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The structure of Pneumocystis carinii dihydrofolate reductase to 1.9 A resolution
J N Champness1, A Achari, S P Ballantine
1Physical Sciences Department, Wellcome Research Laboratories, Beckenham, Kent, UK.
Background:
The fungal pathogen Pneumocystis carinii causes a pneumonia which is an opportunistic infection of AIDS patients. Current therapy 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 the enzyme should form the basis for design of more potent and selective therapeutic agents for treatment of the disease.
Results:
The structure of P. carinii DHFR in complex with reduced nicotinamide adenine dinucleotide phosphate and trimethoprim has accordingly been solved by X-ray crystallography. The structure of the ternary complex has been refined at 1.86 A resolution (R = 0.181). A similar ternary complex with piritrexim (which is a tighter binding, but less selective inhibitor) has also been solved, as has the binary complex holoenzyme, both at 2.5 A resolution.
Conclusions:
These structures show how two drugs interact with a fungal DHFR. A comparison of the three-dimensional structure of this relatively large DHFR with bacterial or mammalian enzyme-inhibitor complexes determined previously highlights some additional secondary structure elements in this particular enzyme species. These comparisons provide further insight into the principles governing DHFR-inhibitor interaction, in which the volume of the active site appears to determine the strength of inhibitor binding.
Insights
Structural analysis of Pneumocystis carinii dihydrofolate reductase (DHFR) reveals insights into drug interactions. This research aids in designing more effective therapies against Pneumocystis pneumonia in AIDS patients.
Area of Science:
- Structural biology
- Biochemistry
- Medical mycology
Background:
- Pneumocystis carinii pneumonia (PCP) is an opportunistic infection common in AIDS patients.
- Current trimethoprim therapy targets dihydrofolate reductase (DHFR) but is a weak inhibitor for P. carinii.
- Understanding P. carinii DHFR structure is crucial for developing improved therapeutics.
Purpose of the Study:
- To determine the three-dimensional structure of P. carinii DHFR.
- To provide a basis for designing more potent and selective therapeutic agents.
- To elucidate drug-enzyme interactions for enhanced anti-PCP drug development.
Main Methods:
- X-ray crystallography was employed to solve the structures.
- Structures of ternary complexes with trimethoprim and piritrexim were determined.
- The binary complex holoenzyme structure was also solved.
Main Results:
- The three-dimensional structure of P. carinii DHFR in complex with NADPH and trimethoprim was refined at 1.86 Å resolution.
- Similar ternary and binary complex structures were solved at 2.5 Å resolution.
- These structures illustrate drug-enzyme interactions within the fungal DHFR.
Conclusions:
- The determined structures reveal how trimethoprim and piritrexim interact with fungal DHFR.
- Comparisons with bacterial and mammalian DHFR highlight unique secondary structures in P. carinii DHFR.
- Active site volume is a key factor influencing DHFR-inhibitor binding strength.