Studies on Molecular Biology of Prostaglandin Biosynthesis by parasitic Trypanosoma

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In this study our aims are the elucidation of the functions and the three-dimensional structures of prostaglandin (PG) synthetic enzymes in parasitic protozoa (Trypanosoma, Leishmania, and Plasmodia), and the development of anti-parasitic drugs. Trypanosoma brucei prostaglandin (PG) F_<2α> synthase (TbPGFS), an aldo-ketoreductase (AKR), catalyzes the NADPH-dependent reduction of the endoperoxide moiety of PGH_2 into PGF_<2α> whose overproduction during trypanosomiasis causes abortion in infected subjects. Here we report the crystal structure of TbPGFS in complex with NADP^+ and citrate at 2.1 Å resolution. TbPGFS adopts a parallel (αβ)_8-barrel fold lacking the protrudent loops. The core active site structure is hydrophobic to bind hydrophobic substrates and contains tyrosine, lysine, histidine and aspartate known as a catalytic tetrad in other aldo-ketoreductases. Mutagenesis identifies that Tyr 52 and Asp 47 are not concerning to the enzyme reaction, showing His 110 and Lys 77 forms the catalytic dyad to perform an acid-assisted reduction in which His 110 transfers its proton to the substrate. These findings identify a novel catalytic mechanism for the biological reduction of the endoperoxide PGH_2 by an aldo-ketoreductase. The structure should allow for rational design of specific inhibitors useful to investigate the physiological roles of TbPGFS in trypanosomes. The reduction of PGH2 has been known for some time and it has also been shown that PGF_<2α> is enzymatically synthesized by a reductive cleavage of the 9, 11-endoperoxide of PGH_2^<43>. However, how the enzyme operates the reductive cleavage of the 9, 11-endoperoxide of PGH_2 has not been elucidated. We provide here the first structural information of a PGH_2 reductase in protozoa enzymes. This structure should now allow the rational design of specific inhibitors that might help investigate the physiological role of TbPGFS in trypanosomes.