During the asexual blood stage, catalyzes the cleavage of denatured host hemoglobin (Hb) or globins (PubMed:16624575). 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. Dimerization causes loss of catalytic activity (PubMed:20435072). Inhibited by pepstatin A (PubMed:16624575). Inhibited by Zn(2+) (PubMed:20435072). Optimum pH is 5.2 (PubMed:16624575). Activity decreases sharply at pH above 6 (PubMed:16624575). Optimum pH is 5.5-7.5 (PubMed:18312598). Probable homodimer; in the zymogen form (Probable). Monomer; in the active form (PubMed:20435072). Acidification disrupts homodimerization (By similarity). Component of the hemozoin formation complex (HFC) composed of falcipain 2, plasmepsins PMII, PMIII/HAP and PMIV, heme detoxifying protein HDP and falcilysin FLN (PubMed:23471987). The HFC complex is involved in hemoglobin degradation and detoxification of heme in the food vacuole during the asexual blood stage (PubMed:23471987). 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 (By similarity). Proteolysis requires an acidic environment (By similarity). Transprocessing may serve as an alternate activation system (PubMed:18312598). No growth defect (PubMed:15513918). However, slight decrease in proliferation and slight increase in doubling time during the asexual blood stage in an amino acid-limited medium (PubMed:15513918). Triple knockout of PMI, PMII and PMIII causes a slight decrease in proliferation during the asexual blood stage in an amino acid-limited medium (PubMed:17581121). 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 (PubMed:17581121). Belongs to the peptidase A1 family. Unlike other plasmepsins, one of the two catalytic aspartates, Asp-157, is replaced with histidine; however, the protein is catalytic active (PubMed:16624575). Unlikely to act as a serine protease (PubMed:18312598). His-157 may stabilizes the catalysis and Asp-337 may act as both an acid and a base during catalysis. It is unclear if PMIII is glycosylated as other members of the same enzyme family, ie. PMI and PMII, are not.