Comparative structural analysis and kinetic properties of lactate dehydrogenases from the four species of human malarial parasites

Parasite lactate dehydrogenase (pLDH) is a potential drug target for new antimalarials owing to parasite dependence on glycolysis for ATP production. The pLDH from all four species of human malarial parasites were cloned, expressed, and analyzed for structural and kinetic properties that might be exploited for drug development. pLDH from Plasmodium vivax, malariae, and ovale exhibit 90-92% identity to pLDH from Plasmodium falciparum. Catalytic residues are identical. Resides 1250 and T246, conserved in most LDH, are replaced by proline in all pLDH. The pLDH contain the same five-amino acid insert (DKEWN) in the substrate specificity loops. Within the cofactor site, pLDH from P. falciparum and P. malariae are identical, while pLDH from P. vivax and P. ovale have one substitution. Homology modeling of pLDH from P. vivax, ovale, and malariae with the crystal structure of pLDH from P. falciparum gave nearly identical structures. Nevertheless, the kinetic properties and sensitivities to inhibitors targeted to the cofactor binding site differ significantly. Michaelis constants for pyruvate and lactate differ 8-9-fold; Michaelis constants for NADH, NAD(+), and the NAD(+) analogue 3-acetylpyridine adenine dinucleotide differ up to 4-fold. Dissociation constants for the inhibitors differ up to 21-fold. Molecular docking studies of the binding of the inhibitors to the cofactor sites of all four pLDH predict similar orientations, with the docked ligands positioned at the nicotinamide end of the cofactor site. pH studies indicate that inhibitor binding is independent of pH in the pH 6-8 range, suggesting that differences in dissociation constants for a specific inhibitor are not due to altered active site pK values among the four pLDH.