The hybrid pathway for heme biosynthesis in the malarial parasite proposes the involvement of parasite genome-coded enzymes of the pathway localized in different compartments such as apicoplast, mitochondria, and cytosol. However, knowledge on the functionality and localization of many of these enzymes is not available. In this study, we demonstrate that porphobilinogen deaminase encoded by the Plasmodium falciparum genome (PfPBGD) has several unique biochemical properties. Studies carried out with PfPBGD partially purified from parasite membrane fraction, as well as recombinant PfPBGD lacking N-terminal 64 amino acids expressed and purified from Escherichia coli cells (⌬PfPBGD), indicate that both the proteins are catalytically active. Surprisingly, PfPBGD catalyzes the conversion of porphobilinogen to uroporphyrinogen III (URO-GEN III), indicating that it also possesses uroporphyrinogen III synthase (UROS) activity, catalyzing the next step. This obviates the necessity to have a separate gene for UROS that has not been so far annotated in the parasite genome. Interestingly, ⌬PfP-BGD gives rise to UROGEN III even after heat treatment, although UROS from other sources is known to be heat-sensitive. Based on the analysis of active site residues, a ⌬PfPBGDL116K mutant enzyme was created and the specific activity of this recombinant mutant enzyme is 5-fold higher than ⌬PfPBGD. More interestingly, ⌬PfPBGDL116K catalyzes the formation of uroporphyrinogen I (UROGEN I) in addition to URO-GEN III, indicating that with increased PBGD activity the UROS activity of PBGD may perhaps become rate-limiting, thus leading to non-enzymatic cyclization of preuroporphyrinogen to URO-GEN I. PfPBGD is localized to the apicoplast and is catalytically very inefficient compared with the host red cell enzyme.Detailed studies of the metabolic pathways in the malarial parasite hold promise for the identification of new antimalarial drug targets (1, 2). Earlier studies in this laboratory had shown that the malarial parasite synthesizes heme de novo, despite acquiring heme from the host red cell hemoglobin in the intraerythrocytic stage. It has been shown that Plasmodium falciparum contains ␦-aminolevulinate dehydratase (ALAD) 2 of dual origin: one species encoded by the parasite genome (PfALAD) and another imported from the host red cell. Inhibition of ALAD activity in the parasite, the second enzyme of the pathway with a specific inhibitor, succinylacetone, leads to inhibition of heme synthesis and death of the parasite, indicating the potential of the pathway as a drug target (3, 4).Heme biosynthesis from glycine and succinyl-CoA involves eight different steps and the genes for all the enzymes of the pathway, with the exception of uroporphyrinogen III synthase (UROS, hemD) have been located on the parasite genome (5). However, a hemD orthologue has been identified in Toxoplasma gondii and is expected to be targeted to the apicoplast, a chloroplast-like organelle in the parasite (6). Based mostly on bioinformatics-based predictions, some e...