RESUMO
Detoxification of heme in Plasmodium depends on its crystallization into hemozoin. This pathway is a major target of antimalarial drugs. The crystalline structure of hemozoin was established by X-ray powder diffraction using a synthetic analog, ß-hematin. Here, we apply emerging methods of in situ cryo-electron tomography and 3D electron diffraction to obtain a definitive structure of hemozoin directly from ruptured parasite cells. Biogenic hemozoin crystals take a striking polar morphology. Like ß-hematin, the unit cell contains a heme dimer, which may form four distinct stereoisomers: two centrosymmetric and two chiral enantiomers. Diffraction analysis, supported by density functional theory analysis, reveals a selective mixture in the hemozoin lattice of one centrosymmetric and one chiral dimer. Absolute configuration has been determined by morphological analysis and confirmed by a novel method of exit-wave reconstruction from a focal series. Atomic disorder appears on specific facets asymmetrically, and the polar morphology can be understood in light of water binding. Structural modeling of the heme detoxification protein suggests a function as a chiral agent to bias the dimer formation in favor of rapid growth of a single crystalline phase. The refined structure of hemozoin should serve as a guide to new drug development.
RESUMO
Electron diffraction from three dimensional crystals, as a technique for solving molecular structures, is rapidly increasing in popularity. The development of methodology and software has borrowed, to great effect, from macromolecular X-ray crystallography. However, standardization lags behind the development of the technique, and practitioners are forced to work with inadequate data formats that are unable to capture a full description of their experiments. This creates obstacles that are increasingly difficult to overcome as experiments become ever faster and the need for data autoprocessing becomes more pressing. We present a data format standard based on best practice from macromolecular crystallography and demonstrate how the adoption of this standard enabled autoprocessing of datasets collected with a high-throughput detector system.