The existence of a Cryptic Allosteric Site on Plasmodium falciparum Malate Dehydrogenase

Abstract

Approximately one million people die from malaria each year, and thus this disease is a prevalent and pressing matter. Plasmodium falciparum, the protozoan parasite responsible for malaria has been subject to intensive study with the goal of developing specific drugs for malaria treatment. Plasmodium falciparum has a distinctive Malate Dehydrogenase (MDH), being tetrameric compared to the canonical dimeric forms of MDH, yet shares the same catalytic and substrate binding site motives as human mitochondrial and cytosolic forms. To investigate the possible existence of a cryptic allosteric site in the P falciparum enzyme, we have used sequence and structural bioinformatics and identified several short sequences,(G175‐L176, Q211‐M216) each containing highly conserved (in Plasmodium and related species) amino acids that are different in the Plasmodium falciparum enzyme than the human mitochondrial or cytosolic forms. To investigate the possibility that these regions form a cryptic allosteric site in Plasmodium falciparum we expressed, purified, using NiNTA Affinity chromatography, and characterized all three forms (Plasmodium falciparum, human cytosolic and human mitochondrial malate dehydrogenases). In terms of kinetic parameters the Plasmodium falciparum enzyme resembled the mammalian mitochondrial isoform, exhibiting similar substrate inhibition with oxaloacetate and weaker affinity for NADH than the mammalian cytosolic isoform. To determine the potential that the Plasmodium falciparum enzyme had a cryptic allosteric site we designed two mutations, one in each of the two significant sequence differences between the human isoforms and the Plasmodium falciparum enzyme (.D176N and R214E ). Each mutation was constructed using Quikchange mutagenesis, transformed into XL Gold cells and expressed. The purified mutant proteins were characterized using enzyme kinetics and size exclusion chromatography and their overall secondary structures compared to the wildtype enzyme using circular dichroism spectroscopy. Stability was compared using CD thermal melts at 222nm and a Fluorescence based Thermal Shift assay. While both mutations showed altered specific activities compared to wildtype, the D176N mutant showed a 90% lower activity than wildtype Plasmodium falciparum malate dehydrogenase. Both mutations significantly lowered affinity for the cofactor NADH, >20fold decrease in affinity. Taken together these results suggest that Plasmodium falciparum malate dehydrogenase may have a highly conserved cryptic allosteric site distinct from any such site in the mammalian isoforms. This Cryptic Allosteric Site could be exploited to develop inhibitors of Plasmodium falciparum malate dehydrogenase that would not impact the human isoforms.Support or Funding InformationThis work was supported by NSF Grants 1726932 and 0448905.