Heme binding to the histidine-rich protein II from Plasmodium falciparum

The histidine-rich protein II (HRP II) from Plasmodium falciparum has been implicated in the formation of hemozoin, a detoxified, crystalline form of ferric protoporphyrin IX (Fe3+-PPIX) produced by the parasite. Fe3+-PPIX titrations coupled with quantitative amino acid analysis showed that HRP II binds 15 Fe3+-PPIX molecules per 30 kDa monomer. Circular dichroism spectroscopy was used to probe the secondary structure of HRP II with and without bound Fe3+-PPIX. These studies have revealed large changes in the secondary structure with Fe3+-PPIX binding, changing from a random coil in the absence of Fe3+-PPIX to a more ordered helical structure in the presence of Fe3+-PPIX. The Fe3+-PPIX-bound HRP II structure most closely resembles a 3(10)-helix. Coincident with this structural change caused by Fe3+-PPIX binding, the formation of an intermolecular disulfide bond occurs between HRP II monomers. In vitro pull-down assays show an interaction between monomers that is dependent on the presence of Fe3+-PPIX. One model that best fits with the data reported here requires formation of 15 intermolecular bishistidyl ligated Fe3+-PPIX molecules arranged in a head to head fashion, which would then allow for the formation of an intermolecular disulfide bond. The structure best able to accommodate these requirements is a 3(10)-helix.