Crystal structures of adenine phosphoribosyltransferase from Leishmania donovani

The enzyme adenine phosphoribosyltransferase (APRT) functions to salvage ad enine by converting it to adenosine-5-monophosphate (AMP), APRT deficiency in humans is a well characterized inborn error of metabolism, and APRT may contribute to the indispensable nutritional role of purine salvage in proto zoan parasites, all of which lack de novo purine biosynthesis. We determine d crystal structures for APRT from Leishmania donovani in complex with the substrate adenine, the product AMP, and sulfate and citrate ions that appea r to mimic the binding of phosphate moieties. Overall, these structures are very similar to each other, although the adenine and AMP complexes show di fferent patterns of hydrogen-bonding to the base, and the active site pocke t opens slightly to accommodate the larger AMP ligand. Whereas AMP adopts a single conformation, adenine binds in two mutually exclusive orientations: one orientation providing adenine-specific hydrogen bonds and the other ap parently positioning adenine for the enzymatic reaction. The core of APRT i s similar to that of other phosphoribosyltransferases, although the adenine -binding domain is quite different. A C-terminal extension, unique to Leish mania APRTs, extends an extensive dimer interface by wrapping around the pa rtner molecule, The active site involves residues from both subunits of the dimer, indicating that dimerization is essential for catalysis.