Pteridine reductase (PTR1) is an
NADPH-dependent
short-chain reductase found in parasitic trypanosomatid protozoans. The
enzyme participates in the salvage of
pterins and represents a target for the development of improved
therapies for
infections caused by these parasites. A series of crystallographic analyses of Leishmania major PTR1 are reported. Structures of the
enzyme in a binary complex with the cofactor
NADPH, and ternary complexes with cofactor and
biopterin, 5,6-dihydrobiopterin, and
5,6,7,8-tetrahydrobiopterin reveal that PTR1 does not undergo any major conformational changes to accomplish binding and processing of substrates, and confirm that these molecules bind in a single orientation at the catalytic center suitable for two distinct reductions. Ternary complexes with cofactor and
CB3717 and
trimethoprim (TOP), potent inhibitors of
thymidylate synthase and
dihydrofolate reductase, respectively, have been characterized. The structure with
CB3717 reveals that the
quinazoline moiety binds in similar fashion to the
pterin substrates/products and dominates interactions with the
enzyme. In the complex with TOP, steric restrictions enforced on the trimethoxyphenyl substituent prevent the
2,4-diaminopyrimidine moiety from adopting the
pterin mode of binding observed in
dihydrofolate reductase, and explain the inhibition properties of a range of
pyrimidine derivates. The molecular detail provided by these complex structures identifies the important interactions necessary to assist the structure-based development of novel
enzyme inhibitors of potential therapeutic value.