Dihydrofolate, binding mode

FIGURE 17.26. Binding of (a) methotrexate to dihydrofolate reductase and (b) a comparison with a model of tetrahydrofolate bound in the same orientation, (c) The binding mode observed for NADPH and methotrexate in the crystalline state. It is seen in (c) that when tetrahydrofolate is bound in the same way as methotrexate, the hydrogen atom cannot be transferred, (d) Therefore the tetrahydrofolate must be bound in a different manner from methotrexate for a reaction to occur. The two orientations of the two ligands are related by a rotation. This has been verified by an X-ray structure determination (Ref. 122). Thus tetrahydrofolate and methotrexate do not bind in the same manner to the enzyme. 

Another enzyme for which X-ray diffraction studies have aided in an analysis of the mode of action is the enzyme dihydrofolate reductase. This catalyzes the reduction of 7,8-dihydrofolate to 5,6,7,8-tetrahydrofolate, an essential coenzyme used in the synthesis of thymidylate, inosinate, and methionine. The antitumor agent methotrexate is a powerful inhibitor of dihydrofolate reductase, causing, on binding, a cellular deficiency of thymidylate (the cause of its antitumor activity). The crystal structures of the enzyme from two bacterial sources—Escherichia coli and Lactobacillus casei—and from chicken liver have been studied (88-90). Both the E. coli and L casei enzymes have been studied as complexes with methotrexate bound at the active site, and, in the case of the . casei enzyme, the cofactor, NADPH, was also present. 

It is evident, therefore, that the 2,4-diaminopyrimidines, like pyrimethamine and trimethoprim, the dihydrotriazines like proguanil (in its active metabolite form), the 2,4-diaminopteridines, and methotrexate, inhibit dihydrofolate reductase. Their selective toxicity to plasmodia may be due to a combination of greater binding to the parasite enzyme and to their selective uptake by parasitised erythrocytes. However, a new antifolic mode of action has recently been proposed for compounds like tetrahydrohomopteroic acid [288], which may inhibit folate metabolism by an action on the feedback... 

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