Structure and mechanism of a transmission blocking vaccine candidate protein Pfs25 from P. falciparum: a molecular modeling and docking study.

Malaria is caused by the protozoan Plasmodium. The parasite Plasmodium completes its life cycle inside two hosts, i. e. human and mosquito. Among all known Plasmodium species, Plasmodium falciparum is known to cause maximum mortality. Various studies done on the mosquito stages of the parasite suggest that the proteins present on the parasite's surface are responsible for its survival under the adverse conditions prevailing in the mosquito midgut. When human blood containing Plasmodium gametocytes enters the mosquito gut, the gametocytes form gametes which then fuse to form zygotes. At this stage two closely related proteins, Pfs25 and Pfs28 are expressed on the surface of the parasite which continue to express up to the young oocyst stage. These proteins present on zygotes, ookinetes and young oocysts of Plasmodium are categorized in P25 and P28 families and are well known malaria vaccine candidate proteins. In this study, we have done sequence analysis, homology modeling and docking studies of a typical member of the P25 family of ookinete surface protein, i.e. Pfs25 from Plasmodium falciparum. We have built a 3D model of Pfs25 based on the X-ray crystallographic structure of Pvs25 from Plasmodium vivax. Also we have modeled the Fv region of the malaria transmission blocking monoclonal antibody 4B7. This antibody is the transmission blocking monoclonal antibody for Pfs25 protein. Pfs25 and 4B7 scFv (single chain variable fragment only) docking results indicate that EGF domain III of the Pfs25 protein interacts with the scFv region of modeled 4B7 antibody forming seven hydrogen bonds out of which six are formed with heavy chain of scFv region. Docking results of Pfs25 with gamma chain of laminin also suggest a possible role of Pfs25 protein in host parasite interaction.