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The crystal structure of dihydrofolate reductase (EC 1.5.1.3) from Escherichia
coli has been solved as the binary complex with NADP+ (the holoenzyme) and as
the ternary complex with NADP+ and folate. The Bragg law resolutions of the
structures are 2.4 and 2.5 A, respectively. The new crystal forms are
nonisomorphous with each other and with the methotrexate binary complex reported
earlier [Bolin, J. T., Filman, D. J., Matthews, D. A., Hamlin, R. C., &
Kraut, J. (1982) J. Biol. Chem. 257, 13650-13662]. In general, NADP+ and folate
binding conform to predictions, but the nicotinamide moiety of NADP+ is
disordered in the holoenzyme and ordered in the ternary complex. A mobile loop
(residues 16-20) involved in binding the nicotinamide is also disordered in the
holoenzyme. We report a detailed analysis of the binding interactions for both
ligands, paying special attention to several apparently strained interactions
that may favor the transition state for hydride transfer. Hypothetical models
are presented for the binding of 7,8-dihydrofolate in the Michaelis complex and
for the transition-state complex.
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