Parasitic protozoa, which are auxotrophic for purines, have evolved a unique metabolic pathway, the purine salvage pathway, to meet their requirements for purines. Since they are unable to synthesize their purines, they must salvage these essential nutrients from their hosts. Numerous enzymes are involved in allowing the parasitic protozoa to obtain the purines it needs from its host, the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRT). Appears to be a key enzyme in purine salvage Salvage used by these parasites. This thesis examines the role of the active site residue lysine 157 within the HGPRT of Leishmania donovani.