The formation of hemozoin crystals is an essential and druggable parasite pathway. However, in the absence of parasite-centered research approaches, the mechanistic basis of this unique biomineralization process remains elusive.
When the parasite proteolytically cleaves the hemoglobin inside the digestive vacuole, the iron-containing co-factor heme is released and rapidly oxidized to hematin. Owing to its pro-oxidant and detergent-like properties, free hematin is highly toxic. Thus, to avoid cell damage, two hematin monomers are linked via reciprocal iron-carboxylate bonds and the dimers assemble into high aspect ratio crystals through intermolecular hydrogen bonding (panel A). The formation of hemozoin is initiated during early parasite development and continues throughout parasite maturation. Upon parasite egress, the crystals are left behind, encapsulated in a residual body (panel B).
Although Robert Virchow and Charles Laveran described the connection between hemozoin and Plasmodium more than a century ago, the mechanisms of heme biomineralization remain elusive. Some studies suggest that hemozoin formation is a spontaneous and autocatalytic process, while others indicate an involvement of parasite proteins and lipids in the nucleation and growth of the crystals. The uncertainty surrounding this issue is alarming, considering that heme biomineralization is the target of the probably most effective antimalarial drug ever, chloroquine. This 4-aminoquinoline interferes with the adsorption of hematin onto growing hemozoin crystals, thereby causing a toxic build-up of unbound hematin. Although the global spread of efflux-mediated drug resistance has limited the utility of chloroquine, the outstanding efficacy of the 4-aminoquinoline antimalarials highlights hemozoin formation as a central Achilles’ heel of Plasmodium. If we can understand the underlying mechanisms, then there is hope for the development of similarly effective drugs targeting this unique parasite pathway. Therefore, we aim to identify and characterize the physicochemical and biogenic factors driving hemozoin formation in P. falciparum. Our studies prioritize experimentation with live malaria parasites over minimalistic crystallization assays and have previously led to the identification of a vacuolar lipocalin, called PV5/PfLCN, that modulates the quantities and architecture of hemozoin in cellulo (panels C-E). Elucidating the molecular functions of such parasite-tailored proteins will lead to a more complete understanding of this unique biomineralization process and might open up new avenues for antimalarial drug development.