by the atypic loop 113-119.

Glutathione transferase of Plasmodium falciparum (PfGST) displays a peculiar dimer to tetramer transition that causes full enzyme inactivation and loss of its ability to sequester parasitotoxic hemin. Furthermore, binding of hemin is modulated by a cooperative mechanism. Site directed mutagenesis, steady-state kinetic experiments and fluorescence anisotropy have been used to verify the possible involvement of the loop 113-119 in the tetramerization process and in the cooperative phenomenon. This protein segment is one the most prominent structural differences between PfGST and other GST isoenzymes. Our results demonstrate that truncation, increased rigidity or even a simple point mutation of this loop cause a dramatic change in the tetramerization kinetics, that becomes hundred times very much slower than in the native enzyme. Furthermore, all mutants lose the positive cooperativity for hemin binding, suggesting that the integrity of this peculiar loop is essential for intersubunit communication. Interestingly, the tetramerization process of the native enzyme, that occurs rapidly when GSH is removed, is prevented not only by GSH but even by oxidized glutathione. This result suggests that protection by PfGST against hemin is independent of the redox status of the parasite cell.