Quaternary Structure and Activity Modulation of HPRT of Trypanosoma cruzi.

VALSECCHI W; SANTOS J; DELFINO JM

Congreso; Reunión Anual de la SAB; 2014

Quaternary Structure and Activity Modulation of HPRT of Trypanosoma cruzi Valsecchi, WM; Santos, J; Delfino, JM. Departamento de Química Biológica, IQUIFIB (UBA-CONICET), FFyB, UBA. Hypoxanthine guanine phosphoribosyl transferase (HPRT) is a globular α/β protein of 221 amino acid residues that catalyzes the transfer of a ribose-1P from phosphoribosyl pyrophosphate (PRPP) to hypoxanthine or guanine bases yielding IMP or GMP, respectively. HPRT has been proposed as a potential target for drugs useful for treating diseases caused by protozoan parasites, given that, while in humans purine nucleotides may be obtained both through the salvage (or recovery) pathway or by de novo synthesis, the salvage pathway is the only one operational in trypanosomatids, therefore becoming essential for their survival. Contrary to the long-standing claim that TcHPRT is a dimer, we have previously shown that this enzyme behaves as a tetramer in solution. In addition, we have crystallized and solved its structure, which allowed us to infer that the C-terminal region (CTR) is flexible and might be involved in the stabilization of its quaternary structure. Here we present evidence on the role of the CTR in the consolidation of the quaternary structure and on the effect of tetramerization on enzymatic activity. Proteolytic removal of the CTR yields a defined dimeric species, possessing increased activity. We also show the inhibitory activity of a set of several bisphosphonates -compounds resembling pyrophosphate, currently in use for the treatment of osteoporosis- and preliminary data on the co-crystallization of these ligands with the protein. A consistent picture of the molecular features determining the inhibitory power emerges from this data. All in all, our results suggest that the CTR represents an important structural element bearing a significant role in both structure and function. Besides, the assay of new inhibitory compounds bears promise for the development of more effective treatments for protozoan diseases.