Two cap residues in the S1 subsite of a Plasmodium falciparum M1-family aminopeptidase promote broad specificity and enhance catalysis.

The aminopeptidase PfA-M1 is a key contributor to peptide catabolism in the human malaria parasite Plasmodium falciparum. PfA-M1 substrate specificity is shaped by the cylindrical S1 subsite, which accommodates the sidechain of the substrate P1 residue. At the top of the S1 subsite are two "cap" residues, E572 and M1034, that are positioned to influence S1 subsite specificity. In this study, we have mutated the cap residues, individually and together, and have evaluated the effects on PfA-M1 specificity and catalytic efficiency. When the P1 residue was too small to engage the cap residues, the mutations had no effect on catalysis. Hydrolysis of dipeptide substrates with a basic P1 residue was significantly impaired in the E572A mutant, most likely due to the loss of a stabilizing salt bridge between E572 and the P1 sidechain. With M1034A, a substantial reduction in catalytic efficiency was observed when the P1 sidechain was large and non-polar. The double E572A/M1034A exhibited significant decreases in catalytic efficiency for most substrates. This effect was not reversed with the polar substitutions E572N/M1034Q, which replaced the PfA-M1 cap residues with those of Escherichia coli aminopeptidase N. Both E572 and M1034 contributed to the binding of the competitive aminopeptidase inhibitor bestatin.