Conformational trimethoprim

The conformations, as determined by crystal structure analyses, of 1-methylweberine, two tetramethoxy weberine analogs, and three polymetho-prims, shown in Fig. 16, are displayed in Figs. 18 and 19. These compounds were synthesized by Takahashi and Brossi (39a) and the crystal structures were determined by J. L. Flippen-Anderson, J. F. Chiang, and I. L. Karle (39b), with the exception of trimethoprim (40) where the three adjacent methoxy groups have the same conformation as shown in Fig. 17(c). 

The binding site for trimetrexate is well defined and was compared with the binding sites in related complexes formed with methotrexate and trimethoprim. No major conform -... 

Another potent inhibitor of DHFR is trimethoprim. The crystal structures of two enzyme-inhibitor complexes (DHFR from Escherichia coli and chicken liver) have been determined [87]. Surprisingly, the inhibitor adopts different conformations in the two proteins. In the chicken liver enzyme, a butterfly-like conformation is observed (t,/t2 = -85°/102 ) in the bacterial enzyme, the aromatic rings are oriented nearly perpendicular to each other ( twisted conformation, r,/T2 = 177°/76°). The inhibitor shows higher affinity for the bacterial enzyme, and the lower energy of the twisted conformation may be partially responsible for this. 

To the small extent to which trimethoprim binds to chicken liver DHFR, X-ray crystallography reveals it to be squeezed into a butterfly conformation, less energetically favourable than the position it occupies in the E. coli enzyme where the two rings are nearly perpendicular to one another (Matthews, 1981). 

Other signals from protons in bound ligands (N2 in NADPH, H6 in trimethoprim) have been detected in the aromatic regions of the spectra of complexes with the deuteriated enzymes. The detection and assignment of such signals should enable more detailed kinetic and conformational information to be obtained from studies of transfer of... 

Multiple conformations have been detected in several other complexes of L. easel DHFR (for example, with NADP and trimethoprim, and with substituted pyrimethamines) and also in complexes with 5. faeelum DHFR and E. eoU DHFR it seems likely that many other protein ligand complexes will exist as mixtures of conformations. Of course, such conformations are more difficult to detect directly if they are in fast exchange. 

Fig. 6. P-NMR spectra (at 4.50 MHz) of E-trimethoprim NADP complex showing the mixture of two conformations of I and II (Gronenbom et al, 1981b). The theoretical spectra for I and II are shown by stick plots.

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