Haemozoin (β‐haematin) biomineralization occurs by self‐assembly near the lipid/water interface

Abstract

Several blood-feeding organisms, including the malaria parasite detoxify haem released from host haemoglobin by conversion to the insoluble crystalline ferriprotoporphyrin IX dimer known as haemozoin. To date the mechanism of haemozoin formation has remained unknown, although lipids or proteins have been suggested to catalyse its formation. We have found that beta-haematin (synthetic haemozoin) forms rapidly under physiologically realistic conditions near octanol/water, pentanol/water and lipid/water interfaces. Molecular dynamics simulations show that a precursor of the haemozoin dimer forms spontaneously in the absence of the competing hydrogen bonds of water, demonstrating that this substance probably self-assembles near a lipid/water interface in vivo.

The mechanism of antimalarial action of the ruthenium(II)–chloroquine complex [RuCl2(CQ)]2

“…This means that the metal complex probably accumulates near water-lipid interface regions within the digestive vacuole of the parasite, rather than deeply inside the aqueous regions where CQ concentrates. This observation, together with the idea that lipids catalyze heme aggregation within plasmodium food vacuoles [47,48] and with the findings of Egan et al [36] that β-hematin spontaneously forms by rapid self-assembly near octanol-water or lipid-water interfaces can explain the enhanced antimalarial activity of complex 1 both in vitro and in vivo, when compared with CQ. Furthermore, the fact that the complex is active against CQ-resistant strains of P. falciparum may also be related to its greater lipophilicity, in line with previous reports indicating a lowered ability of the mutated transmembrane transporter PfCRT to promote the efflux of highly lipophilic drugs [40,51].…”

“…This is important in connection with the proposal that lipids catalyze heme aggregation, probably by increasing heme solubility in acidic environments [47]; neutral lipids that catalyze the formation of hemozoin in vitro have been shown to form lipid bodies associated with plasmodium food vacuoles [48]. Egan et al [36] recently demonstrated that under acidic physiologically realistic conditions β-hematin spontaneously forms by rapid selfassembly near octanol-water, pentanol-water, or lipid-water interfaces. Our partition data (Fig.…”

“…Inhibition of the heme aggregation at the interface of n-octanol-aqueous buffer mixtures was studied following a modification of the method described by Egan et al [36]. To establish a base line for the aggregation process, hemin was dissolved in 0.1 M NaOH solution to generate hematin and acetone was added until the acetone-to-water ratio was 4:6; the final solution contained 15 mg hematin per milliliter.…”

“…4b, when the inhibition assay was conducted in a 1:1 n-octanol-water mixture, a simple model of a waterlipid interface [49], following an adaptation of the procedure described by Egan et al [36] in which both the hematin and the drug are carefully introduced close to the interface, the overall process is much faster (30 min), and more importantly, the activity trend is reversed and complex 1 was significantly more efficient than CQDP (50% inhibition at drug-to-hemin concentration ratios of approximately 1.2 and 2.0, respectively). These results, together with the observation of Egan et al that β-hematin rapidly forms near octanol-water or lipid-water interfaces [36], led to the idea that a drug that concentrates near such interfaces at a pH value typical of the acidic food vacuole of Plasmodium parasites should be in a better position than the highly hydrophilic CQ to inhibit the process of heme aggregation, and this provides an explanation for the higher antimalarial activity of complex 1 compared with CQDP [28].…”

Aligned hemozoin crystals in curved clusters in malarial red blood cells revealed by nanoprobe X-ray Fe fluorescence and diffraction

“…On the premise that hemozoin nucleation is induced via a surface layer of lipid glycerol groups that expose OH, CH 2 , and oxygen lonepairs, nucleation was predicted to occur via the {100} face, which exposes CO 2 H and C─H groups (5). Precipitation of micron-sized crystals of β-hematin from a myristoyl-glycerol solution/water interface was reported by Egan et al (6). The orientation of such crystals at the water surface was identified by grazing incidence X-ray diffraction, revealing crystals floating on their {100} faces, but not the presence of lipid layers (7).…”