Human malaria parasite orotate phosphoribosyltransferase: functional expression, characterization of kinetic reaction mechanism and inhibition profile

Plasmodium falcipartum, the causative agent of the most lethal form of human malaria, relies on de novo pyrimidine biosynthesis. A acne encoding, orotate phosphoribosyltransferase (OPRT), the fifth enzyme of the de novo pathway catalyzing formation of orotidine 5'-monophosphate (OMP) and pyrophosphate (PPi) from 5-phosphoribosyl-l-pyrophosphate (PRPP) and orotate, was identified from P falciparum (pfOPRT). The deduced amino acid sequence for pJOPRT was compared with OPRTs from other organisms and found to be most similar to that of Escherichia coli. The catalytic residues and consensus sequences for substrate binding in the enzyme were conserved among, other organisms. The pjOPRT was exceptional in that it contained a unique insertion of 20 amino acids and an amino-terminal extension of 66 amino acids, making the longest amino acid sequence (281 amino acids with a predicted molecular mass of 33 kDa). The cDNA of the pJOPRT gene was cloned, sequenced and functionally expressed in soluble form. The recombinant pJOPRT was purified from the E coli lysate by two steps, nickel metal-affinity and gel-filtration chromatography. From 11 E coli culture, 1.2-1.5 mg of pure pjOPRT was obtained. SDS-PAGE revealed that the pjOPRT had a molecular mass of 33 kDa and analytical gel-filtration chromatography showed that the enzyme activity eluted at approximately 67 kDa. Using dimethyl suberimidate to cross-link neighboring subunits of the pJOPRT, it was confirmed that the native enzyme exists in a dimeric form. The steady state kinetics of initial velocity and product inhibition studies indicate that the enzyme pJOPRT follows a random sequential kinetic mechanism. Compounds aimed at the pJOPRT nexus may act against the parasite through at least two mechanisms: by directly inhibiting the enzyme activity, or be processed to an inhibitor of thymidylate synthase. This study provides a working system with which to investigate new antimalarial agents targeted against P falciparum OPRT. (C) 2004 Elsevier B.V. All rights reserved.