During the asexual blood stage, catalyzes the cleavage of denatured host hemoglobin (Hb) (By similarity). Digestion of host Hb is an essential step which provides the parasite with amino acids for protein synthesis, and regulates osmolarity (Probable). Hydrolysis of the bonds linking certain hydrophobic residues in hemoglobin or globin. Also cleaves small molecules substrates such as Ala-Leu-Glu-Arg-Thr-Phe-|-Phe(NO2)-Ser-Phe-Pro-Thr. Inhibited by KNI derived compounds KNI-10333 and to a lesser extent KNI-10743. In trophozoites, localizes to the digestive (or food) vacuole, an acidic vacuole where host hemoglobin is digested. Expressed during the asexual blood stage; expression begins in trophozoites and continues in schizonts (at protein level). Proteolytically cleaved into the soluble active mature form by cysteine proteases in the digestive vacuole of trophozoites. Proteolysis requires an acidic environment. Autoprocessing or transprocessing by other plasmepsins such as PMII may serve as an alternate activation system. Quadruple knockout of PMI, PMII, PMIII and PMIV causes a decrease in proliferation, an impaired proliferation in an amino acid-limited medium, a reduced formation of haemozoin and an abnormal accumulation of endosomal vesicles inside the digestive vacuole. Belongs to the peptidase A1 family. It is unclear if PMIV is glycosylated as other members of the same enzyme family, ie. PMI and PMII, are not.