STRUCTURE OF PNEUMOCYSTIS-CARINII DIHYDROFOLATE-REDUCTASE TO 1.9-ANGSTROM RESOLUTION

Background: The fungal pathogen Pneurnocystis carinii causes a pneumon ia which is an opportunistic infection of AIDS patients. Current thera py includes the dihydrofolate reductase (DHFR) inhibitor trimethoprim which is selective but only a relatively weak inhibitor of the enzyme for P. carinii. Determination of the three-dimensional structure of th e enzyme should form the basis for design of more potent and selective therapeutic agents for treatment of the disease. Results: The structu re of P. carinii DHFR in complex with reduced nicotinamide adenine din ucleotide phosphate and trimethoprim has accordingly been solved by X- ray crystallography. The structure of the ternary complex has been ref ined at 1.86 Angstrom resolution (R=0.181). A similar ternary complex with piritrexim (which is a tighter binding, but less selective inhibi tor) has also been solved, as has the binary complex holoenzyme, both at 2.5 Angstrom resolution. Conclusions: These structures show how two drugs interact with a fungal DHFR. A comparison of the three-dimensio nal structure of this relatively large DHFR with bacterial or mammalia n enzyme-inhibitor complexes determined previously highlights some add itional secondary structure elements in this particular enzyme species . These comparisons provide further insight into the principles govern ing DHFR-inhibitor interaction, in which the volume of the active site appears to determine the strength of inhibitor binding.