Binding Isotope Effects for <i>para</i>-Aminobenzoic Acid with Dihydropteroate Synthase from <i>Staphylococcus aureus</i> and <i>Plasmodium falciparum</i>

Dihydropteroate synthase is a key enzyme in folate biosynthesis and is the target of the sulfonamide class of antimicrobials. Equilibrium binding isotope effects and density functional theory calculations indicate that the substrate binding sites for <i>para</i>-aminobenzoic acid on the dihydropteroate synthase enzymes from <i>Staphylococcus aureus</i> and <i>Plasmodium falciparum</i> present distinct chemical environments. Specifically, we show that <i>para</i>-aminobenzoic acid occupies a more sterically constrained vibrational environment when bound to dihydropteroate synthase from <i>P. falciparum</i> relative to that of <i>S. aureus</i>. Deletion of a nonhomologous, parasite-specific insert from the plasmodial dihydropteroate synthase abrogated the binding of <i>para</i>-aminobenzoic acid. The loop specific to <i>P. falciparum</i> is important for effective substrate binding and therefore plays a role in modulating the chemical environment at the substrate binding site